sub2ind

Convert subscripts to a linear index.

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.
• `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 = new Array( len );
var i;
for ( i = 0; i < len; i++ ) {
arr[ i ] = 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 );
}
}
``````