I got error saying IndexError: list index out of range. how do I fix this?

Question

   res = [3, 1, 1, 5, 2, 4, 2, 4, 2, 4, 3, 1, 1, 5, 3]      

   while not i>(len(res)-1):
        if res[i]==res[i+1]:
            answer+=2
            i+=2
        else:
            i+=1

The variable "answer" is supposed to count duplicated numbers that are placed next to each other. For some reason, I get the error saying IndexError: list index out of range. How do I fix this?

Answer 1

How about giving it this approach?

res = [3, 1, 1, 5, 2, 4, 2, 4, 2, 4, 3, 1, 1, 5, 3]
answer = 0
start = 0
while start < len(res):
    if start + 1 < len(res):
        if res[start] == res[start + 1]:
            answer += 1
            start += 2
        else:
            start += 1
    else:
        start += 1
print(answer)