¬ just serendipity 🍀 (not just serendipity)

Given a list v = [1, 2, 3, 4], use a double nested¹ loop to iterate over every pair of its elements, in order.

In other words, the iteration must yield the following deterministic order:

• 1, 2
• 1, 3
• 1, 4
• 2, 3
• 2, 4
• 3, 4

C/C++ style #

Use indexes + range:

for i in range(len(v)):
    for j in range(i + 1, len(v)):
        print(v[i], v[j])

1 2
1 3
1 4
2 3
2 4
3 4

C/C++ style, but iterating directly #

We could use enumerate:

for i, a in enumerate(v):
    for b in v[i + 1:]:
        print(a,b)

It is not an objectively superior improvement to the previous form, but at least we avoid range and len.

itertools #

for a, b in itertools.combinations(v, 2):
  print(a,b)

Discrete Mathematics combinations provides an elegant one-liner for this situation. The second argument controls how many elements to include per tuple, for comparison:

>>> list(itertools.combinations(v, 3))
[(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]