Problem with python nested loops

[TheHimself]

I've written a code like


for i in A:
for j in B
[Do something]


But surprisingly, the inner loop is run only once (for the "first" element of A). I see this by putting print commands. Both A, and B are sets given by itertools.product. It seems that the inner loop somehow empties the set B!

[Bachstelze]

Are we supposed to guess what the code actually does?

[TheHimself]

Here is what I've done so far.


def diffs(n, n_out):
if( n<2 or n_out > n): return

n_in=n-n_out
iindex_set= itertools.product(set([-1,1]), repeat=n_in)
oindex_set= itertools.product(set([-1,1]), repeat=n_out)
m=4-n

diff={}
for I in iindex_set:
print "I=", I
for J in oindex_set:
diff[(I,J)]=0
print "J=", J
if sum(I)-sum(J)!=m:
continue
polygon=[]
print ("Polygon=")
for i,elem in enumerate(I):
polygon.append(i+1)
print i+1
for j,elem in enumerate(J):
polygon.append(n_out-j+i+1)
print n_out-j+i+1

[Bachstelze]

Iterators are not lists. When you have reached the end of an iterator, you can't "go back".


firas@av104151 ~ % cat iter.py
import itertools

iter = itertools.product([-1, 1], repeat=3)
print "pass 1"
for i in iter:
print i
print "pass 2"
for i in iter:
print i
firas@av104151 ~ % python iter.py
pass 1
(-1, -1, -1)
(-1, -1, 1)
(-1, 1, -1)
(-1, 1, 1)
(1, -1, -1)
(1, -1, 1)
(1, 1, -1)
(1, 1, 1)
pass 2

[Bachstelze]

So just make lists out of your iterators:


firas@av104151 ~ % cat iter.py
import itertools

iter = list(itertools.product([-1, 1], repeat=3))
print "pass 1"
for i in iter:
print i
print "pass 2"
for i in iter:
print i
firas@av104151 ~ % python iter.py
pass 1
(-1, -1, -1)
(-1, -1, 1)
(-1, 1, -1)
(-1, 1, 1)
(1, -1, -1)
(1, -1, 1)
(1, 1, -1)
(1, 1, 1)
pass 2
(-1, -1, -1)
(-1, -1, 1)
(-1, 1, -1)
(-1, 1, 1)
(1, -1, -1)
(1, -1, 1)
(1, 1, -1)
(1, 1, 1)

[TheHimself]

Thanks it worked!! :guitar: