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.

Answer 1

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.

Answer 2

One way to do this:

1. Find the first element of the pair your are looking for in the first list:

   p = (1, 4)
   i = a.index(p[0])
   

2. Find the second element of the pair your are looking for in the second list:

   j = b.index(p[1])
   

3. Compute the index in the product list:

   k = i * len(b) + j