Matrix creation quiz

[Steven Schveighoffer]

On Thu, 28 Apr 2011 08:02:40 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> A little quiz. This is related to a recent post of mine in the main D  
> newsgroup, but please don't take a look at that post yet. This is the  
> original function:
>
>
> double[][] matgen(int n) {
>     double[][] a;
>     double tmp = 1.0 / n / n;
>     a.length = n;
>     for (int i = 0; i < n; ++i) a[i].length = n;
>     for (int i = 0; i < n; ++i)
>         for (int j = 0; j < n; ++j)
>             a[i][j] = tmp * (i - j) * (i + j);
>     return a;
> }
>
>
> Second "improved" version:
>
> double[][] matgen(int n) {
>     double tmp = 1.0 / n / n;
>     auto a = new double[][](n, n);
>     foreach (i, row; a)
>         foreach (j, ref x; row)
>             x = tmp * (i - j) * (i + j);
>     return a;
> }
>
>
> But the second nicer version has a bug, do you see it? :-)

I read the other answer, I thought it was because the indexing is  
different, but that's only on initialization.  So I was wrong :)

-Steve