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 |