# dmap
> Apply a unary function accepting and returning double-precision floating-point numbers to each element in a double-precision floating-point strided input array and assign each result to an element in a double-precision floating-point strided output array.
## Usage
```javascript
var dmap = require( '@stdlib/strided/base/dmap' );
```
#### dmap( N, x, strideX, y, strideY, fcn )
Applies a unary function accepting and returning double-precision floating-point numbers to each element in a double-precision floating-point strided input array and assigns each result to an element in a double-precision floating-point strided output array.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var abs = require( '@stdlib/math/base/special/abs' );
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:
dmap( x.length, x, 1, x, 1, abs );
// 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`.
- **fcn**: function to apply.
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 abs = require( '@stdlib/math/base/special/abs' );
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 );
dmap( N, x, 2, y, -1, abs );
// 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' );
var abs = require( '@stdlib/math/base/special/abs' );
// 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 );
dmap( N, x1, -2, y1, 1, abs );
// y0 => [ 0.0, 0.0, 0.0, 6.0, 4.0, 2.0 ]
```
#### dmap.ndarray( N, x, strideX, offsetX, y, strideY, offsetY, fcn )
Applies a unary function accepting and returning double-precision floating-point numbers to each element in a double-precision floating-point strided input array and assigns each result to an element in a double-precision floating-point strided output array using alternative indexing semantics.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var abs = require( '@stdlib/math/base/special/abs' );
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 ] );
dmap.ndarray( x.length, x, 1, 0, y, 1, 0, abs );
// 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 abs = require( '@stdlib/math/base/special/abs' );
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 );
dmap.ndarray( N, x, 2, 1, y, -1, y.length-1, abs );
// 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 dmap = require( '@stdlib/strided/base/dmap' );
function scale( x ) {
return x * 10.0;
}
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 );
dmap.ndarray( x.length, x, 1, 0, y, -1, y.length-1, scale );
console.log( y );
```
* * *
## C APIs
### Usage
```c
#include "stdlib/strided/base/dmap.h"
```
#### stdlib_strided_dmap( N, \*X, strideX, \*Y, strideY, fcn )
Applies a unary function accepting and returning double-precision floating-point numbers to each element in a double-precision floating-point strided input array and assigns each result to an element in a double-precision floating-point strided output array.
```c
#include
static double scale( const double x ) {
return x * 10.0;
}
double X[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 };
double Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
int64_t N = 6;
stdlib_strided_dmap( N, X, 1, Y, 1, scale );
```
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`.
- **fcn**: `[in] double (*fcn)( double )` unary function to apply.
```c
void stdlib_strided_dmap( const int64_t N, const double *X, const int64_t strideX, double *Y, const int64_t strideY, double (*fcn)( double ) );
```
### Examples
```c
#include "stdlib/strided/base/dmap.h"
#include
#include
#include
// Define a callback:
static double scale( const double x ) {
return x * 10.0;
}
int main() {
// Create an input strided array:
double X[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 };
// Create an output strided array:
double Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
// Specify the number of elements:
int64_t N = 6;
// Define the strides:
int64_t strideX = 1;
int64_t strideY = -1;
// Apply the callback:
stdlib_strided_dmap( N, X, strideX, Y, strideY, scale );
// Print the results:
for ( int64_t i = 0; i < N; i++ ) {
printf( "Y[ %"PRId64" ] = %lf\n", i, Y[ i ] );
}
}
```
[@stdlib/array/float64]: https://www.npmjs.com/package/@stdlib/array-float64