langref.org - ruby, clojure, and erlang

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Structure

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.

ruby

if (name=='Bob')
puts "Hello, Bob!"
end

puts "Hello, Bob!" if name=='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).

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"

ruby

if (age > 42)
puts "You are old"
else
puts "You are young"
end

puts (age>42) ? "You are old" : "You are young"

puts "You are #{age > 42 ? "old" : "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]).

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

ruby

if age > 84
puts "You are really ancient"
elsif age > 30
puts "You are middle-aged"
else
puts "You are young"
end

case
when age > 84 then puts "You are really ancient"
when age > 30 then puts "You are middle-aged"
else puts "You are young"
end

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.

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

ruby

def suffixed(number)
last_digit = number.to_s[-1..-1].to_i
suffix = case last_digit
when 1 then 'st'
when 2 then 'nd'
when 3 then 'rd'
else 'th'
end
suffix = 'th' if (11..13).include?(number)
"#{number}#{suffix}"
end

(1..40).each {|n| puts suffixed(n) }

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])

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.

ruby

x=1
while x < 150
puts x
x <<= 1
end

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).

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"

ruby

# Ruby has no DO..WHILE construct. Need to write it as a WHILE
rnd = 0
while (rnd != 6)
rnd = rand(6)+1
print rnd
print "," if (rnd!=6)
end

begin
rnd = rand(6)+1
print rnd
print "," if rnd!=6
end while rnd != 6

# This uses Enumerators, ad it becomes almost functional style...

games = Enumerator.new do |yielder|
yielder.yield rand(6) + 1 while true
end

puts games.take_while {|roll| roll != 6}.join(",")

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).