Erlang Thursday – lists:dropwhile/2

Today’s Erlang Thursday is lists:dropwhile/2.

lists:dropwhile/2 takes a predicate function and a list, and returns a list where the first series of items for which the predicate function returned true have been removed.

lists:dropwhile(fun erlang:is_atom/1, [hello, 'World', foo, 1, 3, 4]).
% [1,3,4]
lists:dropwhile(fun (X) -> X > 0 end, [-1, 0, 1, 2, 3]).
% [-1,0,1,2,3]
lists:dropwhile(fun (X) -> X > 0 end, [-2, -1, 0, 1, 2, 3]).
% [-2,-1,0,1,2,3]
lists:dropwhile(fun (X) -> X < 0 end, [-2, -1, 0, 1, 2, 3]).
% [0,1,2,3]
lists:dropwhile(fun (X) -> X < 0 end, [0, -1, -2, -3, -4, -5]). 
% [0,-1,-2,-3,-4,-5]
lists:dropwhile(fun (X) -> true end, [hello, 'World', foo, 1, 3, bar, 4]). 
% []
lists:dropwhile(fun (X) -> false end, [hello, 'World', foo, 1, 3, bar, 4]).
% [hello,'World',foo,1,3,bar,4]

Unlike lists:filter/2, lists:dropwhile/2 stops checking the list as soon as the predicate function returns false. This means that elements for which the predicate function would return true can still appear in the result list, as if they occur after an element for which the predicate function returns false.

lists:dropwhile(fun erlang:is_atom/1, [hello, 'World', foo, 1, 3, bar, 4]).
% [1,3,bar,4]
lists:filter(fun (X) -> not is_atom(X) end, [hello, 'World', foo, 1, 3, bar, 4]).     
% [1,3,4]
lists:dropwhile(fun (X) -> X < 0 end, [-2, -1, 0, 1, -5, 3, 7]).
% [0,1,-5,3,7]
lists:filter(fun (X) -> X >= 0 end, [-2, -1, 0, 1, -5, 3, 7]).   
% [0,1,3,7]

–Proctor

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