LFE Friday - lists:dropwhile/2
This week's LFE Friday was translated with permission from the
Erlang Thursday
series by Steven Proctor.
This week's translator: Robert Virding.
Today’s LFE Friday 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 #'erlang:is_atom/1 '(hello World 1 3 4))
(1 3 4)
> (lists:dropwhile (lambda (x) (> x 0)) '(-1 0 1 2 3))
(-1 0 1 2 3)
> (lists:dropwhile (lambda (x) (> x 0)) '(-2 -1 0 1 2 3))
(-2 -1 0 1 2 3)
> (lists:dropwhile (lambda (x) (< x 0)) '(-2 -1 0 1 2 3))
(0 1 2 3)
> (lists:dropwhile (lambda (x) (< x 0)) '(0 -1 -2 -3 -4 -5))
(0 -1 -2 -3 -4 -5)
> (lists:dropwhile (lambda (x) 'true) '(hello World 1 3 bar 4))
()
> (lists:dropwhile (lambda (x) 'false) '(hello World 1 3 bar 4))
(hello World 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 #'erlang:is_atom/1 '(hello World foo 1 3 bar 4))
(1 3 bar 4)
> (lists:filter (lambda (x) (not (is_atom x))) '(hello World foo 1 3 bar 4))
(1 3 4)
> (lists:dropwhile (lambda (x) (< x 0)) '(-2 -1 0 1 -5 3 7))
(0 1 -5 3 7)
> (lists:filter (lambda (x) (>= x 0)) '(-2 -1 0 1 -5 3 7))
(0 1 3 7)–Proctor, Robert
