lizaveta - 1 year ago 68

Python Question

I have all pair combinations from some list:

`a = [1,2,3,4,5,6]`

pairs = [pair for pair in itertools.combinations(a,2)]

print pairs

>>>[(1, 2), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), (2, 5), (3, 4), (3, 5), (4, 5)]

How can I know what exactly pair I work with if I know only its index?

Maybe there is some formula? The problem is that my list is too big and I can't just keep in mind all pairs. I need only some particular pairs which I can recognise by index.

Any suggestions?

Thanks!

Answer Source

Here's a way to generate a pair directly from its index. There's probably a more efficient equation for this, but this is what I came up with after a couple of minutes. :)

```
import itertools
def pair_from_index(a, i):
m = n = len(a) - 1
while i >= n:
i -= n
n -= 1
m -= n
return a[m], a[m + i + 1]
# test
a = list('abcdefg')
for i, t in enumerate(itertools.combinations(a, 2)):
print(i, t, pair_from_index(a, i))
```

**output**

```
0 ('a', 'b') ('a', 'b')
1 ('a', 'c') ('a', 'c')
2 ('a', 'd') ('a', 'd')
3 ('a', 'e') ('a', 'e')
4 ('a', 'f') ('a', 'f')
5 ('a', 'g') ('a', 'g')
6 ('b', 'c') ('b', 'c')
7 ('b', 'd') ('b', 'd')
8 ('b', 'e') ('b', 'e')
9 ('b', 'f') ('b', 'f')
10 ('b', 'g') ('b', 'g')
11 ('c', 'd') ('c', 'd')
12 ('c', 'e') ('c', 'e')
13 ('c', 'f') ('c', 'f')
14 ('c', 'g') ('c', 'g')
15 ('d', 'e') ('d', 'e')
16 ('d', 'f') ('d', 'f')
17 ('d', 'g') ('d', 'g')
18 ('e', 'f') ('e', 'f')
19 ('e', 'g') ('e', 'g')
20 ('f', 'g') ('f', 'g')
```

**Update** This function isn't as fast as I thought: I made a silly mistake in my test. Oops! But at least it *does* work properly. :)