Basic C++ problems

Bumping this thread cause I don't want to start a new thread.

I have this code:

Code:

#include <iostream>
#include <iomanip> // for fixed and setprecision
#include "assn.h" // for is_vowel function
using namespace std;

int main () {

int i = 0;
string word = "regal";
char ch = word.at(i);
int wordsize = word.size();
int syllables = 1;

while (i < wordsize) {
    if (is_vowel(ch)) {
    cout << ch << " is a vowel." << endl;
  
        if (i > 0) {
        syllables = syllables + 1;
        }
    }
    
    else {
    cout << ch << " is not a vowel." << endl;
    }
  
    ch = word.at(++i);
}

cout << "Syllables: " << syllables; //ignore this

return 0;

}

For references:

No, I cannot change:
#include <iostream>
#include <iomanip> // for fixed and setprecision
#include "assn.h" // for is_vowel function
using namespace std;

The grading program will check to see if I specifically have this header. It will not take anything else.

The error I get is:

std :: out of range.
what(): basic string at

I figured out that this is caused by i < wordsize, as if I have i < wordsize - 1, I do not get this error; however, it will only analyze "r e g a".
The is_vowel function is a function provided by my profs for this assignment. If needed, I can post the code for the function.

So, my question is, how do I prevent this error from happening while having my program analyze all the letters of any inputted word?