npm package: @stdlib/ndarray-iter-column-entries
Description: Create an iterator which returns [index, column] pairs for each column in a matrix (or stack of matrices).
Author: The Stdlib Authors
Latest version: 0.2.4

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# nditerColumnEntries

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> Create an iterator which returns `[index, column]` pairs for each column in a matrix (or stack of matrices).

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/ndarray-iter-column-entries
```

</section>

<section class="usage">

## Usage

```javascript
var nditerColumnEntries = require( '@stdlib/ndarray-iter-column-entries' );
```

#### nditerColumnEntries( x\[, options] )

Returns an iterator which returns `[index, column]` pairs for each column in a matrix (or stack of matrices).

```javascript
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var array = require( '@stdlib/ndarray-array' );

var x = array( [ [ [ 1, 2 ], [ 3, 4 ] ], [ [ 5, 6 ], [ 7, 8 ] ] ] );
// returns <ndarray>

var iter = nditerColumnEntries( x );

var v = iter.next().value;
// returns [...]

var idx = v[ 0 ];
// returns [ 0, null, 0 ]

var col = ndarray2array( v[ 1 ] );
// returns [ 1, 3 ]

v = iter.next().value;
// returns [...]

idx = v[ 0 ];
// returns [ 0, null, 1 ]

col = ndarray2array( v[ 1 ] );
// returns [ 2, 4 ]

// ...
```

The function accepts the following `options`:

-   **readonly**: boolean indicating whether returned [`ndarray`][@stdlib/ndarray/ctor] views should be read-only. In order to return writable [`ndarray`][@stdlib/ndarray/ctor] views, the input [`ndarray`][@stdlib/ndarray/ctor] must be writable. If the input [`ndarray`][@stdlib/ndarray/ctor] is read-only, setting this option to `false` raises an exception. Default: `true`.

By default, the iterator returns [`ndarray`][@stdlib/ndarray/ctor] views which are **read-only**. To return writable [views][@stdlib/ndarray/slice], set the `readonly` option to `false`.

```javascript
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var array = require( '@stdlib/ndarray-array' );

var x = array( [ [ [ 1, 2 ], [ 3, 4 ] ], [ [ 5, 6 ], [ 7, 8 ] ] ] );
// returns <ndarray>

var iter = nditerColumnEntries( x, {
    'readonly': false
});

var v = iter.next().value;
// returns [...]

var col = ndarray2array( v[ 1 ] );
// returns [ 1, 3 ]

v[ 1 ].set( 0, 10 );

col = ndarray2array( v[ 1 ] );
// returns [ 10, 3 ]
```

The returned [iterator][mdn-iterator-protocol] protocol-compliant object has the following properties:

-   **next**: function which returns an [iterator][mdn-iterator-protocol] protocol-compliant object containing the next iterated value (if one exists) assigned to a `value` property and a `done` property having a `boolean` value indicating whether the [iterator][mdn-iterator-protocol] is finished.
-   **return**: function which closes an [iterator][mdn-iterator-protocol] and returns a single (optional) argument in an [iterator][mdn-iterator-protocol] protocol-compliant object.

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   Each returned index is a Cartesian index (i.e., an array of subscripts/dimension indices). A dimension index equal to `null` indicates that all values along the respective dimension are included in the returned [`ndarray`][@stdlib/ndarray/ctor].
-   If an environment supports `Symbol.iterator`, the returned iterator is iterable.
-   A returned iterator does **not** copy a provided [`ndarray`][@stdlib/ndarray/ctor]. To ensure iterable reproducibility, copy the input [`ndarray`][@stdlib/ndarray/ctor] **before** creating an iterator. Otherwise, any changes to the contents of input [`ndarray`][@stdlib/ndarray/ctor] will be reflected in the returned iterator.
-   In environments supporting `Symbol.iterator`, the function **explicitly** does **not** invoke an ndarray's `@@iterator` method, regardless of whether this method is defined.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var array = require( '@stdlib/ndarray-array' );
var zeroTo = require( '@stdlib/array-base-zero-to' );
var nditerColumnEntries = require( '@stdlib/ndarray-iter-column-entries' );

// Define an input array:
var x = array( zeroTo( 27 ), {
    'shape': [ 3, 3, 3 ]
});

// Create an iterator for returning [index, column] pairs:
var it = nditerColumnEntries( x );

// Perform manual iteration...
var v;
while ( true ) {
    v = it.next();
    if ( v.done ) {
        break;
    }
    console.log( v.value[ 0 ] );
    console.log( ndarray2array( v.value[ 1 ] ) );
}
```

</section>

<!-- /.examples -->

<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="references">

</section>

<!-- /.references -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/ndarray-iter/columns`][@stdlib/ndarray/iter/columns]</span><span class="delimiter">: </span><span class="description">create an iterator which iterates over each column in a matrix (or stack of matrices).</span>
-   <span class="package-name">[`@stdlib/ndarray-iter/entries`][@stdlib/ndarray/iter/entries]</span><span class="delimiter">: </span><span class="description">create an iterator which returns \[index, value] pairs for each element in a provided ndarray.</span>
-   <span class="package-name">[`@stdlib/ndarray-iter/row-entries`][@stdlib/ndarray/iter/row-entries]</span><span class="delimiter">: </span><span class="description">create an iterator which returns \[index, row] pairs for each row in a matrix (or stack of matrices).</span>
-   <span class="package-name">[`@stdlib/ndarray-slice`][@stdlib/ndarray/slice]</span><span class="delimiter">: </span><span class="description">return a read-only view of an input ndarray.</span>

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][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][stdlib], see the main project [repository][stdlib].

#### Community

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---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/ndarray-iter-column-entries/main/LICENSE

[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol

[@stdlib/ndarray/ctor]: https://www.npmjs.com/package/@stdlib/ndarray-ctor

[@stdlib/ndarray/slice]: https://www.npmjs.com/package/@stdlib/ndarray-slice

<!-- <related-links> -->

[@stdlib/ndarray/iter/columns]: https://www.npmjs.com/package/@stdlib/ndarray-iter-columns

[@stdlib/ndarray/iter/entries]: https://www.npmjs.com/package/@stdlib/ndarray-iter-entries

[@stdlib/ndarray/iter/row-entries]: https://www.npmjs.com/package/@stdlib/ndarray-iter-row-entries

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