atan2 : <math.h> library function

The function double atan2(double y, double x); returns the arc tangent of y/x, expressed in radians. Function atan2 takes into account the sign of both arguments in order to determine the quadrant.

Function prototype of atan2

double atan2(double y, double x);
  • y : A floating point value representing an Y-coordinate.
  • x : A floating point value representing an X-coordinate.

Return value of atan2

Function atan2 returns the principal arc tangent of y/x, in the interval [-Pi, +Pi] radians.

C program to show the use of atan2 function

The following program shows the use of atan2 function to calculate inverse tangent of y/x.

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

#define PI 3.14159

int main(){
    double Y, X, radian, degree;
    printf("Enter value of Y and X\n");
    scanf("%lf %lf", &Y, &X);
    
    radian = atan2(Y, X);
    /* 
     *  Radian to degree conversion
     *  One radian is equal to 180/PI degrees.
     */
    degree = radian * (180.0/PI);
    
    printf("The arc tan2 of %0.4lf and %0.4lf is %0.4lf radian\n",
        Y, X, radian);
    printf("The arc tan2 of %0.4lf and %0.4lf is %0.4lf degree\n",
        Y, X, degree);
        
    return 0;
}

Program Output
Enter value of Y and X
5 5
The arc tan2 of 5.0000 and 5.0000 is 0.7854 in radian
The arc tan2 of 5.0000 and 5.0000 is 45.0000 in degree