C asin()

The asin() function takes a single argument (1 ≥ x ≥ -1), and returns the arc sine in radians. Mathematically, asin(x) = sin-1(x).

The asin() function is included in <math.h> header file.

asin() Prototype

double asin(double x);

To find arc sine of type int, float or long double, you can explicitly convert the type to double using cast operator.

 int x = 0;
 double result;
 result = asin(double(x));

Also, two functions asinf() and asinl() were introduced in C99 to work specifically with type float and long double respectively.

float asinf(float x);
long double asinl(long double x);

asin() Parameter

The asin() function takes a single argument in the range of [-1, +1]. It's because the value of sine is in the range of 1 and -1.

asin() Return Value

The asin() functions returns the value in range of [-π/2, +π/2] in radians. If the parameter passed to the asin() function is less than -1 or greater than 1, the function returns NaN (not a number).

Example 1: asin() function with different parameters

#include <stdio.h>
#include <math.h>
int main()
{
    // constant PI is defined
    const double PI =  3.1415926;
    double x, result;

    x =  -0.5;
    result = asin(x);
    printf("Inverse of sin(%.2f) = %.2lf in radians\n", x, result);

    // converting radians to degree
    result = asin(x)*180/PI;
    printf("Inverse of sin(%.2f) = %.2lf in degrees\n", x, result);

    // paramter not in range
    x = 1.2;
    result = asin(x);
    printf("Inverse of sin(%.2f) = %.2lf", x, result);

    return 0;
}

Output

Inverse of sin(-0.50) = -0.52 in radians
Inverse of sin(-0.50) = -30.00 in degrees
Inverse of sin(1.20) = nan

Example 2: asinf() and asinl() function

#include <stdio.h>
#include <math.h>
int main()
{
    float fx, fasinx;
    long double lx, ldasinx;

    // arc sinine of type float
    fx = -0.505405;
    fasinx = asinf(fx);

    // arc sinine of type long double
    lx = -0.50540593;
    ldasinx = asinf(lx);

    printf("asinf(x) = %f in radians\n", fasinx);
    printf("asinl(x) = %Lf in radians", ldasinx);

    return 0;
}

Output

asinf(x) = -0.529851 in radians
asinl(x) = -0.529852 in radians