C cos() library function

The function double cos(double x); returns the cosine of x, expressed in radians.

Function prototype of cos

double cos(double x);
  • x : A floating point value representing an angle in radians.

Return value of cos

It returns the cosine of x.

C program using cos function

The following program shows the use of cos function to calculate cosine of an angle.

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

#define PI 3.14159

int main(){
    double value, radian, degree;
    printf("Enter an angle in degree\n");
    scanf("%lf", &degree);
     *  Degree to radian conversion
     *  One radian is equal to 180/PI degrees.
    radian = degree * (PI/180.0);
    value = cos(radian);
    printf("The cosine of %lf is %0.4lf\n", degree, value);
    return 0;

Enter an angle in degree
The cosine of 45.0000 is 0.7071
Enter an angle in degree
The cosine of 180.0000 is -1.0000