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# flipsign

> Return a [double-precision floating-point number][ieee754] with the magnitude of `x` and the sign of `x*y`.

<section class="usage">

## 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( -3.0, 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
```

</section>

<!-- /.usage -->

<section class="notes">

## 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`; otherwise, behavior may be indeterminate.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var uniform = require( '@stdlib/random/array/uniform' );
var logEachMap = require( '@stdlib/console/log-each-map' );
var flipsign = require( '@stdlib/math/base/special/flipsign' );

var opts = {
    'dtype': 'float64'
};
var x = uniform( 100, -50.0, 50.0, opts );
var y = uniform( 100, -5.0, 5.0, opts );

// Generate random numbers `x` and `y` and flip the sign of `x` only if `y` is negative...
logEachMap( 'x: %0.4f, y: %0.4f => %0.4f', x, y, flipsign );
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

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

#### stdlib_base_flipsign( x, y )

Returns a [double-precision floating-point number][ieee754] with the magnitude of `x` and the sign of `x*y`.

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

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.

```c
double stdlib_base_flipsign( const double x, const double y );
```

</section>

<!-- /.usage -->

<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

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

int main( void ) {
    const 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 );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/math/base/special/copysign`][@stdlib/math/base/special/copysign]</span><span class="delimiter">: </span><span class="description">return a double-precision floating-point number with the magnitude of x and the sign of y.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="links">

[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985

<!-- <related-links> -->

[@stdlib/math/base/special/copysign]: https://github.com/stdlib-js/math/tree/main/base/special/copysign

<!-- </related-links> -->

</section>

<!-- /.links -->
