C in a Nutshell

Name

acos

Synopsis

Calculates the inverse cosine of a number

#include <math.h>
doubleacos( double x );
float acosf( float x );        (C99)
long double acosl( long double x );        (C99)

acos() implements the inverse cosine function, commonly called arc cosine. The argument x must be between −1 and 1, inclusive: −1 ≤ x ≤ 1. If x is outside the function’s domain—that is, greater than 1 or less than −1—the function incurs a domain error.

The return value is given in radians, and is thus in the range 0 ≤ acos(x) ≤ π.

Example

/*
 * Calculate the pitch of a roof given
 * the sloping width from eaves to ridge and
 * the horizontal width of the floor below it.
 */
#define PI 3.141593
#define DEG_PER_RAD (180.0/PI)

double floor_width = 30.0;
double roof_width = 34.6;

double roof_pitch =acos( floor_width / roof_width ) * DEG_PER_RAD ;
printf( "The pitch of the roof is %2.0f degrees.\n", roof_pitch );

This code produces the following output:

The pitch of the roof is 30 degrees.

See Also

The arc cosine functions for complex numbers: cacos(), cacosf(), and cacosl()