nested for loop

[Sean Ross]

"Sean Ross" <sross at connectmail.carleton.ca> wrote in message
news:VDWnc.21679$FH5.581779 at news20.bellglobal.com...
[snip]
> # slightly modified code from
> http://twistedmatrix.com/wiki/python/PostYourCode
> def sequences(n, things):
>     "generates sequences of n items from a set of things"
>     if n == 0:
>         yield []
>     else:
>         for x in things:
>             for y in sequences(n-1, things):
>                 yield [x] + y
>
> def nXn_matrices(n, elements):
>     "generates nXn matrices from elements"
>     for s in sequences(n*n, elements):
>         yield [s[i*n:(i+1)*n] for i in xrange(n)]
[snip]
> [I] believe there are several opportunities for optimization both in the
> code and in the algorithm (for instance, it may be possible to take
> advantage of repetition in the sub-matrices), but I won't be trying that
> now.

Looks like I'll be trying it now after all:

def nXn_matrices2(n, elements):
    for m in sequences(n, list(sequences(n, elements))):
        yield m

This is slightly faster than the first version, but it has more overhead
since it builds a list of the submatrices. That could be problem. It can
probably be avoided, but I haven't figured out how to do it just yet. We'll
see what happens.

Sean