help with modulus in C

[salomelamprecht]

Hi,
I'm struggling to understand the modulus operator and why it isn't returning what I expect it to return in an equation.

Using the below equation, when I have y down as 420 I get b = 2, however when compiling it within a programme b = 1. Please can someone explain to me why this happens?

b = (y % 25) / 10

Thank you,
Salome

[Salem]

No idea. I get 2.
If you're getting 1, post your test case.

$ cat foo.c
#include<stdio.h>

int main(void)
{
  int y = 420;
  int b = (y % 25) / 10;
  printf("b=%d\n",b);
  return 0;
}
$ gcc foo.c
$ ./a.out 
b=2

[salomelamprecht]

No idea. I get 2.
If you're getting 1, post your test case.

$ cat foo.c
#include<stdio.h>

int main(void)
{
  int y = 420;
  int b = (y % 25) / 10;
  printf("b=%d\n",b);
  return 0;
}
$ gcc foo.c
$ ./a.out 
b=2

my output wasn't exactly what I wanted so I have narrowed the problem down to 'b' so changed the code so that only 'b' would be printed rather than the actual answer to the problem. I just need to know why I'm getting 1 in my programme for 'b' rather than 2. Thank you for your help.

#include <cs50.h>
#include <stdio.h>


float get_positive_float(string prompt);
    
int main(void)
{
    float change_required = get_positive_float("How much change? ");
    int y = change_required * 100;
    int a, b, c, d, e;
        
    a = y / 25;
    b = (y % 25) / 10; 
    c = (y - (a * 25) - (b * 10)) / 5;
    d = y - (a * 25) - (b * 10) - (c * 5);
    e = a + b + c + d;
    
    if (y > 0)
    {
        printf("%d\n", b);
    }
}


float get_positive_float(string prompt)
{
    float change_required;
    do
    {
        change_required = get_float("%s", prompt);
    }
    while (change_required <= 0);
    return change_required;
}

[Salem]

$ cat foo.c
#include<stdio.h>

int main(void)
{
  int y = 419;  //!! NOT 420
  int b = (y % 25) / 10;
  printf("b=%d\n",b);
  return 0;
}
$ gcc foo.c
$ ./a.out 
b=1

> int y = change_required * 100;
Floating point numbers are pesky things at times.
You might think you typed in 4.2, and reasonably expect y to be 420.
But I bet you'll find it's really 4.199999999999999 and you have y=419 as a result.

[salomelamprecht]

Of course...thank you. So straight forward, but yet not!