Question

I’ve written a small piece of code that should detect if there are any matching characters in the same place in the 2 strings. If there is , the score in incremented by 1, if there is 2 or more consecutive matching characters , the score is incremented by 3, if there is no matching character, the score is decremented by 1.
The problem is though , when i try to run the code, it gives me a error: string index out of range.
What might be wrong ? thank you very much.

def pairwiseScore(seqA, seqB):
    count = 0
    score = 0
    while count < len(seqA):
        if seqA[count] == seqB[count]:
            score = score + 1
            count = count + 1
            while seqA[count] == seqB[count]:  # This is the line the error occurs
                score = score + 3
                count = count + 1 
        elif seqA[count] != seqB[count]:
            score = score - 1
            count = count + 1
    return score

Answer 1

Indexes are numberd 0 to n.

len(someString)

will give you n + 1.

Let’s say that your string is length 10, and the indexes are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Your while loop checks the condition that count is < 10. So far so good.

Ok now let’s say that count is equal to 9. Immediately within the first while loop, you increment count.

So now count = 10.

Now attempting to access someString[count] will give you an IndexError because the indices only go up to 9.