# Flipsign
> Return a [double-precision floating-point number][ieee754] with the magnitude of `x` and the sign of `x*y`.
## Usage
```javascript
var flipsign = require( '@stdlib/math/base/special/flipsign' );
```
#### flipsign( x, y )
Returns a [double-precision floating-point number][ieee754] with the magnitude of `x` and the sign of `x*y`; i.e., only return `-x` when `y` is a negative number.
```javascript
var z = flipsign( -3.14, 10.0 );
// returns -3.14
z = flipsign( -3.14, -1.0 );
// returns 3.14
z = flipsign( 1.0, -0.0 );
// returns -1.0
z = flipsign( -3.14, -0.0 );
// returns 3.14
z = flipsign( -0.0, 1.0 );
// returns -0.0
z = flipsign( 0.0, -1.0 );
// returns -0.0
```
## Notes
- According to the [IEEE 754][ieee754] 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`, else behavior may be indeterminate.
## Examples
```javascript
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 double-precision floating-point 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 );
}
```
[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985