[Tutor] recursion

noopy ml1 at bitbert.com
Thu Feb 4 21:03:14 EST 2016


I just cannot get my head around the following code, maybe someone could 
explain it to me.

def permutations(items):
     n = len(items)
     if n==0: yield []
         for i in range(len(items)):
             for cc in permutations(items[:i]+items[i+1:]):
                 yield [items[i]]+cc

for p in permutations(list("XYZ")):

I did not figure out how this works in the deepest level of the 
recursion. I mean, when I go down the recursion level
and do the part "for cc in permutations("Z")".

I have a problem with the following understanding:
when I am in the recursion and call "for cc in permutations("Z")", the 
next level of the recursion will do "for cc in permutations([])", which 
will yield an empty list.

But when "for cc in permutations([])" yields an empty list, why does 
"for cc in permutations("Z")" then actually have an item so that "yield 
[items[i]]+cc" will be executed?
So although "for cc in permutations("Z")" does have an item it does 
"yield [items[i]]+cc" - thats the confusion for me. I hope I could 
express myself clearly.

Maybe someone could enlighten me, I would be more than happy since I 
have been thinking for quite some time about this and it does not let me 
sleep, hehe.

thanks in advance,


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