Meaning of the expression n %= 100

Question

I have studied C and came across a very difficult problem that after a lot of trying, I gave up and went to look for an algorithm ready on the internet to study on it, but the expression n %= 100 I could not understand at all. I searched several programming and math sites but all I could find was that the percentage symbol in the middle of two whole numbers returned the rest of the division, and this I already knew, but when there’s an equality together?

If it helps the problem is the 1018 of the URI beginner module.

Here’s the code I’m studying:

#include <stdio.h>
int main()
{
    int n;
    scanf("%d", &n);
    printf("%d\n", n);
    printf("%d nota(s) de R$ 100,00\n", n/100);
    n %= 100;
    printf("%d nota(s) de R$ 50,00\n", n/50);
    n %= 50;
    printf("%d nota(s) de R$ 20,00\n", n/20);
    n %= 20;
    printf("%d nota(s) de R$ 10,00\n", n/10);
    n %= 10;
    printf("%d nota(s) de R$ 5,00\n", n/5);
    n %= 5;
    printf("%d nota(s) de R$ 2,00\n", n/2);
    n %= 2;
    printf("%d nota(s) de R$ 1,00\n", n);
    return 0;
}

Answer 1

The operator % (module) returns the rest of the division between two integers. When we find an expression like x = 5 % 2; that means that x will receive the rest of the 5 for 2, that is to say, x will store the value 1. Mathematically, 5≡xmod2.

It is important to note that % only operates on whole numbers, so any attempt to use it with other type operands, float for example, will result in error.

Already the expression n %= 100; is a form of syntactic sugar equivalent to n = n % 100; and, among other forms, can be rewritten as:

aux = n % 100;
n = aux;

Some other examples of syntatic sugar in C-based languages are:

a += 5;    // a = a + 5
b -= 4;    // b = b - 4
c *= 3;    // c = c * 3
d /= 2;    // d = d / 2