Iterate n times

[Raimo Niskanen]

You can make up numerous variants of this idea. Here is another one:

%% A simple loop construct.
%%
%% Calls 'Fun' with argument 'Start' first and then repeatedly with
%% its returned value (state) until 'Fun' returns 'Stop'. Then
%% the last state value that was not 'Stop' is returned.

iterate(Start, Done, Fun) when function(Fun) ->
    iterate(Start, Done, Fun, Start).

iterate(Done, Done, Fun, I) ->
    I;
iterate(I, Done, Fun, _) ->
    iterate(Fun(I), Done, Fun, I).


Example:
1>iterate([1], done, fun ([5|_])->done; (L)->[hd(L)+1|L] end).
[5,4,3,2,1]

Another thought is to make lists:seq/3 accept a fun as incrementor.

The problem might be that everytime a user comes up with this kind of
construct, it looks slightly different, so there is no obvious
one-size-fits-all solution. Furthermore, if you give the user another
loop construct it is just another way for the user to shoot her/himself
in the foot.

/ Raimo Niskanen, Ericsson UAB, Erlang/OTP



Sean Hinde wrote:
> 
> Does anyone know if this function has a proper name or already exists
> anywhere?
> 
> iter(F, St, 0) ->
>     St;
> iter(F, St, N) when integer(N), N > 1 ->
>     iter(F, F(St, N), N-1).
> 
> Simple examples of usage might be:
> 
> 1> iter(fun(St, N) -> N + St end, 0, 5).
> 15
> 2> iter(fun(St, N) -> [N|St] end, [], 5).
> [1,2,3,4,5]
> 
> Sean
> 
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