LFE Friday - lists:any/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 function of the week is lists:any/2.

lists:any/2 takes a predicate function as the first argument, and a list to iterate over as its second argument. lists:any/2 returns true if the predicate function returns true for any of the elements in the given list, otherwise, lists:any/2 returns false.

> (lists:any #'erlang:is_atom/1 '(1 2 3 4 5 6 7))
false
> (lists:any #'erlang:is_atom/1 '(1 2 3 4 a 6 7))
true
> (lists:any #'erlang:is_atom/1 '(#(1 2) 3 4 a 6 7))
true
> (lists:any (lambda (x) (== (rem x 2) 1)) '(1 2 4))
true
> (lists:any (lambda (x) (== (rem x 2) 1)) '(0 2 4))
false

lists:any/2 is eager, and will return with a result of true as soon as it is found, and will ignore processing the rest of the list.

> (timer:tc 'lists 'any (list (lambda (x) (== (rem x 2) 1)) (lists:seq 2 200000 2))) 
#(171661 false)
> (timer:tc 'lists 'any (list (lambda (x) (== (rem x 2) 0)) (lists:seq 2 200000 2)))
#(19 true)

The lists module also contains a function lists:all/2, similar to lists:any/2, but checks if the predicate function returns true for every element in the supplied list.

> (lists:all #'erlang:is_number/1 '(1 2 3 4 a 6 7))
false
> (lists:all #'erlang:is_number/1 '(1 2 3 4 5 6 7))
true

lists:all/2 is also eager, and will return with a result of false as soon as it is found, and will ignore processing the rest of the list.

> (timer:tc 'lists 'all (list (lambda (x) (== (rem x 2) 0)) (lists:seq 2 200000 2)))
#(170436 true)
> (timer:tc 'lists 'all (list (lambda (x) (== (rem x 2) 1)) (lists:seq 2 200000 2)))
#(18 false)

–Proctor, Robert