python all possible pairs of 2 list elements, and getting the index of that pair

Question

let's say I have two lists:

a = list(1,2,3)
b = list(4,5,6)

So I can have 9 pairs of these list members:

(1,4)
(1,5)
(1,6)

(2,4)
(2,5)
(2,6)

(3,4)
(3,5)
(3,6)

Now, given two list members like above, can I find out the pair's index? Like (1,4) from above would be the 1st pair.

You called the `list()` constructor in the wrong way. It would be simpler to use `[1, 2, 3]` instead of calling `list()`. — Sven Marnach, Nov 23 '11 at 13:22
Hi. I thought list() is the same for lists as print() is for print (in python 2.x)? — , Nov 23 '11 at 13:23
It doesn't work this way. [`list()` expects only one argument.](http://ideone.com/576XG) — Sven Marnach, Nov 23 '11 at 13:26

score 57 · Accepted Answer · answered Nov 23 '11 at 13:22

57

And to complete the answer and stay in the example:

import itertools  

a = [1, 2, 3]
b = [4, 5, 6]
c = list(itertools.product(a, b))

idx = c.index((1,4))

But this will be the zero-based list index, so 0 instead of 1.

answered Nov 23 '11 at 13:22

wal-o-mat

hey. Is it possible without creating a list of combinations first? – Nov 23 '11 at 13:26
Just using the itertools function gives you a plain iterator. If course you could iterate over that then and wait until your desired tuple arrives, counting the iterations. Or you could just find out the index of the first value in the first list, and the index of the second value in the second list and simply start to calculate :) – wal-o-mat Nov 23 '11 at 13:29

score 9 · Answer 2 · answered Nov 23 '11 at 13:29

One way to do this:

Find the first element of the pair your are looking for in the first list:
```
p = (1, 4)
i = a.index(p[0])
```
Find the second element of the pair your are looking for in the second list:
```
j = b.index(p[1])
```
Compute the index in the product list:
```
k = i * len(b) + j
```

2 Answers2