List comprehensions in Erlang

Map and filter in one bracket; generators cross like a matrix.

%% A comprehension maps and filters in one bracket.
main(_) ->
    Lines = lists:seq(1, 10),

    Odd = [L || L <- Lines, L rem 2 =:= 1],
    io:format("Odd-numbered lines: ~p~n", [Odd]),

    Jacks = [{L, L * 100} || L <- Lines, L > 7],
    io:format("High jacks: ~p~n", [Jacks]),

    %% Two generators cross like a patch matrix.
    Patches = [{A, B} || A <- [1, 2], B <- [7, 8]],
    io:format("Possible patches: ~p~n", [Patches]).

How it works

1. [L || L <- Lines, L rem 2 =:= 1] filters as it walks.
2. The template builds tuples as easily as values.
3. Two generators yield every combination.

Keywords and builtins used here

• format
• main
• seq

The run, in numbers

Lines

13

Characters to type

403

Tokens

133

Three-star pace

60 tpm

At the three-star pace of 60 tokens a minute, this run takes about 133 seconds.

Type this snippet

Step 2 of 3 in Recursion & comprehensions, step 9 of 31 in Language basics.

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List comprehensions in other languages

• Haskell
• Python