# flipsign

Return a double-precision floating-point number with the magnitude of `x` and the sign of `x*y`.

## Usage

``````var flipsign = require( '@stdlib/math/base/special/flipsign' );
``````

#### flipsign( x, y )

Returns a double-precision floating-point number with the magnitude of `x` and the sign of `x*y` (i.e., only return `-x` when `y` is a negative number).

``````var z = flipsign( -3.0, 10.0 );
// returns -3.0

z = flipsign( -3.0, -1.0 );
// returns 3.0

z = flipsign( 1.0, -0.0 );
// returns -1.0

z = flipsign( -3.0, -0.0 );
// returns 3.0

z = flipsign( -0.0, 1.0 );
// returns -0.0

z = flipsign( 0.0, -1.0 );
// returns -0.0
``````

## Notes

• According to the IEEE 754 standard, a `NaN` has a biased exponent equal to `2047`, a significand greater than `0`, and a sign bit equal to either `1` or `0`. In which case, `NaN` may not correspond to just one but many binary representations. Accordingly, care should be taken to ensure that `y` is not `NaN`; otherwise, behavior may be indeterminate.

## Examples

``````var randu = require( '@stdlib/random/base/randu' );
var flipsign = require( '@stdlib/math/base/special/flipsign' );

var x;
var y;
var z;
var i;

// Generate random numbers `x` and `y` and flip the sign of `x` only if `y` is negative...
for ( i = 0; i < 100; i++ ) {
x = (randu()*100.0) - 50.0;
y = (randu()*10.0) - 5.0;
z = flipsign( x, y );
console.log( 'x: %d, y: %d => %d', x, y, z );
}
``````

## C APIs

### Usage

``````#include "stdlib/math/base/special/flipsign.h"
``````

#### stdlib_base_flipsign( x, y )

Returns a double-precision floating-point number with the magnitude of `x` and the sign of `x*y`.

``````double v = stdlib_base_flipsign( -3.0, 10.0 );
// returns -3.0

double v = stdlib_base_flipsign( -3.0, -1.0 );
// returns 3.0
``````

The function accepts the following arguments:

• x: `[in] double` number from which to derive a magnitude.
• y: `[in] double` number from which to derive a sign.
``````double stdlib_base_flipsign( const double x, const double y );
``````

### Examples

``````#include "stdlib/math/base/special/flipsign.h"
#include <stdio.h>

int main() {
double x[] = { 3.0, -3.0, 0.0, 0.0/0.0 };

double y;
int i;
for ( i = 0; i < 4; i++ ) {
y = stdlib_base_flipsign( x[ i ], -3.0 );
printf( "flipsign(%lf, %lf) = %lf\n", x[ i ], -3.0, y );
}
}
``````