Why we use -'0' beside charAt(i)?

Question

I read Hello Android book and i dont understand a piece of code this function.

Why we use -'0' beside charAt(i)??

static protected int[] fromPuzzleString(String string) {
    int[] puz = new int[string.length()];
    for (int i = 0; i < puz.length; i++) {
        puz[i] = string.charAt(i) - '0' ;
    }
    return puz; 
 }

Thanks you. Cheers.

Answer 1

Because string.charAt(i) returns char. But puz is int[].

So string.charAt(i) - '0'converts it to an integer.

Answer 2

What you're trying to do is read a char, one at a time, convert it to an int and store it in your array.

The ASCII code for 0 is 48. So what you're actually trying to do is subtract this value from the ASCII value of the char at that particular location and then store the numeric result in the corresponding index.

You can verify it for yourself, if you try to print it out in your loop, something like:

System.out.println((int)string.charAt(i));
System.out.println((int)'0');

Answer 3

You're better off using Character.isDigit(char) and Character.getNumericValue(char) to test and convert a char into an integer value.