Michael Spencer wrote:
> This returns an iterator that 'nests' an arbitrary number of sequences
> (odometer-style).
>
> def nest(*sequences):
> def _nest(outer, inner):
> for outer_item in outer:
> if not isinstance(outer_item, tuple):
> outer_item = (outer_item,)
> for inner_item in inner:
> yield outer_item + (inner_item,)
> return reduce(_nest, sequences)
Nice!
Here's a nonrecursive version. It creates a list of iterators that are
repeating their values just enough times to sychronize the nesting. I
wonder if 'ncycle' would be a useful generalization for itertools'
'cycle' function.
Anton
def ncycle(seq,n):
while True:
for x in seq:
for dummy in xrange(n):
yield x
def cross(*args):
p = 1
R = []
for arg in args[::-1]:
L = list(arg)
R.append(ncycle(L,p))
p *= len(L)
R.reverse()
for dummy in xrange(p):
yield [x.next() for x in R]
def test():
s1='a1','a2','a3','a4'
s2='b1','b2'
s3='c1','c2','c3'
for x in cross(s1,s2,s3):
print x
if __name__=='__main__':
test()
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