Solved Problems
Strings
Printing

Output a string to the console

Write the string "Hello World!" to STDOUT
python
print "Hello World!"
clojure
(println "Hello World!")
erlang
io:format("Hello, World!~n").
fantom
echo("Hello World!")

Retrieve a string containing ampersands from the variables in a url

My PHP script first does a query to obtain customer info for a form. The form has first name and last name fields among others. The customer has put entries such as "Ron & Jean" in the first name field in the database. Then the edit form script is called with variables such as

"http://myserver.com/custinfo/edit.php?mode=view&fname=Ron & Jean&lname=Smith".

The script variable for first name $_REQUEST['firstname'] never gets beyond the "Ron" value because of the ampersand in the data.

I have tried various functions like urldecode but all to no avail. I even tried encoding the url before the view screen is painted so that the url looks like "http://myserver/custinfo/edit.php?mode=view&fname="Ronxxnbsp;xxamp;xxnbsp;Jean"&lname=SMITH". (sorry I had to add the xx to replace the ampersand or it didn't display meaningful url contents the browser sees.)

Of course this fails for the same reasons. What is a better approach?
python
# I'm not really sure this is what the site is for,
# but the one unsolved problem for python was grating me.
# Anyway, I think this is what you're looking for.

from urllib import urlencode

query_dict = {'mode': 'view',
'fname': 'Ron & Jean',
'lname': 'Smith'}

print urlencode(query_dict.items())

# Which will be 'lname=Smith&mode=view&fname=Ron+%26+Jean'.
clojure
(->> {"mode" "view"
"fname" "Ron & Jean"
"lname" "Smith"}
(map #(str (URLEncoder/encode (first %) "UTF-8")
"="
(URLEncoder/encode (second %) "UTF-8")))
(reduce (fn [url e] (str url "&" e))
"http://myserver.com/custinfo/edit.php"))
erlang
% encode ampersand in your string using %XX where XX is hex code for ampersand
% optionally encode spaces for completeness sake to keep URL solid
URL = "http://myserver.com/custinfo/edit.php?mode=view&fname=Ron%20%26%20Jean&lname=Smith",
{_, Query} = string:tokens(URL, "?"),
KeyValuePairs = string:tokens(Query, "&"),...
fantom
encoded := `http://myserver.com/custinfo/edit.php`.plusQuery(
["fname":"Ron & Jean", "lname":"Smith"]).encode
echo(encoded)

string-wrap

Wrap the string "The quick brown fox jumps over the lazy dog. " repeated ten times to a max width of 78 chars, starting each line with "> "

Expected output:
> The quick brown fox jumps over the lazy dog. The quick brown fox jumps over t
> he lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox
> jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The qui
> ck brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy
> dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps o
> ver the lazy dog. The quick brown fox jumps over the lazy dog.
python
def wrap(string, length):

while len(string):
print("> " + string[0:length - 1])
string = string[length - 1:].strip()


wrap("The quick brown fox jumps over the lazy dog. " * 10, 78)
clojure
(defn string-wrap [s]
(if (= 0 (count s))
nil
(lazy-seq (cons (apply str (take 78 s))
(string-wrap (drop 78 s))))))

(let [s (apply str (repeat 10 "The quick brown fox jumps over the lazy dog. "))]
(doseq [line (string-wrap s)]
(println "> " line)))
erlang
wrapper(String, Times, Length) ->
StrList = lists:reverse(formatter(string:copies(String, Times), Length, [])),
lists:foreach(fun(Str) -> io:format("~p~n", [Str]) end, StrList).

formatter([], _Length, Acc) -> Acc;
formatter(String, Length, Acc) when length(String) > Length - 1->
{Head, Tail} = lists:split(Length - 1, String),
formatter(string:strip(Tail), Length, [[$>, $ | Head] | Acc]);
formatter(String, Length, Acc) ->
formatter([], Length, [[$>, $ | String] | Acc]).
fantom
s:=Str[,].fill("The quick brown fox jumps over the lazy dog. ",10).join
while(s.size>0){
echo("> "+s[0..(77.min(s.size))-1])
s=(s.size>77)?s[77..-1].trim : ""
}
Declaration

Define a string containing special characters

Define the literal string "\#{'}${"}/"
python
# yes, Python has way too many forms of string literals :)
print "\\#{'}${\"}/"
print "\\#{'}${"'"'"}/"
print r"""\#{'}${"}/"""
print '\\#{\'}${"}/'
print '\\#{'"'"'}${"}/'
print r'''\#{'}${"}/'''
clojure
(def special "\\#{'}${\"}/")
erlang
Special = "\\#{'}\${\"}/",
fantom
special := Str<|\#{'}${"}/|>

Define a multiline string

Define the string:
"This
Is
A
Multiline
String"
python
text = """This
Is
A
Multiline
String"""
# with proper indentation
text = (
"This\n"
"Is\n"
"A\n"
"Multiline\n"
"String"
)
clojure
(def multiline "This\nIs\nA\nMultiline\nString")
erlang
Text = "This\nIs\nA\nMultiline\nString",
fantom
s := "This
Is
A
Multiline
String"

Define a string containing variables and expressions

Given variables a=3 and b=4 output "3+4=7"
python
class EvalDict(dict):
def __getitem__(s, k):
return eval(k, s)

a=3; b=4
"%(a)d+%(b)d=%(a+b)d" % EvalDict(locals())
a=3; b=4
"%d+%d=%d" % (a, b, a+b)
clojure
(format "%d + %d = %d" a b (+ a b))
erlang
A = 3, B = 4,
io:format("~B+~B=~B~n", [A, B, (A+B)]).
fantom
echo("$a+$b=${a+b}")
Manipulation

Reverse the characters in a string

Given the string "reverse me", produce the string "em esrever"
python
"reverse me"[::-1]
clojure
(require '[clojure.contrib.str-utils2 :as str])
(str/reverse "reverse me")
(apply str (reverse "reverse me"))
erlang
Reversed = lists:reverse("reverse me"),
Reversed = revchars("reverse me"),
fantom
"reverse me".reverse

Reverse the words in a string

Given the string "This is a end, my only friend!", produce the string "friend! only my end, the is This"
python
' '.join(reversed("This is a end, my only friend!".split()))
clojure
(require '[clojure.contrib.str-utils2 :as str])
(str/join " " (reverse (str/split "this is the end, my only friend!" #" ")))
(apply str (interpose " " (reverse (re-seq #"[^\s]+" "This is the end, my only friend!"))))
erlang
Reversed = string:join(lists:reverse(string:tokens("This is the end, my only friend!", " ")), " "),
fantom
"This is a end, my only friend!".split.reverse.join(" ")

Text wrapping

Wrap the string "The quick brown fox jumps over the lazy dog. " repeated ten times to a max width of 78 chars, starting each line with "> ", yielding this result:

> The quick brown fox jumps over the lazy dog. The quick brown fox jumps
> over the lazy dog. The quick brown fox jumps over the lazy dog. The
> quick brown fox jumps over the lazy dog. The quick brown fox jumps
> over the lazy dog. The quick brown fox jumps over the lazy dog. The
> quick brown fox jumps over the lazy dog. The quick brown fox jumps
> over the lazy dog. The quick brown fox jumps over the lazy dog. The
> quick brown fox jumps over the lazy dog.
python
import textwrap
print textwrap.fill("The quick brown fox jumps over the lazy dog. " * 10,
72, initial_indent="> ", subsequent_indent="> ")
clojure
(doseq [line (re-seq #".{0,70} "
(apply str
(repeat 10 "The quick brown fox jumps over the lazy dog. ")))]
(println ">" line))
erlang
TextWrap = textwrap(string:copies(Input, 10), 73 - length(Prefix)),
lists:foreach(fun (Line) -> io:format("~s~n", [string:concat(Prefix, Line)]) end, string:tokens(TextWrap, "\n")).
fantom
buf := Buf()
10.times { buf.writeChars("The quick brown fox jumps over the lazy dog. ") }
buf.flip

out := Env.cur.out
sep := ">"; max := 72 - sep.size - 1
acc := 0; Str? s := null
while ((s = buf.readStrToken) != null)
{
if (acc == 0)
out.print(sep)

acc += s.size
if (acc > max)
{
out.print("\n$sep")
acc = s.size
}
out.print(" $s")
buf.readStrToken(4096) { !it.isSpace }
acc++
}

Remove leading and trailing whitespace from a string

Given the string "  hello    " return the string "hello".
python
assert 'hello' == ' hello '.strip()
clojure
(use 'clojure.contrib.str-utils2)
(trim " hello ")
(clojure.string/trim " hello ")
(.trim " hello ")
erlang
Trimmed = string:strip(S),
fantom
s := " hello ".trim

Simple substitution cipher

Take a string and return the ROT13 and ROT47 (Check Wikipedia) version of the string.
For example:
String is: Hello World #123
ROT13 returns: Uryyb Jbeyq #123
ROT47 returns: w6==@ (@C=5 R`ab
python
# rot13, readable
rot13_tbl = string.maketrans("ABCDEFGHIJKLMabcdefghijklmNOPQRSTUVWXYZnopqrstuvwxyz", "NOPQRSTUVWXYZnopqrstuvwxyzABCDEFGHIJKLMabcdefghijklm")
string.translate("Hello World #123", rot13_tbl)


#
# "a bad programmer can write bad code in any language"
#

# rot13, "clever"
string.translate("Hello World #123", string.maketrans(string.lowercase+string.uppercase, string.lowercase[13:]+string.lowercase[:13]+string.uppercase[13:]+string.uppercase[:13]))

# rot47, very "clever"
''.join([ord(c) in range(33,127) and chr(((ord(c)-33+47)%(127-33))+33) or c for c in "Hello World #123"])

"Hello World #123".encode('rot13')
clojure
(use 'clojure.contrib.cond)

(defn rot13 [s]
(reduce str
(map #(char (let [c (bit-and (int (char %)) 0xDF)]
(+ % (cond-let [i]
(and (>= c (int \A)) (<= c (int \M))) 13
(and (>= c (int \N)) (<= c (int \Z))) -13
true 0))))
(map #(int (char %)) s))))

(defn rot47 [s]
(reduce str
(map #(char (+ % (cond-let [i]
(and (>= % (int \!)) (<= % (int \O))) 47
(and (>= % (int \P)) (<= % (int \~))) -47
true 0)))
(map #(int (char %)) s))))
erlang
rot13(Str) ->
lists:map(fun(A) ->
if
A >= $A, A =< $Z -> ((A - $A + 13) rem 26) + $A;
A >= $a, A =< $z -> ((A - $a + 13) rem 26) + $a;
true -> A
end
end, Str).

rot47(Str) ->
lists:map(fun(A) ->
if
A >= $!, A =< $~ ->
((A - $! + 47) rem 94) + $!;
true -> A
end
end, Str).
fantom
rot := |Str s, |Int c -> Int| remap -> Str|
{
rs := ""
s.each { rs += remap(it).toChar }
return rs
}

rot13 := |Str s -> Str|
{
rot(s) |Int c -> Int|
{
lc := c.lower
c += (lc >= 'a' && lc <= 'm') ? 13
: ((lc >= 'n' && lc <= 'z') ? -13 : 0)
return c
}
}

rot47 := |Str s -> Str|
{
rot(s) |Int c -> Int|
{
c += (c >= '!' && c <= 'O') ? 47
: ((c >= 'P' && c <= '~') ? -47 : 0)
return c
}
}

s := "Hello World #123"
echo("s=$s")
echo("rot13=${rot13(s)}")
echo("rot47=${rot47(s)}")
Case

Make a string uppercase

Transform "Space Monkey" into "SPACE MONKEY"
python
"Space Monkey".upper()
clojure
(.toUpperCase "Space Monkey")
erlang
io:format("~s~n", [string:to_upper("Space Monkey")]).
fantom
s := "Space Monkey".localeUpper

Make a string lowercase

Transform "Caps ARE overRated" into "caps are overrated"
python
"Caps ARE overRated".lower()
clojure
(.toLowerCase "Caps ARE overRated")
erlang
io:format("~s~n", [string:to_lower("Caps ARE overRated")]).
fantom
s := "Caps ARE overRated".localeLower

Capitalise the first letter of each word

Transform "man OF stEEL" into "Man Of Steel"
python
from string import capwords
capwords("man OF stEEL")
' '.join(s.capitalize() for s in "man OF stEEL".split())
"man OF stEEL".title()
clojure
(use 'clojure.contrib.str-utils2)
(join " " (map capitalize (split "man OF stEEL" #" ")))
erlang
Caps = string:join(lists:map(fun(S) -> to_caps(S) end, string:tokens("man OF stEEL", " ")), " "),
fantom
"man OF stEEL".split.map { it.localeLower.localeCapitalize }.join(" ")
Numbers
Mathematical

Find the distance between two points

python
# problem description doesn't say 2D points ;)
from math import sqrt
print sqrt(sum((x-y)**2 for x,y in zip(a, b)))
from math import hypot
print hypot(x2-x1, y2-y1)
clojure
(defstruct point :x :y)

(defn distance
"Euclidean distance between 2 points"
[p1 p2]
(Math/pow (+ (Math/pow (- (:x p1) (:x p2)) 2)
(Math/pow (- (:y p1) (:y p2)) 2))
0.5))

(distance (struct point 0 0) (struct point 1 1)) ; => 1.4142135623730951
(defn distance
"Euclidean distance between 2 points"
[[x1 y1] [x2 y2]]
(Math/sqrt
(+ (Math/pow (- x1 x2) 2)
(Math/pow (- y1 y2) 2))))

(distance [2 2] [3 3])
erlang
Distance = distance({point, 34, 78}, {point, 67, -45}),
io:format("~.2f~n", [Distance]).
Distance = distance(point:new(34, 78), point:new(67, -45)),
io:format("~.2f~n", [Distance]).
fantom
px1 := 34.0f; py1 := 78.0f; px2 := 67.0f; py2 := -45.0f
distance := |Float x1, Float y1, Float x2, Float y2 -> Float|
{ ((x2-x1).pow(2.0f) + (y2-y1).pow(2.0f)).sqrt }

distance(px1, py1, px2, py2)
Output

Zero pad a number

Given the number 42, pad it to 8 characters like 00000042
python
"%08d" % 42
clojure
(defn pad
([x] (if (> 8 (.length (str x))) (pad (str 0 x)) (str x)))
)
(defn pad [x]
(format "%08d" x))
(format "%08d" 42)
erlang
Formatted = io_lib:format("~8..0B", [42]),
io:format("~8..0B~n", [42]).
fantom
formatted := 42.toStr.padl(8, '0')
formatted := 42.toLocale("00000000")

Right Space pad a number

Given the number 1024 right pad it to 6 characters "1024  "
python
"%-6s" % 1024
str(1024).rjust(6)
'{0: <6}'.format(1024)
clojure
(let [s (str 1024)
l (count s)]
(str s (reduce str (repeat (- 6 l) " "))))
erlang
Formatted = io_lib:format("~-6B", [1024]),
io:format("~-6B~n", [1024]).
fantom
formatted := 1024.toStr.padr(6)

Format a decimal number

Format the number 7/8 as a decimal with 2 places: 0.88
python
"%.2f" % (7 / 8.0)
round(7./8., 2)
clojure
(format "%3.2f" (/ 7.0 8))
(* 0.01 (Math/round (* 100 (float (/ 7 8)))))
erlang
Formatted = io_lib:format("~.2f", [7/8]),
io:format("~.2f~n", [7/8]).
fantom
formatted := (7.0/8.0).toLocale("0.00")

Left Space pad a number

Given the number 73 left pad it to 10 characters "        73"
python
"%10s" % 73
clojure
(let [s (str 73)
l (count s)]
(str (reduce str (repeat (- 10 l) " ")) s ))
erlang
Formatted = io_lib:format("~10B", [73]),
io:format("~10B~n", [73]).
fantom
formatted := 73.toStr.padl(10)
Random

Generate a random integer in a given range

Produce a random integer between 100 and 200 inclusive
python
import random
random.randint(100, 200)
# uses best entropy source available (e.g. /dev/urandom, CryptGenRandom, ...)

import random
print random.SystemRandom().randint(100,200)
clojure
(+ (rand-int (- 201 100)) 100)
erlang
RandomInt = gen_rand_integer(100, 200),
fantom
r := Int.random(100..200)

Generate a repeatable random number sequence

Initialise a random number generator with a seed and generate five decimal values. Reset the seed and produce the same values.
python
import random

random.seed(12345)
list1 = [random.randint(1,10) for x in range(5)]

random.seed(12345)
list2 = [random.randint(1,10) for x in range(5)]

assert(list1==list2)
clojure
(dotimes [_ 2]
(let [r (java.util.Random. 12345)]
(dotimes [_ 5]
(println (.nextInt r 100))))
(println))
erlang
setRNG(RNGState),
io:format("~w~n", [lists:map(fun (_) -> gen_rand_integer(100, 200) end, lists:seq(1, 5))]),

setRNG(RNGState),
io:format("~w~n", [lists:map(fun (_) -> gen_rand_integer(100, 200) end, lists:seq(1, 5))]).
fantom
rand := Random.makeSeeded(12345)
first := Int[,].fill(0,5).map { rand.next(100..200) }

rand2 := Random.makeSeeded(12345)
second := Int[,].fill(0,5).map { rand2.next(100..200) }
Regex
Matching

Check if a string matches a regular expression

Display "ok" if "Hello" matches /[A-Z][a-z]+/
python
found = re.match(r'[A-Z][a-z]+', 'Hello')
if found:
print 'ok'
clojure
(if (re-matches #"[A-Z][a-z]+" "Hello")
(println "ok"))
erlang
String = "Hello", Regexp = "[A-Z][a-z]+",
is_match(String, Regexp) andalso (begin io:format("ok~n"), true end).
case re:run("Hello", "[A-Z][a-z]+") of {match, _} -> ok end.
fantom
if (Regex<|[A-Z][a-z]+|>.matches("Hello"))
echo("ok")

Check if a string matches with groups

Display "two" if "one two three" matches /one (.*) three/
python
match = re.match(r'one (.*) three', 'one two three')
if match:
print match.group(1)
clojure
(if-let [groups (re-matches #"one (.*) three" "one two three")]
(println (second groups)))
erlang
case re:run("one two three", "one (.*) three", [{capture, [1], list}]) of {match, Res} -> hd(Res) end.
fantom
m := Regex<|one (.*) three|>.matcher("one two three")
if (m.matches)
echo("${m.group(1)}")
Searching

Check if a string contains a match to a regular expression

Display "ok" if "abc 123 @#$" matches /\d+/
python
found = re.search(r'\d+', 'abc 123 @#$')
if found:
print 'ok'
clojure
(if (re-find #"\d+" "abc 123 @#$")
(println "ok"))
erlang
% Erlang uses 'egrep'-compatible regular expressions, so shortcuts like '\d' not supported
String = "abc 123 @#$", Regexp = "[0-9]+",
is_match(String, Regexp) andalso (begin io:format("ok~n"), true end).
case re:run("abc 123 @#$", "\\d+") of {match, _} -> ok end.
fantom
m := Regex<|\d+|>.matcher("abc 123 @#\$")
if (m.find)
echo("ok")

Loop through a string matching a regex and performing an action for each match

Create a list [fish1,cow3,boat4] when matching "(fish):1 sausage (cow):3 tree (boat):4" with regex /\((\w+)\):(\d+)/
python
map(''.join, re.findall(r"\((\w+)\):(\d+)", "(fish):1 sausage (cow):3 tree (boat):4"))
--------------------------------------------------------------------------
(''.join(m.groups()) for m in re.finditer(r"\((\w+)\):(\d+)", "(fish):1 sausage (cow):3 tree (boat):4"))
clojure
(let [matcher (re-matcher #"\((\w+)\):(\d+)" "(fish):1 sausage (cow):3 tree (boat):4")]
(loop [match (re-find matcher)
lst []]
(if match
(recur (re-find matcher) (conj lst (str (second match) (nth match 2))))
lst)))
erlang
solve(S) ->
R = "\\((\\w+?)\\):(\\d+)",
{match, M} = re:run(S,R, [global, {capture, all_but_first, list}]),
[ A++N || [A, N] <- M].
fantom
m := Regex<|\((\w+)\):(\d+)|>.matcher(s)
list := Str[,]
while (m.find) { list.add("${m.group(1)}${m.group(2)}") }
Replacing

Replace the first regex match in a string with a static string

Transform "Red Green Blue" into "R*d Green Blue" by replacing /e/ with "*"
python
print re.sub(r'e', '*', 'Red Green Blue', 1)
clojure
(.replaceFirst (re-matcher #"e" "Red Green Blue") "*")
erlang
{ok, Replaced, _} = regexp:sub("Red Green Blue", "e", "*"),
re:replace("Red Green Blue", "e", "*", [{return, list}]).
fantom
replaced := Regex<|e|>.split("Red Green Blue",2).join("*")

Replace all regex matches in a string with a static string

Transform "She sells sea shells" into "She X X shells" by replacing /se\w+/ with "X"
python
transformed = re.sub(r'se\w+', 'X', 'She sells sea shells')
clojure
(.replaceAll (re-matcher #"se\w+" "She sells sea shells") "X")
erlang
% Erlang uses 'egrep'-compatible regular expressions, so shortcuts like '\w' not supported
{ok, Replaced, _} = regexp:gsub("She sells sea shells", "se[A-Za-z0-9_]+", "X"),
re:replace("She sells sea shells", "se\\w+", "X", [global, {return, list}]).
fantom
replaced := Regex<|se\w+|>.split("She sells sea shells").join("X")

Replace all regex matches in a string with a dynamic string

Transform "The {Quick} Brown {Fox}" into "The kciuQ Brown xoF" by reversing words in braces using the regex /\{(\w+)\}/.
python
transformed = re.sub(r'\{(\w+)\}',
lambda match: match.group(1)[::-1],
'The {Quick} Brown {Fox}')
clojure
(def *string* "The {Quick} Brown {Fox}")
(def *regex* (re-pattern #"\{(\w+)\}"))

(println
(loop [result ""
src *string*
replace-strs (re-seq *regex* *string*)]
(if (empty? src)
result
(let [[match replacement] (first replace-strs)]
(if (= (first src) (first match))
; At the beginning of a sequence that should be replaced.
; Do replacement of a single match
(recur (str result (apply str (reverse replacement)))
(drop (count match) src)
(rest replace-strs))
; else, just copy one char from the source to the result
(recur (str result (first src))
(rest src)
replace-strs))))))
(clojure.string/replace "The {Quick} Brown {Fox}"
#"\{(\w+)\}"
(fn [[_ word]] (apply str (reverse word))))
erlang
% Erlang regular expressions lack both group capture and backreferences, thus this problem is not directly
% solvable. Presented solution is close, but not on-spec

String = "The {Quick} Brown {Fox}",
{match, FieldList} = regexp:matches(String, "\{([A-Za-z0-9_]+)\}"),

NewString = lists:foldl(fun ({Start, Length}, S) -> replstr(S, lists:reverse(string:substr(S, Start, Length)), Start) end, String, FieldList),
fantom
s := "The {Quick} Brown {Fox}"
m := Regex<|\{(\w+)\}|>.matcher(s)
buf := StrBuf(s.size)
last := 0
while (m.find)
{
buf.add(s[last..m.start-1]).add(m.group(1).reverse)
last = m.end
}
buf.add(s[last..-1])
replaced := buf.toStr
Lists
Declaration

Define an empty list

Assign the variable "list" to a list with no elements
python
list = []
clojure
(list)
'()
erlang
List = [],
fantom
list := [,]

Define a static list

Define the list [One, Two, Three, Four, Five]
python
list = ['One', 'Two', 'Three', 'Four', 'Five']
print list
clojure
(def a '[One Two Three Four Five])
erlang
List = [one, two, three, four, five],
List = ['One', 'Two', 'Three', 'Four', 'Five'],
fantom
list := ["One", "Two", "Three", "Four", "Five"]
Output

Join the elements of a list, separated by commas

Given the list [Apple, Banana, Carrot] produce "Apple, Banana, Carrot"
python
print ", ".join(['Apple', 'Banana', 'Carrot'])
clojure
(apply str (interpose ", " '("Apple" "Banana" "Carrot")))
erlang
Result = string:join(Fruit, ", "),
Result = lists:foldl(fun (E, Acc) -> Acc ++ ", " ++ E end, hd(Fruit), tl(Fruit)),
Result = lists:flatten([ hd(Fruit) | [ ", " ++ X || X <- tl(Fruit)]]).
fantom
["Apple", "Banana", "Carrot"].join(", ")

Join the elements of a list, in correct english

Create a function join that takes a List and produces a string containing an english language concatenation of the list. It should work with the following examples:
join([Apple, Banana, Carrot]) = "Apple, Banana, and Carrot"
join([One, Two]) = "One and Two"
join([Lonely]) = "Lonely"
join([]) = ""
python
def join(*x):
if len(x) <= 2:
return ' and '.join(x)
else:
return ', '.join(x[:-1] + ('and ' + x[-1],))

if __name__ == "__main__":
assert join("Apple", "Banana", "Carrot") == "Apple, Banana, and Carrot"
assert join("One", "Two") == "One and Two"
assert join("Lonely") == "Lonely"
assert join(*[]) == ""
clojure
(defn join [lst]
(cond
(= (count lst) 0) ""
(= (count lst) 1) (first lst)
(= (count lst) 2) (str (first lst) " and " (second lst))
(> (count lst) 2) (loop [lst lst sb (StringBuilder.)]
(if (empty? lst)
(.toString sb)
(recur (rest lst) (.append sb (cond
(> (count lst) 2) (str (first lst) ", ")
(> (count lst) 1) (str (first lst) ", and ")
(= (count lst) 1) (str (first lst)))))))))
(defn join
([lst]
(join lst false))
([lst is-long]
(condp = (count lst)
0 ""
1 (first lst)
2 (str (first lst) (if is-long ",") " and " (second lst))
(str (first lst) ", " (join (rest lst) true)))))
erlang
io:format("~s~n", [join(Fruit)]).

% ------

join([]) -> "";
join([W|Ws]) -> join(Ws, W).

join([], S) -> S;
join([W], S) -> join([], S ++ " and " ++ W);
join([W|Ws], S) -> join(Ws, S ++ ", " ++ W).
%% According to the reference manual, "string is not a data type in Erlang."
%% Instead it has lists of integers. But I/O functions in general accept
%% IO lists, where an IO list is either a list of IO lists or an integer.
%% This gives you O(1) string concatenation.

-module(commalist).
-export([join/1]).

join([]) -> "";
join([W]) -> W;
join([W1, W2]) -> [W1, " and ", W2];
join([W1, W2, W3]) -> [W1, ", ", W2, ", and ", W3];
join([W1|Ws]) -> [W1, ", ", join(Ws)].

fantom
join := |List list -> Str|
{
switch(list.size)
{
case 0: return ""
case 1: return list[0]
case 2: return list.join(" and ")
default: return list[0..-2].join(", ") + ", and " + list[-1]
}
}

echo(join(["Apple", "Banana", "Carrot"]))
echo(join(["One", "Two"]))
echo(join(["Lonely"]))
echo(join([,]))

Produce the combinations from two lists

Given two lists, produce the list of tuples formed by taking the combinations from the individual lists. E.g. given the letters ["a", "b", "c"] and the numbers [4, 5], produce the list: [["a", 4], ["b", 4], ["c", 4], ["a", 5], ["b", 5], ["c", 5]]
python
[(x, y) for y in [1,2] for x in ['a','b','c']]
import itertools
[x for x in itertools.product(["a", "b", "c"], [4, 5])]
clojure
(defn combine [lst1 lst2]
(mapcat (fn [x] (map #(list % x) lst1)) lst2))
(mapcat (fn [x] (map #(list % x) ["a", "b", "c"])) [4, 5])
erlang
Combinations =
lists:foldl(fun (Number, Acc) -> Acc ++ lists:map(fun (Letter) -> {Letter, Number} end, Letters) end, [], Numbers),
Combinations = lists:keysort(2, sofs:to_external(sofs:product(sofs:set(Letters), sofs:set(Numbers))))
[[A, B] || A <- ["a", "b", "c"], B <- [4, 5]].

fantom
[4,5].each |Int i| { ["a","b","c"].each |Str s| { r.add([i,s]) } }

From a List Produce a List of Duplicate Entries

Taking a list:
["andrew", "bob", "chris", "bob"]

Write the code to produce a list of duplicates in the list:
["bob"]
python
import itertools
input = ["andrew", "bob", "chris", "bob"]
input.sort()
output = [k for k, g in itertools.groupby(input, lambda x: x) if len(list(g)) > 1]
clojure
(->> '("andrew" "bob" "chris" "bob")
(group-by identity)
(filter #(> (count (second %)) 1))
(map first))
erlang
{_, Result} = lists:foldl(
fun(X, {Uniq, Dupl}) -> case lists:member(X, Uniq) of
true -> {Uniq,[X | Dupl]};
_ -> {[X | Uniq], Dupl}
end
end,
{[], []},
List),
Fun = fun
([X | Xs], F) -> case lists:member(X, Xs) of
true -> [X | F(Xs, F)];
_ -> F(Xs, F)
end;
([], _) -> []
end,
Result = Fun(List, Fun).
fantom
nameCounts := Str:Int[:] { def = 0 }
["andrew", "bob", "chris", "bob"].each |Str v| { nameCounts[v]++ }
results := nameCounts.findAll |Int v, Str k->Bool| { v > 1 }.keys
echo(results.join(","))
Access

Fetch an element of a list by index

Given the list [One, Two, Three, Four, Five], fetch the third element ('Three')
python
list = ['One', 'Two', 'Three', 'Four', 'Five']
list[2]
clojure
(nth '[One Two Three Four Five] 2)
erlang
Result = lists:nth(3, List),
Result = element(3, list_to_tuple(List)),
{Left, _} = lists:split(3, List), Result = lists:last(Left),
Result = nth0(2, List),
fantom
["One", "Two", "Three", "Four", "Five"][2]
["One", "Two", "Three", "Four", "Five"].get(2)

Fetch the last element of a list

Given the list [Red, Green, Blue], access the last element ('Blue')
python
list = ['Red', 'Green', 'Blue']
list[-1]
clojure
(last '[One Two Three Four Five])
erlang
Result = lists:last(List),
Result = last(List),
Result = hd(lists:reverse(List)),
Result = lists:nth(length(List), List),
fantom
["Red", "Green", "Blue"][-1]
["One", "Two", "Three", "Four", "Five"].last

Find the common items in two lists

Given two lists, find the common items. E.g. given beans = ['broad', 'mung', 'black', 'red', 'white'] and colors = ['black', 'red', 'blue', 'green'], what are the bean varieties that are also color names?
python
beans = ['broad', 'mung', 'black', 'red', 'white']
colors = ['black', 'red', 'blue', 'green']

common = [b for b in beans if b in colors]
beans = ['broad', 'mung', 'black', 'red', 'white']
colors = ['black', 'red', 'blue', 'green']

common = set(beans) & set(colors)
clojure
(use 'clojure.set)

(let [beans '[broad mung black red white]
colors '[black red blue green]]
(intersection (set beans) (set colors)))
erlang
Beans = sets:from_list([broad, mung, black, red, white]), Colors = sets:from_list([black, red, blue, green]),

Common = sets:to_list(sets:intersection(Beans, Colors)),
fantom
beans := ["broad", "mung", "black", "red", "white"]
colors := ["black", "red", "blue", "green"]
echo(beans.intersection(colors))

Display the unique items in a list

Display the unique items in a list, e.g. given ages = [18, 16, 17, 18, 16, 19, 14, 17, 19, 18], display the unique elements, i.e. with duplicates removed.
python
ages = [18, 16, 17, 18, 16, 19, 14, 17, 19, 18]

unique_ages = list(set(ages))
clojure
;; returns a set
(set [18, 16, 17, 18, 16, 19, 14, 17, 19, 18])
;;#{14 16 17 18 19}

;; returns a lazy sequence of the unique elements
(distinct [18, 16, 17, 18, 16, 19, 14, 17, 19, 18])
;;(18 16 17 19 14)


erlang
Ages = sets:to_list(sets:from_list([18, 16, 17, 18, 16, 19, 14, 17, 19, 18])), io:format("~w~n", [Ages]).
lists:usort([18, 16, 17, 18, 16, 19, 14, 17, 19, 18]).
fantom
uniqueAges := [18, 16, 17, 18, 16, 19, 14, 17, 19, 18].unique
echo(uniqueAges)
Modification

Remove an element from a list by index

Given the list [Apple, Banana, Carrot], remove the first element to produce the list [Banana, Carrot]
python
myList = ['Apple', 'Banana', 'Carrot']
print myList
del myList[0]
# or
myList.pop(0) # returns 'Apple'
print myList
clojure
(let [fruit ["Apple" "Banana" "Carrot"]
index 0]
(concat
(take index fruit)
(drop (+ index 1) fruit)))
erlang
Result = tl(List),
[_|Result] = List,
N = 1, {Left, Right} = lists:split(N - 1, List), Result = Left ++ tl(Right),
Result = drop(1, List),
fantom
list := ["Apple", "Banana", "Carrot"]
list.removeAt(0)

Remove the last element of a list

python
myList = ['Apple', 'Banana', 'Carrot']
myList.pop()

clojure
(pop ["Apple" "Banana" "Carrot"])
erlang
Result = init(List),
Result = take(length(List) - 1, List),
Result = lists:reverse(tl(lists:reverse(List))),
fantom
list := ["Apple", "Banana", "Carrot"]
list.removeAt(-1)
list := ["Apple", "Banana", "Carrot"]¨
list.pop

Rotate a list

Given a list ["apple", "orange", "grapes", "bananas"], rotate it by removing the first item and placing it on the end to yield ["orange", "grapes", "bananas", "apple"]
python
l = ["apple", "orange", "grapes", "bananas"]
first, l = l[0], l[1:] + l[:1]
fruit = ['apple', 'orange', 'grapes', 'bananas']
fruit.append(fruit.pop(0))
clojure
(let [fruit ["apple" "orange" "grapes" "bananas"]]
(concat (rest fruit) [(first fruit)])
erlang
N = 1, {Left, Right} = lists:split(N, List), Result = Right ++ Left,
N = 1, Result = rotate(N, List),
fantom
list := ["apple", "orange", "grapes", "bananas"]
list.add(list.removeAt(0))
Manipulation

Gather together corresponding elements from multiple lists

Given several lists, gather together the first element from every list, the second element from every list, and so on for all corresponding index values in the lists. E.g. for these three lists, first = ['Bruce', 'Tommy Lee', 'Bruce'], last = ['Willis', 'Jones', 'Lee'], years = [1955, 1946, 1940] the result should produce 3 actors. The middle actor should be Tommy Lee Jones.
python
first = ['Bruce', 'Tommy Lee', 'Bruce']
last = ['Willis', 'Jones', 'Lee']
years = [1955, 1946, 1940]

actors = zip(first, last, years)

assert len(actors) == 3
assert actors[1] == ('Tommy Lee', 'Jones', 1946)
clojure
(defn gatherer [listOfLists]
(if (empty? (first listOfLists))
() ; the base case for recursion
(cons
(map first listOfLists) ; get the first element of each of the lists
(gatherer (map rest listOfLists)) ; gather all the subsequent ones
)
)
)

(def firstnames '("Bruce" "Tommy Lee" "Bruce"))
(def lastnames '("Willis" "Jones" "Lee"))
(def years '(1955 1946 1940))

(println (gatherer [firstnames lastnames years]))

; -> ((Bruce Willis 1955) (Tommy Lee Jones 1946) (Bruce Lee 1940))
(def firstnames ["Bruce" "Tommy Lee" "Bruce"])
(def lastnames ["Willis" "Jones" "Lee"])
(def years [1955 1946 1940])
(println (map (fn [f l y] [f l y]) firstnames lastnames years))
erlang
First = ['Bruce', 'Tommy Lee', 'Bruce'], Last = ['Willis', 'Jones', 'Lee'], Years = [1955, 1946, 1940],

Result = lists:zip3(First, Last, Years),
fantom
r := [,]
first.size.times |Int i| { r.add([first[i], last[i], years[i]]) }

echo(r)

List Combinations

Given two source lists (or sets), generate a list (or set) of all the pairs derived by combining elements from the individual lists (sets). E.g. given suites = ['H', 'D', 'C', 'S'] and faces = ['2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A'], generate the deck of 52 cards, confirm the deck size and check it contains an expected card, say 'Ace of Hearts'.
python
suites = ('H', 'D', 'C', 'S')
faces = ('2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A')
deck = [(face,suite) for suite in suites for face in faces]
assert len(deck) == 52
assert ('A', 'H') in deck
clojure
(def suites ["H" "D" "C" "S"])
(def faces [2 3 4 5 6 7 8 9 10 "J" "Q" "K" "A"])
(defn listCards [] (for [s suites f faces] [f s]))
(some (partial = ["A" "H"]) (listCards))
; -> true
(count (listCards))
; -> 52
erlang
Cards = lists:foldl(fun (Suite, Acc) -> Acc ++ lists:flatmap(fun (Face) -> [{Suite, Face}] end, Faces) end, [], Suites),

io:format("Deck has ~B cards~n", [length(Cards)]),
IsMember = lists:member({h, 'A'}, Cards),
io:format("~s~n", [if IsMember -> "Deck contains 'Ace of Hearts'" ; true -> "'Ace of Hearts' not in deck" end]),
Cards = sofs:to_external(sofs:product(sofs:set(Suites), sofs:set(Faces))),

io:format("Deck has ~B cards~n", [length(Cards)]),
IsMember = lists:member({h, 'A'}, Cards),
io:format("~s~n", [if IsMember -> "Deck contains 'Ace of Hearts'" ; true -> "'Ace of Hearts' not in deck" end]),
Deck2 = [{S, V} || S <- [d, c, h, s], V <- [2, 3, 4, 5, 6, 7, 8, 9, 10, 'J', 'Q', 'K', 'A']],
52 = length(Deck2),
true = lists:member({h, 'A'}, Deck2).

fantom
r := [,]
["2","3","4","5","6","7","8","9","10","J","Q","K","A"].each |Str c|
{ ["H","D","C","S"].each |Str s| { r.add([c,s]) } }

q := ["A","H"]
result := r.contains(q)
echo("Deck size=${r.size}, contains $q? -> $result")

Perform an operation on every item of a list

Perform an operation on every item of a list, e.g.
for the list ["ox", "cat", "deer", "whale"] calculate
the list of sizes of the strings, e.g. [2, 3, 4, 5]
python
print map(lambda x: len(x), ["ox", "cat", "deer", "whale"])
print [len(x) for x in ['ox', 'cat', 'deer', 'whale']]
clojure
(map count ["ox" "cat" "deer" "whale"])
erlang
lists:map(fun (X) ->length(X) end, List).
fantom
["ox", "cat", "deer", "whale"].map { it.size }

Split a list of things into numbers and non-numbers

Given a list that might contain e.g. a string, an integer, a float and a date,
split the list into numbers and non-numbers.
python
import re
data = '34234aff340980adf0e0fa0fefl' ## or ''.join(array)

nonDigits = re.findall(re.compile('\D'), data)
digits = re.findall(re.compile('\d'), data)


clojure
(def jumble [3 "Bill" 5.7 '("A" "B" "C")]) ; int, string, float, list

(defn numberNonNumberSorter [jumbledList]
(if (empty? jumbledList)
(hash-map :numbers [], :nonnumbers []) ; recursion base case - return two empty lists
(let [head (first jumbledList)] ; let <head> be the first element in the list
(let [tailresult (numberNonNumberSorter (rest jumbledList))] ; tailresult applies recursively to the remainder
(if (number? head) ; is head a number?
(hash-map
:numbers (cons head (tailresult :numbers)) ; add <head> to the numbers
:nonnumbers (tailresult :nonnumbers)) ; leave nonnumbers the same
(hash-map
:numbers (tailresult :numbers) ; leave numbers the same
:nonnumbers (cons head (tailresult :nonnumbers))) ; add <head> to nonnumbers
)
)
)
)
)

(println (numberNonNumberSorter jumble))

; -> {:nonnumbers (Bill (A B C)), :numbers (3 5.7)}
(group-by number? ["hello" 42 3.14 (Date.)])
erlang
% Wrapped call to the auxiliary function
number_split(Xs) ->
number_split(Xs, [], []).

% The auxiliary function
number_split([], Num, NonNum) ->
{Num, NonNum};
number_split([X|Xs], Num, NonNum) ->
case is_number(X) of
true ->
number_split(Xs, [X|Num], NonNum);
false ->
number_split(Xs, Num, [X|NonNum])
end.
List = ["hello", 25, 3.14, calendar:local_time()],
{Numbers, NonNumbers} = lists:partition(fun(E) -> is_number(E) end, List)
fantom
things := ["hello", 25, 3.14, Time.now]
numbers := things.findType(Num#)
nonNumbers := things.exclude { numbers.contains(it) }
Testing

Test if a condition holds for all items of a list

Given a list, test if a certain logical condition (i.e. predicate) holds for all items of the list.
python
all(x > 1 for x in [2,3,4])
clojure
(every? #(> % 1) [2 3 4])
erlang
Result = lists:all(Pred, List).
fantom
echo([2,3,4].all{ it>1 })

Test if a condition holds for any items of a list

Given a list, test if a certain logical condition (i.e. predicate) holds for any items of the list.
python
any(x > 3 for x in [2, 3, 4])
clojure
; The standard library in Clojure has "not-any?" but (oddly enough) no "any?"
(defn any? [pred coll]
((complement not-any?) pred coll))

(any? #(> % 3) [2 3 4])
(some #(> % 3) [2 3 4])
erlang
Result = lists:any(Pred, List).
fantom
echo([2,3,4].any{ it==4 })
Maps
Declaration

Define an empty map

python
map = {}
clojure
(def m {})
erlang
Map = dict:new(),
Map = orddict:new(),
Map = gb_trees:empty(),
Map = ets:new(the_map_name, [set, private, {keypos, 1}]),
fantom
map := [:]

Define an unmodifiable empty map

python
import collections
EmptyDict = collections.namedtuple("EmptyDict", "")
e = EmptyDict()
clojure
; Clojure maps are immutable
(def m {})
erlang

% Erlang data structures are immutable - updating a 'map' sees a modified copy created
Map = dict:new(),
fantom
map := [:].ro

Define an initial map

Define the map {circle:1, triangle:3, square:4}
python
shapes = {'circle': 1, 'square': 4, 'triangle': 2}
clojure
(def m '{circle 1 triangle 1 square 4})
erlang
Map = dict:from_list([{circle, 1}, {triangle, 3}, {square, 4}]),
Map0 = dict:new(),

% Erlang variables are 'single-assignment' i.e. they cannot be reassigned
Map1 = dict:store(circle, 1, Map0),
Map2 = dict:store(triangle, 3, Map1),
Map3 = dict:store(square, 4, Map2),
Map0 = gb_trees:empty(),

Map1 = gb_trees:enter(circle, 1, Map0),
Map2 = gb_trees:enter(triangle, 3, Map1),
Map3 = gb_trees:enter(square, 4, Map2),
Map = gb_trees:from_orddict(lists:keysort(1, [{circle, 1}, {triangle, 3}, {square, 4}])),
Map = ets:new(the_map_name, [ordered_set, private, {keypos, 1}]),
ets:insert(Map, [{circle, 1}, {triangle, 3}, {square, 4}]),
fantom
map := ["circle":1, "triangle":2, "square":4]
Access

Check if a key exists in a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print "ok" if an pet exists for "mary"
python
pets = dict(joe='cat', mary='turtle', bill='canary')
if ("mary" in pets) print "ok"
clojure
(if (contains? '{joe cat mary turtle bill canary} 'mary)
(println "ok"))
erlang
dict:is_key(mary, Pets) andalso begin io:format("ok~n"), true end.
IsMember = ets:member(Pets, mary), if (IsMember) -> io:format("ok~n") ; true -> false end.
case gb_trees:lookup(mary, Pets) of none -> false ; _ -> io:format("ok~n") end.
fantom
map := ["joe":"cat", "mary":"turtle", "bill":"canary"]
if (map.containsKey("mary")) echo("ok")

Retrieve a value from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print the pet for "joe" ("cat")
python
print pets['joe']
clojure
(def pets '{joe cat mary turtle bill canary})

(println (get pets 'joe))
erlang
dict:is_key(joe, Pets) andalso begin io:format("~w~n", [dict:fetch(joe, Pets)]), true end.
case dict:find(joe, Pets) of error -> false ; {ok, Pet} -> io:format("~w~n", [Pet]) end.
IsMember = ets:member(Pets, joe), if (IsMember) -> io:format("~w~n", [ets:lookup_element(Pets, joe, 2)]) ; true -> false end.
case ets:match(Pets, {joe, '$1'}) of [] -> false ; [[Pet]] -> io:format("~w~n", [Pet]) end.
case gb_trees:lookup(joe, Pets) of none -> false ; {value, Pet} -> io:format("~w~n", [Pet]) end.
fantom
map := ["joe":"cat", "mary":"turtle", "bill":"canary"]
pet := map["joe"]
echo("pet=$pet")

Add an entry to a map

Given an empty pets map, add the mapping from "rob" to "dog"
python
pets['rob'] = 'dog'
clojure
(assoc {} 'rob 'dog)
erlang
Pets1 = dict:store(rob, dog, Pets0).
ets:insert(Pets, {rob, dog}).
Pets1 = gb_trees:enter(rob, dog, Pets0).
fantom
map["rob"] = "dog"

Remove an entry from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} remove the mapping for "bill" and print "canary"
python
print pets.pop('bill')
clojure
; Maps are immutable
; The following expression will return a new map without the 'bill key
(let [pets '{joe cat mary turtle bill canary}]
(println (get pets 'bill))
(dissoc pets 'bill))
erlang
Pet = dict:fetch(bill, Pets0), Pets1 = dict:erase(bill, Pets0), io:format("~w~n", [Pet]),
Pet = ets:lookup_element(Pets, bill, 2), ets:delete(Pets, bill), io:format("~w~n", [Pet]),
{value, Pet} = gb_trees:lookup(bill, Pets0), Pets1 = gb_trees:delete(bill, Pets0), io:format("~w~n", [Pet]),
fantom
pet := map.remove("bill")
echo ("pet=$pet")
Algorithms

Create a histogram map from a list

Given the list [a,b,a,c,b,b], produce a map {a:2, b:3, c:1} which contains the count of each unique item in the list
python
from collections import defaultdict
h = defaultdict(int)
for k in "abacbb":
h[k] += 1

h = {}
for k in "abacbb":
h[k] = h.setdefault(k, 0) + 1
from collections import Counter
h = Counter("abacbb")
print(h)
clojure
(let [l '[a b a c b b]]
(loop [m {}
d (distinct l)]
(let [item (first d)]
(if (zero? (count d))
m
(recur
(assoc m
item
(count
(filter #(= item %) l)))
(rest d))))))
(->> [:a :b :a :c :b :b]
(group-by identity)
(reduce (fn [m e] (assoc m (first e) (count (second e)))) {}))
(reduce conj {} (for [[x xs] (group-by identity "abacbb")] [x (count xs)]))
(frequencies ["a","b","a","c","b","b"])
(frequencies '[a b a c b b])
erlang
% Imperative Solution
Histogram = histogram(List),
% Functional (1) Solution
Histogram = histogram(List),
lists:foldl(fun(Elem, OldDict) ->
dict:update_counter(Elem, 1, OldDict)
end,
dict:new(),
[a,b,a,c,b,b])).
fantom
list := ["a","b","a","c","b","b"]
map := [Str:Int][:]
list.each |Str s, Int i| { if(!map.containsKey(s)) map.add(s,1); else map[s] = ++map[s] }
echo (map)

Categorise a list

Given the list [one, two, three, four, five] produce a map {3:[one, two], 4:[four, five], 5:[three]} which sorts elements into map entries based on their length
python
c = defaultdict(list)
for k in ["one", "two", "four", "three", "five"]:
c[len(k)].append(k)
from itertools import groupby
lst = ["one", "two", "four", "three", "five"]
c = dict((k, list(g)) for k,g in
groupby(sorted(lst, key=lambda x: len(x)), key=lambda x: len(x)))
print(c)
clojure
(loop [m {}
l ["one" "two" "three" "four" "five"]]
(if (zero? (count l))
m
(let [item (first l)
key (count item)]
(recur
(assoc m key (cons item (get m key [])))
(rest l)))))
(group-by count ["one" "two" "three" "four" "five"])
erlang
% Imperative Solution
CatList = categorise(List),
% Functional (1) Solution
CatList = categorise(List),
fantom
list := ["one", "two", "three", "four", "five"]
map := [Int:List][:]
list.each { List l := map[it.size] ?: [,]; map[it.size] = l.add(it) }
echo(map)
Structure
Conditionals

Perform an action if a condition is true (IF .. THEN)

Given a variable name, if the value is "Bob", display the string "Hello, Bob!". Perform no action if the name is not equal.
python
if name == 'Bob':
print 'Hello, Bob!'
clojure
(def person "Bob")
(if (= person "Bob")
(println "Hello, Bob!"))
erlang
if (Name == "Bob") -> io:format("Hello, ~s!~n", [Name]) ; true -> false end.
case Name of "Bob" -> io:format("Hello, ~s!~n", [Name]) ; _ -> false end.
Name == "Bob" andalso (begin io:format("Hello, ~s!~n", [Name]), true end).
fantom
if (name=="Bob") echo("Hello, Bob!")

Perform different actions depending on a boolean condition (IF .. THEN .. ELSE)

Given a variable age, if the value is greater than 42 display "You are old", otherwise display "You are young"
python
if age > 42:
print 'You are old'
else:
print 'You are young'
print age > 42 and 'You are old' or 'You are young'
clojure
(def age 41)
(if (> age 42) "You are old" "You are young")
erlang
if Age > 42 -> io:format("You are old~n") ; true -> io:format("You are young~n") end.
Message = if Age > 42 -> "old" ; true -> "young" end, io:format("You are ~s~n", [Message]).
case Age > 42 of true -> io:format("You are old~n") ; false -> io:format("You are young~n") end.
case Age of _ when Age > 42 -> io:format("You are old~n") ; _ -> io:format("You are young~n") end.
Message = case Age of _ when Age > 42 -> "old" ; _ -> "young" end, io:format("You are ~s~n", [Message]).
Age > 42 andalso (begin io:format("You are old~n"), true end) orelse (begin io:format("You are young~n"), true end).
(fun (X) when X > 42 -> io:format("You are old~n"); (_) -> io:format("You are young~n") end)(Age).
(fun () when Age > 42 -> io:format("You are old~n"); () -> io:format("You are young~n") end)().
io:format("You are ~s~n", [if Age > 42 -> "old" ; true -> "young" end]).
fantom
if (age > 42)
echo("You are old")
else
echo("You are young")
echo((age > 42) ? "You are old" : "You are young")

Perform different actions depending on several boolean conditions (IF .. THEN .. ELSIF .. ELSE)

python
if age > 84:
print 'You are really ancient'
elif age > 30:
print 'You are middle-aged'
else:
print 'You are young'
clojure
(println
(condp <= age
84 "You are really ancient"
30 "You are middle aged"
"You are young"))
erlang
if
Age > 84 -> io:format("You are really ancient~n");
Age > 30 -> io:format("You are middle-aged~n");
true -> io:format("You are young~n")
end.
case Age of
_ when Age > 84 -> io:format("You are really ancient~n");
_ when Age > 30 -> io:format("You are middle-aged~n");
true -> io:format("You are young~n")
end.
fantom
if (age > 84)
echo("You are really ancient")
else if (age > 30)
echo("You are middle-aged")
else
echo("You are young")

Replacing a conditional with many branches with a switch/case statement

Many languages support more compact forms of branching than just if ... then ... else such as switch or case or match. Use such a form to add an appropriate placing suffix to the numbers 1..40, e.g. 1st, 2nd, 3rd, 4th, ..., 11th, 12th, ... 39th, 40th
python
def affix(num):
num = num == 1 and str(num) + 'st' or num == 2 and str(num) + 'nd' or \
num == 3 and str(num) +'rd' or str(num) + 'th'
return num

print [affix(x) for x in xrange(1,41)]

clojure
(def n 112)

(println (str n
(let [rem (mod n 100)]
(if (and (>= rem 11) (<= rem 13))
"th"
(condp = (mod n 10)
1 "st"
2 "nd"
3 "rd"
"th")))))
erlang
Suffix = case Num of
N when N > 10, N < 20 -> "th";
N when N rem 10 =:= 1 -> "st";
N when N rem 10 =:= 2 -> "nd";
N when N rem 10 =:= 3 -> "rd";
_ -> "th"
end,
io_lib:format("~w~s", [Num, Suffix])
fantom
suffix := |Int n -> Str|
{
if ((4..20).contains(n % 100))
return "th"

switch((n.toStr)[-1])
{
case '1': return "st"
case '2': return "nd"
case '3': return "rd"
default: return "th"
}
}

(1..40).each { echo("$it${suffix(it)}") }
Loops

Perform an action multiple times based on a boolean condition, checked before the first action (WHILE .. DO)

Starting with a variable x=1, Print the sequence "1,2,4,8,16,32,64,128," by doubling x and checking that x is less than 150.
python
x = 1
while x < 150:
print '%s, ' % x,
x *= 2
clojure
(take-while #(< % 150) (iterate #(* 2 %) 1))
erlang
X = 1, print_while_X_less_150(X).
Pred = fun (X) -> X < 150 end,
Action = fun (X) -> io:format("~B,", [X]), X * 2 end,
X = 1,

while_do(Pred, Action, X).
fantom
x := 1
while (x < 150) {
Env.cur.out.print("$x,")
x *= 2
}
echo

Perform an action multiple times based on a boolean condition, checked after the first action (DO .. WHILE)

Simulate rolling a die until you get a six. Produce random numbers, printing them until a six is rolled. An example output might be "4,2,1,2,6"
python
import random, itertools

def dice():
while True:
yield random.randint(1,6)

print ", ".join(str(d) for d in itertools.takewhile(lambda x: x < 6, dice()))
clojure
(loop [r (rand-int 6)]
(if (= r 5)
nil
(do
(println r)
(recur (rand-int 6)))))
erlang
Pred = fun (DiceRoll) -> DiceRoll =/= 6 end,
Action = fun (DiceRoll) -> io:format("~B,", [DiceRoll]), dice_roll() end,

do_while(Pred, Action, dice_roll()).
-module(dice).
-export([start/0]).

start() ->
roll(dice_roll()).

roll(6) ->
io:format("6~n", []);
roll(N) ->
io:format("~B,", [N]),
roll(dice_roll()).

dice_roll() -> random:uniform(6).
fantom
rnd := 0
while(rnd != 6) {
rnd = Int.random(1..6)
Env.cur.out.print(rnd)
if (rnd != 6)
Env.cur.out.print(",")
}
echo

Perform an action a fixed number of times (FOR)

Display the string "Hello" five times like "HelloHelloHelloHelloHello"
python
print "Hello" * 5
for i in range(5):
print "Hello"
clojure
(dotimes [_ 5]
(print "Hello"))
erlang
dotimes(5, fun () -> io:format("Hello") end).
lists:foreach(fun (_) -> io:format("Hello") end, lists:seq(1, 5)).
fantom
5.times { Env.cur.out.print("Hello") }
for (i := 0; i < 5; i++)
Env.cur.out.print("Hello")
(1..5).each { Env.cur.out.print("Hello") }

Perform an action a fixed number of times with a counter

Display the string "10 .. 9 .. 8 .. 7 .. 6 .. 5 .. 4 .. 3 .. 2 .. 1 .. Liftoff!"
python
print " .. ".join(str(i) for i in range(10, 0, -1)), ".. liftoff!"
clojure
(dotimes [i 10]
(print (str (- 10 i) " .. ")))

(println "Liftoff!")
erlang
fromto(10, 1, -1, fun (X) -> io:format("~B .. ", [X]) end), io:format("Liftoff!~n").
lists:foreach(fun (X) -> io:format("~B .. ", [X]) end, lists:seq(10, 1, -1)), io:format("Liftoff!~n").
fantom
(10..1).each { Env.cur.out.print("$it .. ") }
Env.cur.out.print("Liftoff!")
for (i := 10; i >= 1; i--)
Env.cur.out.print("$i .. ")
Env.cur.out.print("Liftoff!")
Files
Reading

Read the contents of a file into a string

python
contents = open('myFile.txt', 'rt').read()
clojure
(slurp "/tmp/foobar")
erlang
Text = readfile("Solution607.erl"),
Text = readfile("Solution608.erl"),
fantom
contents := File(`file.text`).readAllStr

Process a file one line at a time

Open the source file to your solution and print each line in the file, prefixed by the line number, like:
1> First line of file
2> Second line of file
3> Third line of file
python
for no, line in enumerate(open(__file__)):
print "{0}> {1}".format(no+1, line.rstrip())
clojure
(defn read-line-by-line [fn]
(reduce str (map (partial format "%d> %s\n")
(iterate inc 1)
(read-lines fn))))
erlang
Reader = fun (IODevice) -> io:get_line(IODevice, "") end,
Worker = fun (Line, N) -> io:format("~B> ~s", [N, Line]), N + 1 end,

while_not_eof("Solution609.erl", Reader, Worker, 1).
Reader = fun (Filename) -> {ok, Contents} = file:read_file(Filename), Contents end,
Transformer = fun (Line, N) -> string:concat(string:concat(integer_to_list(N), "> "), Line) end,
Printer = fun (Line) -> io:format("~s~n", [Line]) end,

Lines = string:tokens(binary_to_list(Reader("Solution610.erl")), "\n"),
NewLines = lists:zipwith(Transformer, Lines, lists:seq(1, length(Lines))),
lists:foreach(Printer, NewLines).
fantom
File(`input.text`).readAllLines.each |Str s, Int i| { echo("${i+1}> $s") }
Writing

Write a string to a file

python
open('test.txt', 'wt').write('Hello World!')
clojure
(with-out-writer "output.txt" (println "Hello file!"))
erlang
Line = "This line overwites file contents!\n",
{ok, IODevice} = file:open("test.txt", [write]), file:write(IODevice, Line), file:close(IODevice).
fantom
File(`out.txt`).out.writeChars("some text").flush

Append to a file

python
open('test.txt', 'at').write('Hello World!\n')
clojure
(with-out-append-writer "output.txt" (println "This is appended to the file"))
erlang
Line = "This line appended to file!\n",
{ok, IODevice} = file:open("test.txt", [append]), file:write(IODevice, Line), file:close(IODevice).
fantom
File(`out.txt`).out(true).writeChars("some text").flush
Directories

Process each file in a directory

python
import os
results = (process(f) for f in os.listdir(".") if os.path.isfile(f))
clojure
; (defn process-file [f] "process one file" body...)
(map process-file (.listFiles (File. ".")))
erlang
% File basenames only - many tasks require absolute paths to work
lists:foreach(fun (FileOrDirPath) -> Worker(FileOrDirPath) end, file:list_dir(Directory)).
% Absolute paths provided - will accomodate most tasks
lists:foreach(fun (FileOrDirPath) -> Worker(FileOrDirPath) end, list_dir_path(Directory)).
fantom
File(`./`).list.each { process(it) }

Process each file in a directory recursively

python
import os
results = (process(os.path.join(p, n)) for p,d,l in os.walk(".") for n in l)
clojure
; (defn process-file [f] "process one file" body...)
(map process-file (file-seq (File. ".")))
erlang
filelib:fold_files(Directory, ".*", true, fun (FileOrDirPath, Acc) -> Worker(FileOrDirPath), Acc end, []).
process_dir(Directory, Worker).
fantom
File(`./`).walk { process(it) }
Dates
Parsing

Parse a date and time from a string

Given the string "2008-05-06 13:29", parse it as a date representing 6th March, 2008 1:29:00pm in the local time zone.
python
import time
time.strptime("2008-05-06 13:29", "%Y-%m-%d %H:%M")
clojure
(.. (SimpleDateFormat. "yyyy-MM-dd HH:mm")
(parse "2008-05-06 13:29"))
erlang
% AFAIK, no datetime-parsing library exists; 'parse_to_datetime' is a simplistic, problem-specific hack
LocalDateTime = erlang:universaltime_to_localtime(parse_to_datetime("2008-05-06 13:29:34")),
fantom
dt := DateTime.fromLocale("2008-05-06 13:29", "YYYY-MM-DD hh:mm")
Creation

Display the current date and time

Create a Date object representing the current date and time. Print it out.
If you can also do this without creating a Date object you can show that too.
python
from datetime import datetime
print datetime.utcnow()
clojure
(import 'java.util.Date)

(println (str (Date.)))
erlang
io:format("~p~n", [calendar:local_time()])
fantom
echo(DateTime.now)
OOP
Basics

Define a class

Declare a class named Greeter that takes a string on creation and greets using this string if you call the "greet" method.
python
class Greeter(object):
""" Greet someone.
"""
def __init__(self, whom):
self._whom = whom
def greet(self):
print "Hello, %s!" % self._whom

Greeter("world").greet()
clojure
(defprotocol IGreeter
(greet [this]))

(deftype Greeter [whom]
IGreeter
(greet [this]
(println (str "Hello, " whom))))

(greet (Greeter. "world"))
(defn greeter [whom]
{:whom whom})

(defn greet [g]
(println (str "Hello, " (:whom g))))

(greet (greeter "world"))
erlang
Greeter = make_greeter("world!"),
Greeter(greet).
fantom
class Greeter
{
private Str whom
new make(Str whom) { this.whom = whom }
Void greet() { echo("Hello, $whom") }
}

Greeter("world").greet
Networking
HTTP

Check your language appears on the langref.org site

Your language name should appear within the HTML found at the http://langreg.org main page.
python
from urllib import urlopen
print urlopen('http://langref.org').read().find('python') >= 0 and 'found' or 'not found'
clojure
(def *url* "http://langref.org/")
(def *lang* "clojure")

(with-open [ stream (.openStream (URL. *url*)) ]
(let [ body (str (line-seq (BufferedReader. (InputStreamReader. stream)))) ]
(str "Language " *lang* " does "
(if-not (re-matches (re-pattern (str ".*" *url* *lang* ".*")) body) "not ")
"exist")))
erlang
URL = "http://langref.org/", Language = "erlang", Regexp = ".*" ++ URL ++ Language ++ ".*",

case http:request(URL) of
{ok, {_, _, Body}} ->
case regexp:first_match(Body, Regexp) of
{match, _, _} -> io:format("Language ~s exists @ ~s~n", [Language, URL]);
_ -> false
end;
{error, ErrorInfo} -> throw("Error: " ++ http:format_error(ErrorInfo))
end,
fantom
language := "Fantom"
url := `http://langref.org/`

response := WebClient(url).getStr
if (Regex.fromStr("\\b$language.lower\\b").matcher(response).find)
echo("Language $language appears at ${url}.")
XML
Parsing

Process an XML document

Given the XML Document:

<shopping>
  <item name="bread" quantity="3" price="2.50"/>
  <item name="milk" quantity="2" price="3.50"/>
</shopping>

Print out the total cost of the items, e.g. $14.50
python
from xml.dom.minidom import parseString
document = parseString(
"""<shopping>
<item name="bread" quantity="3" price="2.50"/>
<item name="milk" quantity="2" price="3.50"/>
</shopping>""").documentElement
total = sum([float(item.getAttribute('price')) *
int(item.getAttribute('quantity'))
for item in document.getElementsByTagName('item')])
print '$%.2f' % total
clojure
(println (format "Total cost of items are $%#.2f"
(->> (xml-seq (parse *xml-input-stream*))
(filter #(= :item (:tag %))) ; Remove all but the item tags
(map :attrs) ; Keep the attributes
(map (fn [e] (str "(* " (:quantity e) " " (:price e) ")"))) ; Get the total price as a sexp
(map read-string) ; "(* quantity price)" -> (* quantity price)
(map eval) ; (* quantity price) -> quantity*price
(apply +)))) ; Sum all elements
erlang
-include_lib("xmerl/include/xmerl.hrl").
-export([get_total/1]).

get_total(ShoppingList) ->
{XmlElt, _} = xmerl_scan:string(ShoppingList),
Items = xmerl_xpath:string("/shopping/item", XmlElt),
Total = lists:foldl(fun(Item, Tot) ->
[#xmlAttribute{value = PriceString}] = xmerl_xpath:string("/item/@price", Item),
{Price, _} = string:to_float(PriceString),
[#xmlAttribute{value = QuantityString}] = xmerl_xpath:string("/item/@quantity", Item),
{Quantity, _} = string:to_integer(QuantityString),
Tot + Price*Quantity
end,
0, Items),
io:format("$~.2f~n", [Total]).
fantom
sum := 0.0
root := XParser(File(`shop.xml`).in).parseDoc.root
if (root.name == "shopping")
{
root.elems.each
{
if (it.name == "item")
{
quantity := Int.fromStr(it.get("quantity"))
price := Decimal.fromStr(it.get("price"))
sum += quantity * price;
}
}
}
echo("\$$sum")
Creation

create some XML programmatically

Given the following CSV:

bread,3,2.50
milk,2,3.50

Produce the equivalent information in XML, e.g.:

<shopping>
  <item name="bread" quantity="3" price="2.50" />
  <item name="milk" quantity="2" price="3.50" />
</shopping>
python
from xml.dom import minidom

csv = """bread,3,2.50
milk,2,3.50"""

doc = minidom.Document()
shopping = doc.createElement("shopping")

for line in csv.split("\n"):
name, quantity, price = line.split(",")
el = doc.createElement("item")
el.setAttribute("name", name)
el.setAttribute("quantity", quantity)
el.setAttribute("price", price)
shopping.appendChild(el)

print shopping.toprettyxml()
from xml.etree.ElementTree import Element, SubElement, tostring

csv = """bread,3,2.50
milk,2,3.50"""

root = Element('shopping')

for line in csv.split("\n"):
name, quantity, price = line.split(",")
SubElement(root,'item', {'name' : name,
'quantity' : quantity,
'price' : price })

print tostring(root)
clojure
(defn list->xml-item [lst]
(let [[name quantity price] (map str lst)]
{:tag :item
:attrs {:name name
:quantity quantity
:price price}}))

(defn cvs->xml [r]
(->> (map #(read-string (str "(" % ")")) (line-seq r))
(map list->xml-item)
(assoc {:tag :shopping} :content)
(emit)
(with-out-str)))

(println (cvs->xml *cvs-reader*))
erlang
to_xml(ShoppingList) ->
Items = lists:map(fun(L) ->
[Name, Quantity, Price] = string:tokens(L, ","),
{item, [{name, Name}, {quantity, Quantity}, {price, Price}], []}
end, string:tokens(ShoppingList, "\n")),
xmerl:export_simple([{shopping, [], Items}], xmerl_xml).
fantom
sum := 0.0
rows := CsvInStream(File(`shop.csv`).in).readAllRows

doc := XDoc()
doc.root = XElem("shopping")
{
root := it
rows.each |Str[] row|
{
root.add(XElem("item")
{
XAttr("name", row[0]),
XAttr("quantity", row[1]),
XAttr("price", row[2])
})
}
}

os := File(`shop.xml`).out
doc.write(os)
os.close
Algorithms
Geometry

Find all Pythagorean triangles with length or height less than or equal to 20

Pythagorean triangles are right angle triangles whose sides comply with the following equation:

a * a + b * b = c * c

where c represents the length of the hypotenuse, and a and b represent the lengths of the other two sides. Find all such triangles where a, b and c are non-zero integers with a and b less than or equal to 20. Sort your results by the size of the hypotenuse. The expected answer is:

[3, 4, 5]
[6, 8, 10]
[5, 12, 13]
[9, 12, 15]
[8, 15, 17]
[12, 16, 20]
[15, 20, 25]
python
from math import sqrt

a = 1
ret = []
while a <= 20:
b = 1
while b <= 20:
c = sqrt((a**2)+(b**2))
if int(c) == c and sorted([a,b,int(c)]) not in ret:
ret.append(sorted([a,b,int(c)]))
b +=1
a +=1
print ret


or if you wanna get snarky..

print sorted(set([tuple(sorted((a,b,int(sqrt((a**2)+(b**2)))))) for a in xrange(1,21) for \
b in xrange(1,21) if int(sqrt((a**2)+(b**2))) == sqrt((a**2)+(b**2))]))

clojure
(defn pythagorean [a b c] (= (+ (* a a) (* b b)) (* c c)))

(defn intsqrt [cc]
(. (. Math sqrt cc) intValue)
)

(defn triples [maxSize]
(filter not-empty
(for [a (range 1 20) b (range a 20)]
(let [c (intsqrt (+ (* a a) (* b b)))]
(if (pythagorean a b c)
[a b c]
()
)))))

(triples 20)
; -> ([3 4 5] [5 12 13] [6 8 10] [8 15 17] [9 12 15] [12 16 20] [15 20 25])

(defn sortByHypotenuse [triples]
(sort-by #(first (rest (rest %))) triples)
)

(sortByHypotenuse (triples 20))
; -> ([3 4 5] [6 8 10] [5 12 13] [9 12 15] [8 15 17] [12 16 20] [15 20 25])
(doseq [pt (sort-by #(% 2)
(for [a (range 1 21)
b (range a 21)
:let [aa+bb (+ (* a a) (* b b))
c (Math/round (Math/sqrt aa+bb))]
:when (= aa+bb (* c c))]
[a b c]))]
(println pt))
erlang
find_all_pythagorean_triangles(L) ->
lists:sort(fun({_, _, H1}, {_, _, H2}) -> H1 =< H2 end,
[ { X, Y, Z } ||
X <- lists:seq(1,L),
Y <- lists:seq(1,L),
Z <- lists:seq(1,2*L),
X*X + Y*Y =:= Z*Z,
Y > X,
Z > Y
]).

main(_) ->
List = find_all_pythagorean_triangles(20).
fantom
triangles := [,]
(1..20).each |Int a|
{
(a..20).each |Int b|
{
c := (a.pow(2) + b.pow(2)).toFloat.sqrt
if (c % c.toInt == 0.0f && !triangles.contains([b,a,c]))
triangles.add([a,b,c.toInt])
}
}
triangles.sort |Int[] x, Int[] y -> Int| { x[2]-y[2] }
echo(triangles)
Arithmetic

Greatest Common Divisor

Find the largest positive integer that divides two given numbers without a remainder. For example, the GCD of 8 and 12 is 4.

python
def gcd_recursive(i, j):
if min(i, j) == 0:
return max(i, j)
else:
return gcd_recursive(min(i, j), abs(i - j))

def gcd_iterative(i, j):
while min(i, j) != 0:
i, j = min(i, j), abs(i - j)
return max(i, j)

if __name__ == "__main__":
print gcd_recursive(8, 12)
print gcd_iterative(8, 12)
from fractions import gcd
print gcd(8, 12)
clojure
(defn gcd [a b]
(if (zero? b)
a
(recur b (mod b a))))
erlang
-module(gcd).
-export([gcd/2]).

gcd(A, 0) -> A;
gcd(A, B) -> gcd(B, A rem B).
fantom
gcd := |Int a, Int b -> Int| {
pair := [a, b].sort
while (pair.first != 0)
pair.set(1, pair.last % pair.first).swap(0, 1)
return pair.last
}
echo(gcd(12, 8)) // a>b, result == 4
echo(gcd(1029, 1071)) // a<b, result == 21
Parallel
Threads

Create a multithreaded "Hello World"

Create a program which outputs the string "Hello World" to the console, multiple times, using separate threads or processes.

Example:

-Output-

Thread one says Hello World!
Thread two says Hello World!
Thread four says Hello World!
Thread three says Hello World!

-Notice that the threads can print in any order.
python
#!/usr/bin/python
from threading import Thread
Nthread = ['one','two','three','four']
def ThreadSpeaks(number):
print "Thread", number, "says Hello World!"
if __name__ == "__main__":
for n in range(0,len(Nthread)):
th =Thread(target=ThreadSpeaks, args=(Nthread[n],))
th.start()
clojure
(doseq [msg ["one" "two" "three" "four"]]
(future (println "Thread" msg "says Hello World!")))
(dorun (pmap #(println (str "Thread " % " says Hello World!")) '("one" "two" "three" "four")))
(dorun (map (fn [n] (.start (Thread. #(println (str "Thread " n " says Hello World!")))))
'("one" "two" "three" "four")))
erlang
-module(spam).
-export([spam/1]).

spam(N) when N<5 ->
spawn(fun() -> io:format("Hello World from thread ~p~n",[N]) end),
spam(N+1);
spam(_) -> void.
fantom
pool := ActorPool()
["one", "two", "three", "four"].each
{
a := Actor(pool) |Str name| { echo("Thread $name says Hello World!") }
a.send(it)
}