Modulus of two float or double numbers using C

In C, to find the modulus (remainder) of two floating-point or double numbers, we use the remainder() function from the math library. Unlike the % operator which only works with integers, remainder() handles floating-point arithmetic.

Syntax

double remainder(double x, double y);
float remainderf(float x, float y);
long double remainderl(long double x, long double y);

Parameters

• x − The numerator (dividend)
• y − The denominator (divisor)

Return Value

Returns the floating-point remainder of x/y. The remainder is calculated as:

remainder(x, y) = x - rquote * y

where rquote is x/y rounded to the nearest integral value.

Example

Note: To use remainder() function, you need to link with the math library using -lm flag during compilation.

#include <stdio.h>
#include <math.h>

int main() {
    double x = 14.5, y = 4.1;
    double res = remainder(x, y);
    printf("Remainder of %.1lf/%.1lf is: %.1lf<br>", x, y, res);
    
    x = -34.50;
    y = 4.0;
    res = remainder(x, y);
    printf("Remainder of %.1lf/%.1lf is: %.1lf<br>", x, y, res);
    
    x = 7.5;
    y = 2.3;
    res = remainder(x, y);
    printf("Remainder of %.1lf/%.1lf is: %.1lf<br>", x, y, res);
    
    return 0;
}

Remainder of 14.5/4.1 is: -1.9
Remainder of -34.5/4.0 is: 1.5
Remainder of 7.5/2.3 is: 0.6

Key Points

• The remainder() function uses IEEE remainder, which can return negative values
• For division by zero, the function returns NaN (Not a Number)
• The result has the same sign as the numerator in some cases, depending on the rounding
• Use remainderf() for float and remainderl() for long double types

Conclusion

The remainder() function provides IEEE-compliant floating-point remainder calculation in C. It handles both positive and negative numbers correctly and is the standard way to find modulus of floating-point numbers.