[erlang-questions] lists:takewhile and lists:splitwith
Richard O'Keefe
<>
Thu Feb 3 22:29:56 CET 2011
On 3/02/2011, at 10:56 PM, maruthavanan s wrote:
>
> Hi,
>
> The document says lists:takewhile(Pred,List) would return tuple of lists which would split the List when Pred returns true.
No it doesn't. This is what the documentation says:
takewhile(Pred, List1) -> List2
Types:
Pred = fun(Elem) -> bool()
Elem = term()
List1 = List2 = [term()]
Takes elements Elem from List1 while Pred(Elem) returns true,
that is, the function returns the longest prefix of the list
for which all elements satisfy the predicate.
>
> I tried the below.
>
> lists:takewhile(fun(A)-> A==3 end,[1,2,3,4,5]).
>
> it gives me an empty result..
And indeed the longest prefix of [1,2,3,4,5] such that all elements
are the number 3 really and truly is [].
> What should I do to get {[1,2],[3,4,5]}
Write your own function.
What you want is
Pred, [X1,...,Xk] => {[Y1,...,Ym],[Z1,...,Zn]}
such that [Y1,...,Ym]++[Z1,...,Zn] is [X1,...,Xk]
and all(Pred, [Y1,...,Ym]
and n = 0 or not Pred(Z1).
split_at_first(Pred, List) ->
split_at_first_loop(Pred, List, []).
split_at_first_loop(Pred, [], Reversed_Prefix) ->
{lists:reverse(Reversed_Prefix), []};
split_at_first_loop(Pred, [X|Xs], Reversed_Prefix) ->
case Pred(X)
of true -> {lists:reverse(Reversed_Prefix), [X|Xs]}
; false -> split_at_first_loop(Pred, Xs, [X|Reversed_Prefix])
end.
You should probably ask yourself whether [Ym,...,Y1] will do just as
well as [Y1,...,Ym].
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