Return the minimum number from the given array using while loop Java

Hi guys. How are you? =)

I’m new to Java and currently

I have a task to create a method, it takes one parameter sum – the amount of money to be given out, and returns the minimum number of banknotes that can be given out this amount.

Only While loop can be used.

I made it with for loop, but I can’t find where I made mistake in while loop.
Could you please give me hint or advice?
Thank you!

public class ATM {

    public static int countBanknotes(int sum) {
        int[] notes = new int[] { 500, 200, 100, 50, 20, 10, 5, 2, 1 };
        int[] noteCounter = new int[9];
        int amount = 0;

        for (int i = 0; i < 9; i++) {
            if (sum >= notes[i]) {
                noteCounter[i] = sum / notes[i];
                sum -= noteCounter[i] * notes[i];
            }
        }
        for (int i = 0; i < 9; i++) {
            if (noteCounter[i] != 0) {
                amount += noteCounter[i];
            }
        }
        return amount;
    }

    public static void main(String[] args) {

        // 6 (500 + 50 + 20 + 5 + 2 + 1
        int sum = 578;
        System.out.print(ATM.countBanknotes(sum));
    }
}

And while loop

public class JustATest {

    public static int countBanknotes(int sum) {
        int[] notes = new int[] { 500, 200, 100, 50, 20, 10, 5, 2, 1 }; 
        int[] noteCounter = new int[9]; 
        int amount = 0; 
        int i = 0;  
        
        while ( i < 9 ) {
            if (sum >= notes[i]) {
                i++;
                noteCounter[i] = sum / notes[i];
                sum -= noteCounter[i] * notes[i];           
            }
        }
        while ( i < 9 ) {           
            if (noteCounter[i] != 0) {
                i++;
                amount += noteCounter[i];           
            }
        }
        return amount;
    }

    public static void main(String[] args) {

        // Should be 6 (500 + 50 + 20 + 5 + 2 + 1)
        int sum = 578;
        System.out.print(JustATest.countBanknotes(sum));
    }
}

>Solution :

You need to reinitialize your i variable between the loops, or use another variable, like j.

Furthermore, you should not have your i++ inside the if statements in your while loops. Otherwise, there are scenarios where the counter is never incremented and you will have an endless loop. That is bad!
Put your i++ in the bottom of the while loop outside the if statement like this:

while ( i < 9 ) {
    if (sum >= notes[i]) {
        noteCounter[i] = sum / notes[i];
        sum -= noteCounter[i] * notes[i];           
    }
    i++;
}
int j = 0;
while ( j < 9 ) {           
    if (noteCounter[j] != 0) {
        amount += noteCounter[j];           
    }
    j++;
}

This way, the counters are always incremented no matter what, and you will not have endless loops.
I included a fix for your problem in the code above as well.