# signbit
> Return a boolean indicating if the sign bit for a [double-precision floating-point number][ieee754] is on (true) or off (false).
## Usage
```javascript
var signbit = require( '@stdlib/number/float64/base/signbit' );
```
#### signbit( x )
Returns a `boolean` indicating if the sign bit for a [double-precision floating-point number][ieee754] is on (`true`) or off (`false`).
```javascript
var bool = signbit( 4.0 );
// returns false
bool = signbit( -9.14e-307 );
// returns true
bool = signbit( 0.0 );
// returns false
bool = signbit( -0.0 );
// returns true
```
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var signbit = require( '@stdlib/number/float64/base/signbit' );
var sign;
var x;
var i;
for ( i = 0; i < 100; i++ ) {
x = ( randu()*100.0 ) - 50.0;
sign = signbit( x );
sign = ( sign ) ? 'true' : 'false';
console.log( 'x: %d. signbit: %s.', x, sign );
}
```
* * *
## C APIs
### Usage
```c
#include "stdlib/number/float64/base/signbit.h"
```
#### stdlib_base_float64_signbit( x )
Returns an integer indicating whether the sign bit for a double-precision floating-point number is on (`1`) or off (`0`).
```c
#include
int8_t out = stdlib_base_float64_signbit( 3.14 );
```
The function accepts the following arguments:
- **x**: `[in] double` input value.
```c
int8_t stdlib_base_float64_signbit( const double x );
```
### Examples
```c
#include "stdlib/number/float64/base/signbit.h"
#include
#include
#include
int main( void ) {
double x[] = { 3.14, -3.14, 0.0, -0.0, 4.0, 1.0, -1.0, 1.0e308, -1.0e308 };
int8_t out;
int i;
for ( i = 0; i < 9; i++ ) {
stdlib_base_float64_signbit( x[ i ], &out );
printf( "%lf => signbit: %" PRId8 "\n", x[ i ], out );
}
}
```
[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985
[@stdlib/number/float32/base/signbit]: https://github.com/stdlib-js/number/tree/main/float32/base/signbit