In trying to combine a set of lists together, I landed upon
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/302478 , which
does the job beautifully. The problem is I really want to understand
how it does it, and I can't quite get my brain wrapped around it.
I'll try to explain, step by step, what I think it's doing, and if
someone would be so kind as to try to show me where I'm missing
something, I'd be grateful.
Okay, so here's the function:
1 def combine(*seqin):
2 def rloop(seqin, listout, comb):
3 if seqin:
4 for item in seqin[0]:
5 newcomb = comb + [item]
6 rloop(seqin[1:], listout, newcomb)
7 else:
8 listout.append(comb)
9 listout = []
10 rloop(seqin, listout, [])
11 return listout
We'll start with the example Mr. Klaffenbach gives in the
documentation on the function: combine((1, 2), (3, 4)) . Line 10 is
the first thing to act upon the list of seqin, looking like this:
rloop(((1, 2), (3, 4)), [], [])
So we go into the rloop function. There is something left in the
seqin, so we pass the if statement and hit the for loop. seqin[0] ==
(1, 2), and comb == [], so newcomb == [1], the first item, and we're
sent back into the rloop with the following new setup:
rloop(((3, 4)), [], [1])
This time around we pass the if statement again and the first item in
(3, 4) is 3. comb == [1], so newcomb == [1, 3]. We get sent back to
the top of the rloop with the following new setup:
rloop((), [], [1, 3])
Now we fail the if statement, so comb is appended to listout and
listout == [[1, 3]], which is the first combination to be returned.
Great. But, as far as I can see, the whole function should stop right
here. How does it produce the rest of the combinations? I suspect
I'm misunderstanding something about the for loop, or the way seqin is
handed into the rloop function. Any help is much appreciated.
gsf
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