Counting the number of vowels in a string

Question

I am writing a function that returns the number of vowels for a given string, here is the code:

int isVowel(string sequence)
{   
    int numberOfVowels = 0;         //Initialize number of vowels to zero
    string vowels = "aeiouAEIOU";   //Possible vowels, including capitals
    for (int i = 0; i <= sequence.length(); i++)
    {
        for (int j = 0; j <= vowels.length(); j++)
        {
            if (sequence[i] == vowels[j])
            {
                numberOfVowels += 1;
            }
        }
    }
    return numberOfVowels;
}

This returns answers that are off by one. For example, input of "a" returns 2, input of "aa" returns 3, etc.

Answer 1

i <= sequence.length()

<= is almost never correct in any for loop since C++ uses 0-based indices. Instead, you should do

i < sequence.length()