Iterating over multiple collections in parallel

[Jarrett Billingsley]

"Koroskin Denis" <2korden at gmail.com> wrote in message 
news:op.udp4pcjxenyajd at proton.creatstudio.intranet...
> If found myself solving the same problem again and again:
> how to implement simultaneous iteration over two (or more collections)?
>
> suppose, we have the arrays:
> int[] a = [1, 2, 3];
> int[] b = [1, 4, 9];
>
> and would like to multiply them per-component, like this:
>
> int[] c = new int[a.length];
> for (int i = 0; i < a.length; ++j) {
>     c[i] = a[i] * b[i];
> }
>
> Since foreach is the prefered (and sometimes the only) way to iterate over 
> collection, there should be a way to iterate over multiple collections 
> simultaneously, like (just an example) this:
>
> int[] c = new int[a.length];
> foreach (i : a; j : b; ref k : c) {
>     k = a + b;
> }
>
> But this isn't supported (yet?)
> More complicated example would be an iterating over two (or more) 
> collection with *no* random access iterators available, opApply only.
>
> I spend a lot of time on this and have found no solution how to do it with 
> existing D feature set, but this is surely doable with a few 
> inter-function gotos and an exception if collections sizes don't match. 
> It's just a small layer written in asm, nothing more.
>
> How about an enhancement proposal? :)

It's something Walter mentioned before, but without any kind of public 
"plan" of these ideas he has, there's no way to know if it's even on his 
plate.

I think the syntax he used was something like this:

foreach(a, b; c)(d, e; f) { blah }

In any case, it's difficult in the general case to do parallel iteration 
with D's inside-out iterators.  To do it generally, you'd have to use 
coroutines.  But you can currently write a templated zip to iterate over 
arrays simultaneously, only because they're so easy to iterate over.  I'm 
sure downs will post something horrid-looking that you can look over.