[Tutor] smarter way of looping

Kent Johnson kent37 at tds.net
Thu Jan 26 14:48:51 CET 2006

Elderely Geek wrote:
> Hi,
> I`m a python newbie and could use some help.
> I often do a loop over arbitrary number of sequences. I do it like this:
> for elem1 in seq1:
>     for elem2 in seq2:
>         do whatever seq1,seq2
> this isn`t nice I think. Is there some way I can say 
> myiterator=geniousfunction(seq1,seq2,seq3)
> and then myiterator.next()  and have it return the corresponding elemnts 
> from all sequences like if
> seq1 = ['a','b']
> seq2 = [1,2]
> I could get
> a 1
> a 2
> b 1
> b 2

This is a perennial topic on comp.lang.python and there are many clever 
ways to do it. The most straightforward way is to use a recursive 
generator. The idea is, at each step, combine each element of the first 
sequence with each sequence generated from the remaining sequences. Here 
is one way to do it:

def combine(*seqs):
   if not seqs:
     yield []
     for item in seqs[0]:
       for subitem in combine(*seqs[1:]):
         yield [item] + subitem

seq1 = ['a','b']
seq2 = [1,2]

for seq in combine(seq1, seq2):
   print seq

print list(combine(seq1, seq2))

This uses some fairly advanced bits of Python:
arbitrary argument lists (I can't find a good doc for this)
generators: http://www.python.org/doc/2.3.5/whatsnew/section-generators.html
and recursion.

> also, how do you loop over all the elements that can be returned by 
> myiterator?

As in the example above, you can iterate it with a for loop, or convert 
to a list.

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