gapxsum

Add a constant to each strided array element and compute the sum.

Usage

var gapxsum = require( '@stdlib/blas/ext/base/gapxsum' );

gapxsum( N, alpha, x, stride )

Adds a constant to each strided array element and computes the sum.

var x = [ 1.0, -2.0, 2.0 ];
var N = x.length;

var v = gapxsum( N, 5.0, x, 1 );
// returns 16.0

The function has the following parameters:

N: number of indexed elements.
x: input Array or typed array.
stride: index increment for x.

The N and stride parameters determine which elements in x are accessed at runtime. For example, to access every other element in x,

var floor = require( '@stdlib/math/base/special/floor' );

var x = [ 1.0, 2.0, 2.0, -7.0, -2.0, 3.0, 4.0, 2.0 ];
var N = floor( x.length / 2 );

var v = gapxsum( N, 5.0, x, 2 );
// returns 25.0

Note that indexing is relative to the first index. To introduce an offset, use typed array views.

var Float64Array = require( '@stdlib/array/float64' );
var floor = require( '@stdlib/math/base/special/floor' );

var x0 = new Float64Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

var N = floor( x0.length / 2 );

var v = gapxsum( N, 5.0, x1, 2 );
// returns 25.0

gapxsum.ndarray( N, alpha, x, stride, offset )

Adds a constant to each strided array element and computes the sum using alternative indexing semantics.

var x = [ 1.0, -2.0, 2.0 ];
var N = x.length;

var v = gapxsum.ndarray( N, 5.0, x, 1, 0 );
// returns 16.0

The function has the following additional parameters:

offset: starting index for x.

While typed array views mandate a view offset based on the underlying buffer, the offset parameter supports indexing semantics based on a starting index. For example, to access every other value in x starting from the second value

var floor = require( '@stdlib/math/base/special/floor' );

var x = [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ];
var N = floor( x.length / 2 );

var v = gapxsum.ndarray( N, 5.0, x, 2, 1 );
// returns 25.0

Notes

If N <= 0, both functions return 0.0.
Depending on the environment, the typed versions (dapxsum, sapxsum, etc.) are likely to be significantly more performant.

Examples

var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var Float64Array = require( '@stdlib/array/float64' );
var gapxsum = require( '@stdlib/blas/ext/base/gapxsum' );

var x;
var i;

x = new Float64Array( 10 );
for ( i = 0; i < x.length; i++ ) {
    x[ i ] = round( randu()*100.0 );
}
console.log( x );

var v = gapxsum( x.length, 5.0, x, 1 );
console.log( v );