Python for loop multi-dimensional array indexing issue

Question

I have the first two strings in a string list strs that is guaranteed to have length of at least 2. I want to compare their letters against each other, and perform a task when their letters are identical. Here is the code I am using:

 iter_len = len(strs[1])
 if (len(strs[0]) >= len(strs[1])):
 iter_len = len(strs[0])
 
 for i in range (0, iter_len, 1):
 if (strs[0][i] == strs[1][i]):
 [do a thing]

However, when I run this, I get a IndexError: string index out of range for the if (strs[0][i] == strs[1][i]): line. I don't quite understand why this is happening, as the first if statement should ensure that iter_len is the minimum length between strs[0] and strs[1], and should prevent the index from going over. Any advice is much appreciated.

Answer 1

If you need to iterate through two sequences in lockstep, it's generally best practice to use zip:

for a,b in zip(strs[0], strs[1]):
 if a == b:
 #[do a thing]

This will deal with the issue of one sequence being longer than the other.

If you need the index, you can use enumerate with your zip:

for i, (a, b) in enumerate(zip(strs[0], strs[1])):
 if a == b:
 #[do a thing with i]