# dabs
> Compute the [absolute value][@stdlib/math/base/special/abs] for each element in a double-precision floating-point strided array.
## Usage
```javascript
var dabs = require( '@stdlib/math/strided/special/dabs' );
```
#### dabs( N, x, strideX, y, strideY )
Computes the [absolute value][@stdlib/math/base/special/abs] for each element in a double-precision floating-point strided array `x` and assigns the results to elements in a double-precision floating-point strided array `y`.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
// Compute the absolute values in-place:
dabs( x.length, x, 1, x, 1 );
// x => [ 2.0, 1.0, 3.0, 5.0, 4.0, 0.0, 1.0, 3.0 ]
```
The function accepts the following arguments:
- **N**: number of indexed elements.
- **x**: input [`Float64Array`][@stdlib/array/float64].
- **strideX**: index increment for `x`.
- **y**: output [`Float64Array`][@stdlib/array/float64].
- **strideY**: index increment for `y`.
The `N` and `stride` parameters determine which elements in `x` and `y` are accessed at runtime. For example, to index every other value in `x` and to index the first `N` elements of `y` in reverse order,
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var floor = require( '@stdlib/math/base/special/floor' );
var x = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
var y = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );
var N = floor( x.length / 2 );
dabs( N, x, 2, y, -1 );
// y => [ 5.0, 3.0, 1.0, 0.0, 0.0, 0.0 ]
```
Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][@stdlib/array/float64] views.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var floor = require( '@stdlib/math/base/special/floor' );
// Initial arrays...
var x0 = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
var y0 = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );
// Create offset views...
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var y1 = new Float64Array( y0.buffer, y0.BYTES_PER_ELEMENT*3 ); // start at 4th element
var N = floor( x0.length / 2 );
dabs( N, x1, -2, y1, 1 );
// y0 => [ 0.0, 0.0, 0.0, 6.0, 4.0, 2.0 ]
```
#### dabs.ndarray( N, x, strideX, offsetX, y, strideY, offsetY )
Computes the [absolute value][@stdlib/math/base/special/abs] for each element in a double-precision floating-point strided array `x` and assigns the results to elements in a double-precision floating-point strided array `y` using alternative indexing semantics.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0 ] );
var y = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0 ] );
dabs.ndarray( x.length, x, 1, 0, y, 1, 0 );
// y => [ 1.0, 2.0, 3.0, 4.0, 5.0 ]
```
The function accepts the following additional arguments:
- **offsetX**: starting index for `x`.
- **offsetY**: starting index for `y`.
While [`typed array`][@stdlib/array/float64] views mandate a view offset based on the underlying `buffer`, the `offsetX` and `offsetY` parameters support indexing semantics based on starting indices. For example, to index every other value in `x` starting from the second value and to index the last `N` elements in `y`,
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var floor = require( '@stdlib/math/base/special/floor' );
var x = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
var y = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );
var N = floor( x.length / 2 );
dabs.ndarray( N, x, 2, 1, y, -1, y.length-1 );
// y => [ 0.0, 0.0, 0.0, 6.0, 4.0, 2.0 ]
```
## Examples
```javascript
var round = require( '@stdlib/math/base/special/round' );
var randu = require( '@stdlib/random/base/randu' );
var Float64Array = require( '@stdlib/array/float64' );
var dabs = require( '@stdlib/math/strided/special/dabs' );
var x = new Float64Array( 10 );
var y = new Float64Array( 10 );
var i;
for ( i = 0; i < x.length; i++ ) {
x[ i ] = round( (randu()*200.0) - 100.0 );
}
console.log( x );
console.log( y );
dabs.ndarray( x.length, x, 1, 0, y, -1, y.length-1 );
console.log( y );
```
* * *
## C APIs
### Usage
```c
#include "stdlib/math/strided/special/dabs.h"
```
#### stdlib_strided_dabs( N, \*X, strideX, \*Y, strideY )
Computes the absolute value for each element in a double-precision floating-point strided array `X` and assigns the results to elements in a double-precision floating-point strided array `Y`.
```c
#include
double X[] = { -1.0, -2.0, -3.0, -4.0, -5.0, -6.0, -7.0, -8.0 };
double Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
int64_t N = 4;
stdlib_strided_dabs( N, X, 2, Y, 2 );
```
The function accepts the following arguments:
- **N**: `[in] int64_t` number of indexed elements.
- **X**: `[in] double*` input array.
- **strideX**: `[in] int64_t` index increment for `X`.
- **Y**: `[out] double*` output array.
- **strideY**: `[in] int64_t` index increment for `Y`.
```c
void stdlib_strided_dabs( const int64_t N, const double *X, const int64_t strideX, double *Y, const int64_t strideY );
```
### Examples
```c
#include "stdlib/math/strided/special/dabs.h"
#include
#include
int main() {
// Create an input strided array:
double X[] = { -1.0, -2.0, -3.0, -4.0, -5.0, -6.0, -7.0, -8.0 };
// Create an output strided array:
double Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
// Specify the number of elements:
int64_t N = 4;
// Specify the stride lengths:
int64_t strideX = 2;
int64_t strideY = 2;
// Compute the absolute value element-wise:
stdlib_strided_dabs( N, X, strideX, Y, strideY );
// Print the result:
for ( int i = 0; i < 8; i++ ) {
printf( "Y[ %i ] = %lf\n", i, Y[ i ] );
}
}
```
[@stdlib/array/float64]: https://www.npmjs.com/package/@stdlib/array-float64
[@stdlib/math/base/special/abs]: https://www.npmjs.com/package/@stdlib/math/tree/main/base/special/abs