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

Here's an approach with itertools.combinations to have a generic number of levels -

map(tuple,(N-1-np.array(list(combinations(range(N),M))))[::-1])

Or a bit twisted one with same method -

map(tuple,np.array(list(combinations(range(N-1,-1,-1),M)))[::-1])

, where N : number of elements and M : number of levels.

Sample run -

In [446]: N = 5
     ...: for i in range(N):
     ...:     for j in range(i):
     ...:         for k in range(j):  # Three levels here
     ...:             print(i, j, k)
     ...:             
(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)

In [447]: N = 5; M = 3

In [448]: map(tuple,(N-1-np.array(list(combinations(range(N),M))))[::-1])
Out[448]: 
[(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)]