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Erlang
1
Print a literal string on standard output
io:format("~s~n", ["Hello, world!"])
2
Loop to execute some code a constant number of times
lists:foreach(
  fun(_) ->
    io:format("Hello~n")
  end, lists:seq(1, 10)).
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
-spec procedure() -> _.
procedure() -> io:format("#YOLO!~n").
4
Create a function which returns the square of an integer
-spec square(integer()) -> integer().
square(X) when is_integer(X) -> X * X.
5
Declare a container type for two floating-point numbers x and y
-module(points).
-export([new/2, x/1, y/1]).

-opaque point() :: #{x => float(), y => float()}.
-export_type([point/0]).

-spec new(float(), float()) -> point().
new(X, Y) -> #{x => X, y => Y}.

-spec x(point()) -> float().
x(#{x := X}) -> X.

-spec y(point()) -> float().
y(#{y := Y}) -> Y.
Alternative implementation:
PointAsAMap = #{x => X, y => Y}.
Alternative implementation:
PointAsATuple = {X, Y}.
6
Do something with each item x of an array-like collection items, regardless indexes.
[do_something(X) || X <- Items]
Alternative implementation:
lists:foreach(fun do_something/1, Items).
7
Print each index i with its value x from an array-like collection items
WithIndex =
  lists:zip(lists:seq(1, length(Items)), Items),
io:format("~p~n", [WithIndex]).
8
Create a new map object x, and provide some (key, value) pairs as initial content.
X = #{one => 1, "two" => 2.0, <<"three">> => [i, i, i]}.
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
-type binary_tree(T) ::
	#{ data := T
	 , left := binary_tree(T)
	 , right := binary_tree(T)
	 }.
10
Generate a random permutation of the elements of list x
[Y||{_,Y} <- lists:sort([ {rand:uniform(), N} || N <- X])].
11
The list x must be non-empty.
ktn_random:pick(X)
Alternative implementation:
lists:nth(rand:uniform(length(X)), X).
12
Check if the list contains the value x.
list is an iterable finite container.
lists:member(X, List).
Alternative implementation:
member(_, []) -> false;
member(Value, [H|T]) -> 
  case H of
    Value -> true;
    _ -> member(T)
  end.
Alternative implementation:
member(_, []) -> false;
member(Value, [H|_]) where Value =:= H -> true;
member(Value, [_|T]) -> member(Value, T).
13
Access each key k with its value x from an associative array mymap, and print them.
maps:fold(
	fun(K, V, ok) ->
		io:format("~p: ~p~n", [K, V])
	end, ok, MyMap).
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
A + (B - A) * rand:uniform().
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
crypto:rand_uniform(A, B)
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
-record( node,{
	value         :: any(),
	children = [] :: [node#{}]
}).
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
lists:reverse(List)
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
-spec search(T, [[T]]) -> {pos_integer(), pos_integer()}.
search(X, M) -> search(X, M, 1).

search(_, [], _) -> throw(notfound);
search(X, [R|Rs], RN) ->
  case search_row(X, R) of
    notfound -> search(X, Rs, RN+1);
    CN -> {RN, CN}
  end.

search_row(X, Row) -> search_row(X, Row, 1).

search_row(_, [], _) -> notfound;
search_row(X, [X|_], CN) -> CN;
search_row(X, [_|Elems], CN) -> search_row(X, Elems, CN+1).
21
Swap the values of the variables a and b
fun1(A, B) ->
	do:something(),
	fun2(B, A).

fun2(A, B) ->
	now(A, is, B),
	and(B, is, A),
	keep:moving().
22
Extract the integer value i from its string representation s (in radix 10)
I = list_to_integer(S).
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
S = io_lib:format("~.2f", [X]).
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
S = unicode:characters_to_binary("ネコ"),
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
X = [{R * 1.0, C * 1.0} || R <- lists:seq(1, M), C <- lists:seq(1, N)].
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
X = array(M, N, P).

-spec array(pos_integer(), pos_integer(), pos_integer()) -> [[[float()]]].
array(M, N, P) -> [array(M, N)  || _ <- lists:seq(1, P)].
array(M, N) -> [array(M) || _ <- lists:seq(1, N)].
array(M) -> [rand:uniform() || _ <- lists:seq(1, M)].
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
sort_by_birth(ListOfMaps) ->
  lists:sort(
    fun(A, B) ->
      maps:get(birth, A, undefined) =< maps:get(birth, B, undefined)
    end, ListOfMaps).

sort_by_birth(ListOfRecords) -> lists:keysort(#item.birth, ListOfRecords).
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
{Left, [_|Right]} = lists:split(I-1, Items),
Left ++ Right.
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
lists:foreach(fun(I) -> spawn(?MODULE, _f, [I]) end, lists:seq(1, 1000)).
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
f(0) -> 1;
f(I) -> I * f(I - 1).
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
-module  (int_power).
-export  ([exp/2]).

exp (X, N) -> exp (X, N, 1).

exp (_X, 0, Y) -> Y;
exp (X, N, Y) when N rem 2 =:= 0 ->
  exp (X * X, N div 2, Y);
exp (X, N, Y) ->
  exp (X * X, N div 2, X * Y).
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
-spec compose(fun((A) -> B), fun((B) -> C)) -> fun((A) -> C).
compose(F, G) -> fun(X) -> G(F(X)) end.
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
-spec compose(fun((X) -> Y), fun((Y) -> Z)) -> fun((X) -> Z).
compose(F, G) -> fun(X) -> G(F(X)) end.
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
T = string:sub_string(I, J-1).
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
Ok = string:str(S, Word) > 0.
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
T = lists:reverse(S)
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
find_negative_in([]) -> undefined;
find_negative_in([Row|Rows]) -> find_negative_in(Row, Rows).

find_negative_in([], Rows) -> find_negative_in(Rows);
find_negative_in([Pos|Values], Rows) when Pos >= 0 ->
	find_negative_in(Values, Rows);
find_negative_in([Neg|_], _) -> Neg.
44
Insert the element x at position i in the list s. Further elements must be shifted to the right.
{Left, Right} = lists:split(I-1, S),
Left ++ [X|Right].
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
timer:sleep(5000).
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
[A, B, C, D, E | _] = S,
T = [A, B, C, D, E].
Alternative implementation:
T = string:slice(S, 0, 5).
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
[T5, T4, T3, T2, T1 | _] = lists:reverse(S),
T = [T1, T2, T3, T4, T5].
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
S = "Strings
may span
across multiple lines"
"and they can"
"have as many portions"
"as you want"
"all of them quoted".
49
Build list chunks consisting in substrings of input string s, separated by one or more space characters.
Chunks = string:tokens(S, [$\s]).
50
Write a loop that has no end clause.
loop() ->
	do:something(),
	loop().
51
Determine whether the map m contains an entry for the key k
maps:is_key(K, M).
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
Y = string:join(X, ",").
54
Calculate the sum s of the integer list or array x.
S = lists:sum(X).
55
Create the string representation s (in radix 10) of the integer value i.
S = integer_to_list(I).
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
run_n_times(Fun, N) ->
    Self = self(),
    Task = fun (I) -> spawn(?MODULE, run_task_and_notify_completion, [Self, Fun, I]) end,
    lists:foreach(Task, lists:seq(1, N)),
    wait_for_n_tasks(N).

run_task_and_notify_completion(ParentPid, Fun, Arg) ->
    Fun(Arg),
    ParentPid ! done.

wait_for_n_tasks(0) ->
    io:format("Finished~n");
wait_for_n_tasks(N) when N > 0 ->
    receive
        done ->
            ok
    end,
    wait_for_n_tasks(N-1).

run_n_times(F, 1000).
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
Y = [I || I <- X, P(X)].
Alternative implementation:
Y = lists:filter(P, X).
58
Create string lines from the content of the file with filename f.
{ok, Lines} = file:read_file(F).
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
io:format(standard_error, "~p is negative~n", [X]).
61
Assign to the variable d the current date/time value, in the most standard type.
D = calendar:local_time().
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
X2 = binary:replace(X, Y, Z, [global]).
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
main() ->
    main(init:get_plain_arguments()).

main(ARGV) ->
    io:format("~s~n", [lists:join(" ", ARGV)]).
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
gcd(A,B) when A == 0; B == 0 -> 0;
gcd(A,B) when A == B -> A;
gcd(A,B) when A > B -> gcd(A-B, B);
gcd(A,B) -> gcd(A, B-A).

lcm(A,B) -> (A*B) div gcd(A, B).
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
S = io_lib:format("~.2B~n", [X]).
78
Execute a block once, then execute it again as long as boolean condition c is true.
do_while(Block, C) ->
  case C(Block()) of
    true -> do_while(Block, C);
    false -> ok
  end.
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
countOccurence(List1, List2) ->
        countOccurence(List1, List2, 0).

countOccurence(_, [], Count) ->
        Count;
countOccurence(List1, [_ | Rest] = List2, Count) ->
        case (lists:prefix(List1, List2)) of
                true ->
                        countOccurence(List1, Rest, Count + 1);
                false ->
                        countOccurence(List1, Rest, Count)
        end.

countOccurence("ab", "abcddababa").
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
erlang:halt(0).
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
error(badarg).
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
B = string:find(S, Prefix) =:= S.
Alternative implementation:
prefix([], _) -> true;
prefix([Ch | Rest1], [Ch | Rest2]) ->
        prefix(Rest1, Rest2);
prefix(_, _) -> false.

prefix("abc", "abcdef").
98
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
Timestamp = {Seconds div 1000000, Seconds rem 1000000, 0},
calendar:now_to_universal_time(Timestamp).
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
D = erlang:localtime(),
{{Year, Month, Day}, {_Hour, _Minute, _Second}} = D,
X = lists:flatten(io_lib:format("~4..0w-~2..0w-~2..0w", [Year, Month, Day])).  % "2017-07-02"
100
Sort elements of array-like collection items, using a comparator c.
lists:sort(C, List).
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
Blank = string:is_empty(string:trim(S)).
116
Remove all occurrences of string w from string s1, and store the result in s2.
S2 = string:replace(S1, W, "", all).
117
Set n to the number of elements of the list x.
N = length(X).
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
Y = sets:from_list(X).
119
Remove duplicates from the list x.
Explain if the original order is preserved.
S = lists:usort(X)
124
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
bsearch([], _) -> -1;
bsearch([H|_T], X) when X < H -> -1;
bsearch(List, X) -> 
  bsearch(List, X, 0, length(List)).

bsearch(_List, _X, First, Last) when Last < First -> -1;
bsearch(List, X, First, Last) -> 
  Middle = First + (Last - First) div 2,
  Item = lists:nth(Middle, List),
  case Item of
    X -> Middle;
    _Less when X < Item -> bsearch(List, X, First, Middle);
    _More -> bsearch(List, X, Middle + 1, Last)
  end.
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
timer:tc(Fun).
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
if
	C1 -> f1();
	C2 -> f2();
	C3 -> f3()
end.
133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
Re = re:compile(Word, [caseless, unicode]),
Ok = re:run(S, Re) =/= nomatch.
134
Declare and initialize a new list items, containing 3 elements a, b, c.
Items = [A,B,C].
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
{_,Rest} = string:to_integer(S),
B = Rest == "".
Alternative implementation:
Str = "21334",
[Ch || Ch <- Str, Ch < $0 orelse Ch > $9] == [].
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
S = io_lib:fwrite("~.16B",[X]).
150
Remove the last character from the string p, if this character is a forward slash /
removeTrailingSlash([$/]) -> [];
removeTrailingSlash([]) -> [];
removeTrailingSlash([Ch | Rest]) ->
        [Ch] ++ removeTrailingSlash(Rest).
152
Create string s containing only the character c.
S = [C]
157
Initialize a constant planet with string value "Earth".
Planet = "Earth".
165
Assign to the variable x the last element of the list items.
x = lists:last(items),
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
AB = A ++ B.
169
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
N = string:length(S)
175
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
hexChar(Num) when Num < 10 andalso Num >= 0->
    $0 + Num;
hexChar(Num) when Num < 16 ->
    $a + Num - 10.

toHex([Byte | Rest]) ->
    [hexChar(Byte div 16), hexChar(Byte rem 16)] ++ toHex(Rest);
toHex([]) -> [].
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
isInsideRect(X1, Y1, X2, Y2, PX, PY) when X1 =< PX andalso PX =< X2 andalso Y1 =< PY andalso PY =< Y2 -> true;
isInsideRect(_, _, _, _, _, _) -> false.
186
Exit a program cleanly indicating no error to OS
erlang:exit(0).
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
Foo = os:getenv("FOO","none").
213
Compare four strings in pair-wise variations. The string comparison can be implemented with an equality test or a containment test, must be case-insensitive and must apply Unicode casefolding.
code (C) when $a =< C andalso C =< $z -> C - $a;
code (C) when $A =< C andalso C =< $Z -> C - $A;
code(C) -> C.

isSame([C | Rest1], [C | Rest2]) when is_integer(C)->
        isSame(Rest1, Rest2);
isSame([C1 | Rest1], [C2 | Rest2]) ->
        case (code(C1) - code(C2)) of
                0 -> isSame(Rest1, Rest2);
                _ -> false
        end;
isSame([], []) -> true;
isSame(_, _) -> false.
Alternative implementation:
code (C) when $a =< C andalso C =< $z -> C - $a;
code (C) when $A =< C andalso C =< $Z -> C - $A;
code(C) -> C.

isSame([C | Rest1], [C | Rest2]) when is_integer(C)->
        isSame(Rest1, Rest2);
isSame([C1 | Rest1], [C2 | Rest2]) ->
        (code(C1) == code(C2)) andalso isSame(Rest1, Rest2);
isSame([], []) -> true;
isSame(_, _) -> false.
218
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
C = lists:flatten([A -- B] ++ [B -- A]).
219
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
singleSpace(Text) ->
        singleSpace(0, Text).

singleSpace(_, []) -> [];
singleSpace(32, [32 | Rest]) ->
        singleSpace(32, Rest);
singleSpace(32, [Ch | Rest]) ->
        [Ch] ++ singleSpace(Ch,  Rest);
singleSpace(Last, [Ch | Rest]) ->
                [Ch] ++ singleSpace(Ch, Rest).


%%singleSpace("this is  a      text  with        multiple spaces").
237
Assign to c the result of (a xor b)
C = A bxor B
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
{A, B, C} = {42, "hello", 5.0}.
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
erlang:list_to_integer(S,2).
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
X = case Condition() of
	true -> "a";
	false -> "b"
end
254
Replace all exact occurrences of "foo" with "bar" in the string list x
[case Elem of "foo" -> "bar"; _ -> Elem end || Elem <- X].
262
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4
trailingZeros(Num) ->
        trailingZeros(Num, 0).

trailingZeros(Num, Count) when Num band 1 == 0 ->
        trailingZeros(Num div 2, Count + 1);
trailingZeros(_, Count) -> Count.
265
Set the integer variable i to 42 and calculate its parity (i.e. 0 if it contains an even number of bits, 1 if it contains an odd number of bits).
parity(Number) -> parity(Number, 0).

parity(Number, Count) when Number band 1 == 1 ->
        parity(Number bsr 1, Count + 1);
parity(Number, Count) when Number > 0 ->
        parity(Number bsr 1, Count);
parity(_, Count) ->
        Count rem 2.
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
S = lists:concat(lists:duplicate(N, V)).
269
Given the enumerated type t with 3 possible values: bike, car, horse.
Set the enum value e to one of the allowed values of t.
Set the string s to hold the string representation of e (so, not the ordinal value).
Print s.
toString(Atom) when Atom == bike orelse Atom == car orelse Atom == horse ->
        erlang:atom_to_list(Atom).

%% E = horse, S = toString(E).
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
[Ch || Ch <- "This is a\tstring with\nwhite\rspaces", Ch /= 8, Ch /= 9, Ch /= 10, Ch /= 13, Ch /= 32 ].