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50kudos/w21list.erl

Created February 23, 2017 08:19
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  1. 50kudos created this gist Feb 23, 2017.
    64 changes: 64 additions & 0 deletions w21list.erl
    View file
    Original file line number Diff line number Diff line change
    @@ -0,0 +1,64 @@
    -module(w21list).
    -export([double/1, evens/1, median/1, mode/1]).

    double([]) -> [];
    double([H|T]) ->
    [H * 2 | double(T)].

    evens([]) -> [];
    evens([H|T]) ->
    case H rem 2 of
    0 ->
    [H | evens(T)];
    _ ->
    evens(T)
    end.

    median([]) -> [];
    median(L) ->
    Sorted = bubble_sort(L),
    M = length(Sorted) div 2,

    case length(Sorted) rem 2 of
    0 ->
    (lists:nth(M, Sorted) + lists:nth(M + 1, Sorted)) / 2;
    _ ->
    lists:nth(M + 1, Sorted)
    end.

    mode([]) -> [];
    mode(L) ->
    S = bubble_sort(L),
    mode(S, {hd(S),1}, []).

    mode([_H|[]], {Mode,Count}, ACC) ->
    [{Mode,Count} | ACC];
    mode([H|T], {Mode,Count}, ACC) ->
    case hd(T) of
    H ->
    mode(T, {Mode,Count+1}, ACC);
    _ ->
    mode(T, {hd(T), 1}, [{Mode,Count} | ACC])
    end.


    % For saving human time complexity,
    % we go for the bubble sort for now!
    %
    % We intend not to use any lists:function for the sake of learning.
    % And this is a mix of direct and tail recursions.

    % Direct recursion
    bubble([]) -> [];
    bubble([H1,H2|T]) when H1 > H2 ->
    [H2 | bubble([H1|T])];
    bubble([H|T]) ->
    [H | bubble(T)].

    % Tail recursion without optimization on bubble_sort.
    bubble_sort(L) ->
    bubble_sort(L, L).
    bubble_sort([], S) ->
    S;
    bubble_sort([_H|T], ACC) ->
    bubble_sort(T, bubble(ACC)).