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sub2ind

Convert subscripts to a linear index.

Installation

npm install @stdlib/ndarray-base-sub2ind

Alternatively,

• To load the package in a website via a script tag without installation and bundlers, use the ES Module available on the esm branch (see README).
• If you are using Deno, visit the deno branch (see README for usage intructions).
• For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the umd branch (see README).

The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.

To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.

Usage

var sub2ind = require( '@stdlib/ndarray-base-sub2ind' );

sub2ind( shape, strides, offset, ...subscripts, mode )

Converts subscripts to a linear index.

var shape = [ 2, 2 ];
var strides = [ 2, 1 ];
var offset = 0;
var mode = [ 'throw' ];

var idx = sub2ind( shape, strides, offset, 1, 0, mode );
// returns 2

The function supports the following modes:

• throw: specifies that the function should throw an error when a subscript exceeds array dimensions.
• normalize: specifies that the function should normalize negative subscripts and throw an error when a subscript exceeds array dimensions.
• wrap: specifies that the function should wrap around subscripts exceeding array dimensions using modulo arithmetic.
• clamp: specifies that the function should set subscripts exceeding array dimensions to either 0 (minimum index) or the maximum index along a particular dimension.

var shape = [ 2, 2 ];
var strides = [ 2, 1 ];
var offset = 0;

var mode = [ 'wrap' ];
var idx = sub2ind( shape, strides, offset, -2, 0, mode );
// returns 0

mode = [ 'clamp' ];
idx = sub2ind( shape, strides, offset, 10, 10, mode );
// returns 3

Notes

• When provided fewer modes than dimensions, the function recycles modes using modulo arithmetic.

  var shape = [ 2, 2, 2 ];
  var strides = [ 4, 2, 1 ];
  var offset = 0;
  var mode = [ 'wrap', 'clamp' ];
  
  var idx = sub2ind( shape, strides, offset, -2, 10, -1, mode );
  // returns 3

• When provided a stride array containing negative strides, if an offset is greater than 0, the function treats subscripts as mapping to a linear index in an underlying data buffer for the array, thus returning a linear index from the perspective of that buffer. If an offset is equal to 0, the function treats subscripts as mapping to a linear index in an array view, thus returning a linear index from the perspective of that view.

  Dims: 2x2
  Buffer: [ 1, 2, 3, 4 ]
  
  View = [ a00, a01,
           a10, a11 ]
  
  Strides: 2,1
  Offset: 0
  
  View = [ 1, 2,
           3, 4 ]
  
  Strides: 2,-1
  Offset: 1
  
  View = [ 2, 1,
           4, 3 ]
  
  Strides: -2,1
  Offset: 2
  
  View = [ 3, 4,
           1, 2 ]
  
  Strides: -2,-1
  Offset: 3
  
  View = [ 4, 3,
           2, 1 ]
  

  var shape = [ 2, 2 ];
  var strides = [ -2, 1 ];
  var offset = 2;
  var mode = [ 'throw' ];
  
  // From the perspective of a view...
  var idx = sub2ind( shape, strides, 0, 0, 0, mode );
  // returns 0
  
  idx = sub2ind( shape, strides, 0, 0, 1, mode );
  // returns 1
  
  idx = sub2ind( shape, strides, 0, 1, 0, mode );
  // returns 2
  
  idx = sub2ind( shape, strides, 0, 1, 1, mode );
  // returns 3
  
  // From the perspective of an underlying buffer...
  idx = sub2ind( shape, strides, offset, 0, 0, mode );
  // returns 2
  
  idx = sub2ind( shape, strides, offset, 0, 1, mode );
  // returns 3
  
  idx = sub2ind( shape, strides, offset, 1, 0, mode );
  // returns 0
  
  idx = sub2ind( shape, strides, offset, 1, 1, mode );
  // returns 1

  In short, from the perspective of a view, view data is always ordered.

Examples

var discreteUniform = require( '@stdlib/random-base-discrete-uniform' );
var shape2strides = require( '@stdlib/ndarray-base-shape2strides' );
var strides2offset = require( '@stdlib/ndarray-base-strides2offset' );
var numel = require( '@stdlib/ndarray-base-numel' );
var randu = require( '@stdlib/random-base-randu' );
var sub2ind = require( '@stdlib/ndarray-base-sub2ind' );

var shape = [ 3, 3 ];
var strides = shape2strides( shape, 'row-major' );
var mode = [ 'throw' ];
var len = numel( shape );

var arr = [];
var i;
for ( i = 0; i < len; i++ ) {
    arr.push( i );
}

var offset;
var idx;
var row;
var j;
var n;
var m;
for ( i = 0; i < 20; i++ ) {
    j = discreteUniform( 0, shape.length-1 );
    strides[ j ] = ( randu() < 0.5 ) ? -1 : 1;
    offset = strides2offset( shape, strides );

    console.log( '' );
    console.log( 'Dimensions: %s.', shape.join( 'x' ) );
    console.log( 'Strides: %s.', strides.join( ',' ) );
    console.log( 'View:' );
    for ( n = 0; n < shape[ 0 ]; n++ ) {
        row = '  ';
        for ( m = 0; m < shape[ 1 ]; m++ ) {
            idx = sub2ind( shape, strides, offset, n, m, mode );
            row += arr[ idx ];
            if ( m < shape[ 1 ]-1 ) {
                row += ', ';
            }
        }
        console.log( row );
    }
}

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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

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License

See LICENSE.

Copyright

Copyright © 2016-2025. The Stdlib Authors.