howto handle nested for

[Laszlo Nagy]

On 2012-09-28 16:39, Neal Becker wrote:
> I know this should be a fairly basic question, but I'm drawing a blank.
>
> I have code that looks like:
>   
>    for s0 in xrange (n_syms):
>          for s1 in xrange (n_syms):
>              for s2 in xrange (n_syms):
>                  for s3 in xrange (n_syms):
>                      for s4 in range (n_syms):
>                          for s5 in range (n_syms):
>
> Now I need the level of nesting to vary dynamically.  (e.g., maybe I need to add
> for  s6 in range (n_syms))
>
> Smells like a candidate for recursion.  Also sounds like a use for yield.  Any
> suggestions?
>
In your example, it seem that the iterable of the for loop is always the 
same: range(n_sysms). It seems to be a number. Is that true? If that is 
so, then here is something useful:

import copy

class MultiLevelIterator(object):
     def __init__(self,levels,n):
         assert(levels>0)
         assert(n>0)
         self.levels = levels
         self.values = [0]*levels
         self.n = n

     def __iter__(self):
         return self

     def next(self):
         res = copy.copy(self.values)
         idx = self.levels-1
         while idx>=0:
             self.values[idx]+=1
             if self.values[idx]>=self.n:
                 self.values[idx] = 0
                 idx-=1
             else:
                 return res
         raise StopIteration

i = MultiLevelIterator(2,3)
for values in i:
     print values

This will print:

[0, 0]
[0, 1]
[0, 2]
[1, 0]
[1, 1]
[1, 2]
[2, 0]
[2, 1]