Notoriously Psychedelic Modules

# npm

## @stdlib/blas-ext-base-gnansumpw 0.0.10 • Public • Published

# gnansumpw

Calculate the sum of strided array elements, ignoring `NaN` values and using pairwise summation.

## Installation

`npm install @stdlib/blas-ext-base-gnansumpw`

## Usage

`var gnansumpw = require( '@stdlib/blas-ext-base-gnansumpw' );`

#### gnansumpw( N, x, stride )

Computes the sum of strided array elements, ignoring `NaN` values and using pairwise summation.

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

var v = gnansumpw( N, x, 1 );
// returns 1.0```

The function has the following parameters:

The `N` and `stride` parameters determine which elements in `x` are accessed at runtime. For example, to compute the sum of 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, NaN, NaN ];
var N = floor( x.length / 2 );

var v = gnansumpw( N, x, 2 );
// returns 5.0```

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 = gnansumpw( N, x1, 2 );
// returns 5.0```

#### gnansumpw.ndarray( N, x, stride, offset )

Computes the sum of strided array elements, ignoring `NaN` values and using pairwise summation and alternative indexing semantics.

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

var v = gnansumpw.ndarray( N, x, 1, 0 );
// returns 1.0```

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 calculate the sum of 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, NaN, NaN ];
var N = floor( x.length / 2 );

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

## Notes

• If `N <= 0`, both functions return `0.0`.
• In general, pairwise summation is more numerically stable than ordinary recursive summation (i.e., "simple" summation), with slightly worse performance. While not the most numerically stable summation technique (e.g., compensated summation techniques such as the Kahan–Babuška-Neumaier algorithm are generally more numerically stable), pairwise summation strikes a reasonable balance between numerical stability and performance. If either numerical stability or performance is more desirable for your use case, consider alternative summation techniques.
• Depending on the environment, the typed versions (`dnansumpw`, `snansumpw`, 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 gnansumpw = require( '@stdlib/blas-ext-base-gnansumpw' );

var x;
var i;

x = new Float64Array( 10 );
for ( i = 0; i < x.length; i++ ) {
if ( randu() < 0.2 ) {
x[ i ] = NaN;
} else {
x[ i ] = round( randu()*100.0 );
}
}
console.log( x );

var v = gnansumpw( x.length, x, 1 );
console.log( v );```

## References

• Higham, Nicholas J. 1993. "The Accuracy of Floating Point Summation." SIAM Journal on Scientific Computing 14 (4): 783–99. doi:10.1137/0914050.

## Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

#### Community ### Install

`npm i @stdlib/blas-ext-base-gnansumpw`

### Repository

github.com/stdlib-js/blas-ext-base-gnansumpw

stdlib.io

62

0.0.10