Python loop through list levels

Question

I am trying to loop through and merge of two lists that look like this:

T1 = [[['a',  1], ['c', 3]], [['e',  5], ['g', 7]], [['i',  9],['j', 11]]]
T2 = [[['m',  1], ['n', 5]], [['q',  7], ['r', 7]], [['t',  9],['u', 11]]]

So for example, T1[0] = [['a', 1], ['c', 3]] T2[0] = [['m', 1], ['n', 5]]

I am writing what is essentially a merge algorithm, where I compare the second element in each list in T1[0][0] (the 1), with the second value in T2[0][0] (so 1 again). Essentially what I want is T1[0][0][0] and T2[0][0][0]. If they match, I merge the two values into a tuple {'a', 1, 'm'} (can also be a list if it makes things easier).

My confusion is stemming from the fact that I don't know how to best loop through T1 and T2 without running it 3 times: (T1[0], T2[0]), (T1[1], T2[1]), (T1[2], T2[2])

Is there a way to elegantly loop through T1 and T2 so that I can go through each sublist in T1 and T2 and perform the check and merge?

Note: the data is already sorted, and I am not looking to use libraries. I want to know if I can do this myself.

##Update## So I know it is a for loop, however the below code is as far as i can get, and it is throwing me a list index out of range as shown below

result = []
i = j = 0

while True:
    for s_T1 in T1:
        for s_T2 in T2:
            if s_T1[i][1] < s_T2[j][1]: ### error at this line
                i = i + 1

            elif  s_T1[i][1] > s_T2[j][1]:
                j = j + 1

            elif s_T1[i][1] == s_T2[j][1]:
                result.append(s_T1[i][0], str(s_T1[i][0), s_T2[j][0])
                i = i + 1
                j = j + 1
                
            if i == len(T1) or j == len(T2):
                break

return result

What the code should return is:

[[['a', 1, 'm']], [['g', 7, 'q'], ['g', 7, 'r']], [['i', 9, 't'], ['j', 11, 'u']]]

So, look at the integer in ['e', 5] in T1, compare it to ['q', 7] in T2, since 5 is smaller than 7, move to the next value in T1. if they match, append. And the other way around - if the value in T1 is greater than the value in T2, move to the next value in T1.

Answer 1

zip ties together multiple iterables so you can iterate through them in lock-step, which is what it seems you're looking for:

>>> result = []
>>> for i, j in zip(T1, T2):
...   sub = []
...   for x, y in zip(i, j):
...     if x[1] == y[1]:
...       sub.append((x[0], x[1], y[0]))
...   if sub:
...     result.append(sub)
...
>>> result
[[('a', 1, 'm')], [('g', 7, 'r')], [('i', 9, 't'), ('j', 11, 'u')]]

Answer 2

This should work:

def task(T1,T2):
    result = []
    for i in range(len(T1)):
        for k in range(2):
            for l in range(2):
                if T1[i][k][1]==T2[i][l][1]:
                    result.append([T1[i][k][0], T1[i][k][1],T2[i][l][0]]) 
    return result

task(T1,T2)

When run it results to:

[['a', 1, 'm'], ['g', 7, 'q'], ['g', 7, 'r'], ['i', 9, 't'], ['j', 11, 'u']]

as indicated in your question