value in a recursive loop
In a recursive loop, I would like to change the value of a variable:
loop(N) when N > ... ->
N;
loop(N) ->
case ... of
N+1
...
end,
...
case ... of
N-1
...
end,
...
loop(N).
How t开发者_开发百科o "pass" the new value of N ?
Since you can't change the value of N
once assigned, you need to call loop
with the new value:
loop(N) ->
loop(N+1).
Alternatively, create a temporary variable to hold its new value before passing it into the recursive loop
invocation.
loop(N) ->
NewN = N+1,
loop(NewN).
If this results in a lot of repetition in your code, you might want to separate the looping construct from the logic which produces the new value of N:
loop(N) ->
NewN = logic(N),
loop(NewN).
logic(N) ->
N+1.
You simply call the function with a new value:
$ cat fac.erl
-module(fac).
-export([fac/1]).
fac(0) -> 1;
fac(N) when N > 0 -> N*fac(N-1).
$ erl
Erlang R13B03 (erts-5.7.4) [source] [64-bit] [smp:8:8] [rq:8] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.7.4 (abort with ^G)
1> c(fac).
{ok,fac}
2> fac:fac(10).
3628800
3> q().
ok
4> $
Shamelessly stolen from: http://learnyousomeerlang.com/recursion#hello-recursion
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