modulo returning wrong value in Greedy

Question

The following code in my Greedy program (getting float from user simplified for forum purposes!):

change = GetFloat();
// convert to int
int cInt = round(change*100);

// define arraw with values of coins
int coinTypes[4] = {25, 10, 5, 1};
int coinTypeAmt = sizeof(coinTypes)/sizeof(int);

for (int i = 0; i < coinTypeAmt; i++)
{
    int remainder = cInt % coinTypes[i];
    printf("  remainder: %d\n", remainder);
}

Why will remainder be always wrong for the first value (25) if I input a value smaller than 0.25 cents in the prompt?

0.15 for example will give me a remainder of 15
0.03 - remainder will be 3
0.02 - remainder will be 2

Now that I write this, I realize the remainder is always the same as cInt if cInt is smaller than 25.

Any other values seem to work fine...

Why is that??

Thanks! Best, Daniel

Answer 1

That's how the remainder operator just works. You may think of it that way:

given a = 3 and b = 2, the expression

a % b // results in 1

because a 3 can have a maximum of a single (i.e., one) 2 in it. Now what's the remainder? It's the difference between 3 and (2 x 1) and that's definitely 1.

Now let a = 2 and b = 3, the expression

2 % 3 // results in 2

because, in the same sense, 2 has zero 3s in it. What's the remainder? It's the difference between 2 and (3 x 0) and that's definitely 2.

You may play with other values as well, here are a couple more examples

let a = 9 and b = 2

9 % 2 // results in 1

because 9 has a maximum of four twos (i.e., 2 x 4) and the remainder is 9 - (2 x 4) = 1

let a = 2 and b = 9

2 % 9 // results in 2

because 2 has zero nines (i.e., 0 x 9) and the remainder is 2 - (0 x 9) = 2.