D Recurrences

[Timon Gehr]

Ben Grabham wrote:
> Hey,
>
> Shouldn't both these programs output the fibonnacci numbers? Only the
> first one does.
>
> import std.range;
> import std.stdio;
> int main() {
>  auto a = recurrence!("a[n-1] + a[n-2]")(0,1);
>  int i = 0;
>  foreach(int n; a) {
>   if(i++ > 20) break;
>   writefln("%d", n);
>  }
>  return 0;
> }
>
>
>
> import std.range;
> import std.stdio;
> int main() {
>  auto a = recurrence!("a[n-1] + (n < 2 ? 0 : a[n-2])")(1);
>  int i = 0;
>  foreach(int n; a) {
>   if(i++ > 20) break;
>   writefln("%d", n);
>  }
>  return 0;
> }

You use the function recurrence incorrectly in the second example: You may only
use a[n-1] to a[n-x] in your formula where x is the number of arguments given to
the template function. It would be nice if the library could enforce this, but (at
least currently) it cannot.


Timon