Why does hard-coding in numbers work, but using variables with the same values, doesn't?-CodePudding

I'm writing a program that is a game where you have to guess whichever random 4 digit integer that the computer generates. I'm having trouble with the input validation - more specifically, my isDigit() method, which aims to make sure all inputs from userGuess are digits 0 - 9:

    private static boolean isDigit(char indexPos) { // verifies if the input char is a digit 0-9 or not
        if(indexPos == 0 || indexPos == 1 || indexPos == 2 || indexPos == 3 || indexPos == 4 || indexPos == 5 || indexPos == 6 || indexPos == 7 || indexPos == 8 || indexPos == 9) {
            return true;
        }
        else {
            return false;
        }
    }

If I hardcode in things like

isDigit(1)

it evaluates as true, but if I plug in variables, it returns false. Mainly, this returns false, even when all index positions of userGuess are digits 0-9:

isDigit(userGuess.charAt(i)) // returns false even when index pos i is [0-9]

I'm utterly confused and stumped as to why this happens. For context, here's the whole if and for loop situation for the validation:

numGuess  ;
System.out.print("Guess "   numGuess   ": ");
userGuess = kbd.nextLine();
System.out.println("DEBUG: userGuess.length() : "   userGuess.length());

// validate that the input is a valid input (a 4 digit natural number, incl 0000)
if(userGuess.length() < 4 || userGuess.length() >= 5){
   System.out.println("Please enter a valid input, which is a 4 digit natural number (postive integer, including 0000).");
}
else if(userGuess.length() == 4) {
   for(int i = 0;  i <= 3; i  ) {
     if(isDigit(userGuess.charAt(i)) == true) { 
        validateCount  ;
     }
}

I also have, what I assume to be the same issue, on validating if userGuess.length() == 4. If I output userGuess.legnth() on the console, it provides the correct number, and when == 4, this if loop doesn't run, even though the condition is met:

if (userGuess.length() < 4 || userGuess.length() >= 5) {
   System.out.println("Please enter a valid input, which is a 4 digit natural number (postive integer, including 0000).");
} else if (userGuess.length() == 4) { // avoid out of bounds errors for inputs less than 4 digits
   for (int i = 0; i <= 3; i  ) { // userGuess will never have an index above 3, as it is a 4 digit number
if (isDigit(userGuess.charAt(i)) == true && userGuess.length() == 4) { // verify that the userInput is a 4 digit integer
    validateCount  ;
}

CodePudding user response：

You are comparing the char indexPos with the integers from 0 to 9. Try comparing indexPos with '0','1','2',... That way you compared if the char indexPos with another char.

CodePudding user response：

If you are really working with characters, then you could do this.

private static boolean isDigit(char indexPos) { 
    return Character.isDigit(indexPos);
}

But then, you don't really need a new method. In any event I would suggest you work with actual ints as the argument name indexPos suggests.

CodePudding user response：

Just pass an integer instead of a char and it will work.

private static boolean isDigit(int indexPos) {
    if(indexPos == 0 || indexPos == 1 || indexPos == 2 || indexPos == 3 || indexPos == 4 || indexPos == 5 || indexPos == 6 || indexPos == 7 || indexPos == 8 || indexPos == 9) {
        return true;
    }
    else {
        return false;
    }
}

You also can write the method a lot shorter:

private static boolean isDigit(int indexPos) {
   return indexPos>=0 && indexPos<=9;
}