Iterating over multiple indices with i > j ( > k) in a pythonic way

Question

i need to iterate over a tuple of indices. all indices must be in the range [0, N) with the condition i > j. The toy example I present here deal

Answer 1

You can use product from itertools if you don't mind the inefficiency of throwing out most of the generated tuples. (The inefficiency gets worse as the repeat parameter increases.)

>>> from itertools import product
>>> for p in ((i,j) for (i,j) in product(range(5), repeat=2) if i > j):
...   print p
...
(1, 0)
(2, 0)
(2, 1)
(3, 0)
(3, 1)
(3, 2)
(4, 0)
(4, 1)
(4, 2)
(4, 3)
>>> for p in ((i,j,k) for (i,j,k) in product(range(5), repeat=3) if i > j > k):
...   print p
...
(2, 1, 0)
(3, 1, 0)
(3, 2, 0)
(3, 2, 1)
(4, 1, 0)
(4, 2, 0)
(4, 2, 1)
(4, 3, 0)
(4, 3, 1)
(4, 3, 2)

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Update: Instead of tuple unpacking, using indexing for the filter. This allows the code to be written a little more compactly. Only my_filter needs to be changed for tuples of varying sizes.

from itertools import product, ifilter
def my_filter(p):
    return p[0] > p[1] > p[2]

for p in ifilter(my_filter, product(...)):
    print p