atan : <math.h> library function

The function double atan(double x); returns the arc tangent of x, expressed in radians. Arc tangent is the inverse operation of tangent.

Function prototype of atan

double atan(double x);
  • x : A floating point value whose arc tangent(inverse tangent) to be computed, in the interval [-INFINITY, INFINITY].

Return value of atan

Function atan returns the principal arc tangent of x, in the interval [-Pi/2, +Pi/2] radians.

C program to show the use of atan function

The following program shows the use of atan function to calculate inverse of tangent.

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

#define PI 3.14159

int main(){
    double input, radian, degree;
    /* Range of tan function is -INFINITY to +INFINITY */
    printf("Enter a number between -INFINITY to +INFINITY\n");
    scanf("%lf", &input);
    
    radian = atan(input);
    /* 
     *  Radian to degree conversion
     *  One radian is equal to 180/PI degrees.
     */
    degree = radian * (180.0/PI);
    
    printf("The arc tan of %0.4lf is %0.4lf in radian\n",
        input, radian);
    printf("The arc tan of %0.4lf is %0.4lf in degree\n",
        input, degree);
        
    return 0;
}

Program Output
Enter a number between -INFINITY to +INFINITY
1.0
The arc tan of 1.0000 is 0.7854 in radian
The arc tan of 1.0000 is 45.0000 in degree
Enter a number between -INFINITY to +INFINITY
9999.0
The arc tan of 9999.0000 is 1.5707 in radian
The arc tan of 9999.0000 is 89.9943 in degree