The Modulo Operator in C

1. Use % Modulo Operator to Calculate Remainder in Division in C
2. Use % Modulo Operator to Implement Leap Year Checking Function in C
3. Use % Modulo Operator to Generate Random Numbers in the Given Integer Range in C

This article will demonstrate multiple methods of how to use the modulo operator in C.

Use % Modulo Operator to Calculate Remainder in Division in C

Modulo % is one of the binary arithmetic operators in the C language. It produces the remainder after the division of two given numbers. Modulo operator can’t be applied to floating-point numbers like float or double. In the following example code, we showcase the simplest possible case for using the % operator, printing modulus 9 results of the given int array.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
  int arr[8] = {10, 24, 17, 35, 65, 89, 55, 77};

  for (int i = 0; i < 8; ++i) {
    printf("%d/%d yields the remainder of - %d\n", arr[i], 9, arr[i] % 9);
  }

  exit(EXIT_SUCCESS);
}

Output:

10/9 yields the remainder of - 1
24/9 yields the remainder of - 6
17/9 yields the remainder of - 8
35/9 yields the remainder of - 8
65/9 yields the remainder of - 2
89/9 yields the remainder of - 8
55/9 yields the remainder of - 1
77/9 yields the remainder of - 5

Use % Modulo Operator to Implement Leap Year Checking Function in C

Alternatively, we can use % operator to implement more complex functions. The next example code demonstrates isLeapYear boolean function that checks if the given year is a leap or not. Note that a year is considered a leap year if its value is divisible by 4 but is not divisible by 100. Additionally, if the year value is divisible by 400, it’s a leap year.

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

bool isLeapYear(int year) {
  if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
    return true;
  else
    return false;
}

int main(void) {
  uint year = 2021;
  isLeapYear(year) ? printf("%d is leap\n", year)
                   : printf("%d is not leap\n", year);

  exit(EXIT_SUCCESS);
}

Output:

2021 is not leap

Use % Modulo Operator to Generate Random Numbers in the Given Integer Range in C

Another useful feature of the modulo operator is to limit the upper floor of numbers during the random number generation process.

Suppose we have a function that generates the random integer. In that case, we can take the remainder of the division between the returned number and the value we need to be the maximum (defined as MAX macro in the following example).

Note that using srand and rand functions for random number generation is not a robust method, and applications requiring quality random numbers should employ other facilities.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define COUNT 10
#define MAX 100

int main(void) {
  srand(time(NULL));
  for (int i = 0; i < COUNT; i++) {
    printf("%d, ", rand() % MAX);
  }
  printf("\b\b  \n");

  exit(EXIT_SUCCESS);
}

Output:

3, 76, 74, 93, 51, 65, 76, 31, 61, 97