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Erlang
1
Print Hello World
Print a literal string on standard output 
io:format("~s~n", ["Hello, world!"])
2
Print Hello 10 times
Loop to execute some code a constant number of times 
lists:foreach(
  fun(_) ->
    io:format("Hello~n")
  end, lists:seq(1, 10)).
3
Create a procedure
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
Create a function which returns the square of an integer 
-spec square(integer()) -> integer().
square(X) when is_integer(X) -> X * X.
5
Create a 2D Point data structure
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
Iterate over list values
Do something with each item x of the list (or array) items, regardless indexes. 
[do_something(X) || X <- Items]
Alternative implementation:
lists:foreach(fun do_something/1, Items).
7
Iterate over list indexes and values
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 map (associative array)
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
Create a Binary Tree data structure
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
Shuffle a list
Generate a random permutation of the elements of list x 
[Y||{_,Y} <- lists:sort([ {rand:uniform(), N} || N <- X])].
11
Pick a random element from a list
The list x must be non-empty. 
ktn_random:pick(X)
Alternative implementation:
lists:nth(rand:uniform(length(X)), X).
12
Check if list contains a value
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
Iterate over map keys and values
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 uniformly a random floating point number in [a..b)
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b. 
A + (B - A) * rand:uniform().
15
Pick uniformly a random integer in [a..b]
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b. 
crypto:rand_uniform(A, B)
17
Create a Tree data structure
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 a list
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering. 
lists:reverse(List)
20
Return two values
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 values
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
Convert string to integer
Extract the integer value i from its string representation s (in radix 10) 
I = list_to_integer(S).
23
Convert real number to string with 2 decimal places
Given a real number x, create its string representation s with 2 decimal digits following the dot. 
S = io_lib:format("~.2f", [X]).
24
Assign to string the japanese word ネコ
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
Create a 2-dimensional array
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
Create a 3-dimensional array
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 by a property
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 item from list, by its index
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
Parallelize execution of 1000 independent tasks
Launch the concurrent execution of the 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
Recursive factorial (simple)
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
Integer exponentiation by squaring
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
First-class function : compose
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
First-class function : generic composition
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
Extract a substring
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
Check if string contains a word
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise. 
Ok = string:str(S, Word) > 0.
41
Reverse a string
Create the string t containing the same characters as the string s, in reverse order.
The original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory. 
T = lists:reverse(S)
43
Break outer loop
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 element in list
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
Pause execution for 5 seconds
Sleep for 5 seconds in current thread, before proceeding with the next instructions. 
timer:sleep(5000).
46
Extract beginning of string (prefix)
Create the string t consisting of the 5 first characters of the 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
Extract string suffix
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
Multi-line string literal
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
Split a space-separated string
Build list chunks consisting in substrings of the string s, separated by one or more space characters. 
Chunks = string:tokens(S, [$\s]).
50
Make an infinite loop
Write a loop that has no end clause. 
loop() ->
	do:something(),
	loop().
51
Check if map contains key
Determine whether the map m contains an entry for the key k 
maps:is_key(K, M).
53
Join a list of strings
Concatenate elements of string list x joined by the separator ", " to create a single string y. 
Y = string:join(X, ",").
54
Compute sum of integers
Calculate the sum s of the integer list or array x. 
S = lists:sum(X).
55
Convert integer to string
Create the string representation s (in radix 10) of the integer value i. 
S = integer_to_list(I).
56
Launch 1000 parallel tasks and wait for completion
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
Filter list
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
Extract file content to a string
Create the string lines from the content of the file with filename f. 
{ok, Lines} = file:read_file(F).
59
Write to standard error stream
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
Get current date
Assign to the variable d the current date/time value, in the most standard type. 
D = calendar:local_time().
63
Replace fragment of a string
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
Echo program implementation
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 LCM
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
Binary digits from an integer
Create the string s of integer x written in base 2.

E.g. 13 -> "1101" 
S = io_lib:format("~.2B~n", [X]).
78
"do while" loop
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
Count substring occurrences
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
Stop program
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it. 
erlang:halt(0).
89
Handle invalid argument
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
Check string prefix
Set the 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
Epoch seconds to date object
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
Format date YYYY-MM-DD
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 by a comparator
Sort elements of array-like collection items, using a comparator c. 
lists:sort(C, List).
110
Check if string is blank
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 occurrences of word from string
Remove all occurrences of string w from string s1, and store the result in s2. 
S2 = string:replace(S1, W, "", all).
117
Get list size
Set n to the number of elements of the list x. 
N = length(X).
118
List to set
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
Deduplicate list
Remove duplicates from the list x.
Explain if the original order is preserved. 
S = lists:usort(X)
124
Binary search for a value in sorted array
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 function call duration
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration. 
timer:tc(Fun).
131
Successive conditions
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
Case-insensitive string contains
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
Create a new list
Declare and initialize a new list items, containing 3 elements a, b, c. 
Items = [A,B,C].
137
Check if string contains only digits
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
Hexadecimal digits of an integer
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7" 
S = io_lib:fwrite("~.16B",[X]).
150
Remove trailing slash
Remove the last character from the string p, if this character is a forward slash / 
removeTrailingSlash([$/]) -> [];
removeTrailingSlash([]) -> [];
removeTrailingSlash([Ch | Rest]) ->
        [Ch] ++ removeTrailingSlash(Rest).
152
Turn a character into a string
Create string s containing only the character c. 
S = [C]
157
Declare constant string
Initialize a constant planet with string value "Earth". 
Planet = "Earth".
165
Last element of list
Assign to the variable x the last element of the list items. 
x = lists:last(items),
166
Concatenate two lists
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
String length
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
Bytes to hex string
From the 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
Check if point is inside rectangle
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 program cleanly
Exit a program cleanly indicating no error to OS 
erlang:exit(0).
205
Get an environment variable
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
Case-insensitive string compare
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
List intersection
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
Replace multiple spaces with single space
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
Xor integers
Assign to c the result of (a xor b) 
C = A bxor B
249
Declare and assign multiple variables
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
Parse binary digits
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13 
erlang:list_to_integer(S,2).
252
Conditional assignment
Assign to the variable x the string 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 value in list
Replace all exact occurrences of "foo" with "bar" in the string list x 
[case Elem of "foo" -> "bar"; _ -> Elem end || Elem <- X].
262
Count trailing zero bits
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
Even parity bit
Calculate the parity p of the integer variable i : 0 if it contains an even number of bits set, 1 if it contains an odd number of bits set. 
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
Repeated string
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
Enum to String
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
Remove all white space characters
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 ].