Warning when comparing integers of different signs

Question

I've two very similar functions in 2 programs as follows.

/*
 * Function: isalphaString
 * Usage: isalphaString(str);
 * ----------------------------
 * Judge if all characters in the string is alpha.
 */
bool isalphaString(string str) {
    for (int i = 0; i < str.size(); i++) {
        if (! isalpha(str[i]))
            return false;
    }
    return true;
}

the second function:

bool containsNonAlpha(string boardText) {
    for (int i = 0; i < boardText.size(); i++) {
        if (!isalpha(boardText[i])) {
            return true;
        }
    }
    return false;
}

but get two different compiling results. The first one I have no compiling warnings, but the second one I received a warning: "comparison of integers of different signs" . Not sure why this happens.

Answer 1

You can't compare an int with std::string::size_type, you need to change this line:

for (int i = 0; i < boardText.size(); i++)

to:

for (string::size_type i = 0; i < boardText.size(); i++)

altought you can use a range-loop in C++11 that is much simpler:

for(char &c : boardText) {
    check_if_alpha(c);
}