View Category
Lists
Declaration

Define an empty list

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

Define a static list

Define the list [One, Two, Three, Four, Five]
python
list = ['One', 'Two', 'Three', 'Four', 'Five']
print list
erlang
List = [one, two, three, four, five],
List = ['One', 'Two', 'Three', 'Four', 'Five'],
clojure
(def a '[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'])
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)]]).
clojure
(apply str (interpose ", " '("Apple" "Banana" "Carrot")))

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(*[]) == ""
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)].

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)))))

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])]
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]].

clojure
(defn combine [lst1 lst2]
(mapcat (fn [x] (map #(list % x) lst1)) lst2))
(mapcat (fn [x] (map #(list % x) ["a", "b", "c"])) [4, 5])

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]
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).
clojure
(->> '("andrew" "bob" "chris" "bob")
(group-by identity)
(filter #(> (count (second %)) 1))
(map first))
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]
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),
clojure
(nth '[One Two Three Four Five] 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]
erlang
Result = lists:last(List),
Result = last(List),
Result = hd(lists:reverse(List)),
Result = lists:nth(length(List), List),
clojure
(last '[One Two Three Four Five])

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)
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)),
clojure
(use 'clojure.set)

(let [beans '[broad mung black red white]
colors '[black red blue green]]
(intersection (set beans) (set 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))
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]).
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)


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
erlang
Result = tl(List),
[_|Result] = List,
N = 1, {Left, Right} = lists:split(N - 1, List), Result = Left ++ tl(Right),
Result = drop(1, List),
clojure
(let [fruit ["Apple" "Banana" "Carrot"]
index 0]
(concat
(take index fruit)
(drop (+ index 1) fruit)))

Remove the last element of a list

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

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

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))
erlang
N = 1, {Left, Right} = lists:split(N, List), Result = Right ++ Left,
N = 1, Result = rotate(N, List),
clojure
(let [fruit ["apple" "orange" "grapes" "bananas"]]
(concat (rest fruit) [(first fruit)])
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)
erlang
First = ['Bruce', 'Tommy Lee', 'Bruce'], Last = ['Willis', 'Jones', 'Lee'], Years = [1955, 1946, 1940],

Result = lists:zip3(First, Last, Years),
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))

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
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).

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

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']]
erlang
lists:map(fun (X) ->length(X) end, List).
clojure
(map count ["ox" "cat" "deer" "whale"])

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)


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)
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.)])
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])
erlang
Result = lists:all(Pred, List).
clojure
(every? #(> % 1) [2 3 4])

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])
erlang
Result = lists:any(Pred, List).
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])