Compute the arctangent, determining the quadrant
#include <math.h>
double atan2( double y,
double x );
float atan2f( float y,
float x );
long double atan2l( long double y,
long double x );
These functions compute the value of the arctangent (in radians) of y/x, using the signs of both arguments to determine the quadrant of the return value. No errors occur if both arguments are zero, or if y is zero.
To check for error situations, use feclearexcept() and fetestexcept(). For example:
The arctangent of y/x, in the range [-π, π].
| If y is: | And x is: | These functions return: | Errors: |
|---|---|---|---|
| ±0.0 | Negative or -0.0 | π, with the same sign as y (see below) | — |
| ±0.0 | Positive or +0.0 | 0.0 (see below) | — |
| ±Inf | Finite | π/2, with the same sign as y | — |
| ±Inf | -Inf | 3π/4, with the same sign as y | — |
| ±Inf | Inf | π/4, with the same sign as y | — |
| Negative | ±0.0 | -π/2 | — |
| Positive | ±0.0 | +π/2 | — |
| Finite and positive | -Inf | +π | — |
| Finite and negative | -Inf | -π | — |
| Finite and positive | Inf | 0.0 | — |
| Finite and negative | Inf | -0.0 | — |
| Any value | NaN | x | — |
| NaN | Any value | y | — |
These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.
#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>
int main( void )
{
int except_flags;
feclearexcept(FE_ALL_EXCEPT);
printf( "%f\n", atan2( .5, 1. ) );
except_flags = fetestexcept(FE_ALL_EXCEPT);
if(except_flags) {
/* An error occurred; handle it appropriately. */
}
return EXIT_SUCCESS;
}
produces the output:
0.463648
| Safety: | |
|---|---|
| Cancellation point | No |
| Interrupt handler | No |
| Signal handler | No |
| Thread | Yes |