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# nditerColumns

> Create an iterator which iterates over each column in a matrix (or stack of matrices).

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## Usage

```javascript
var nditerColumns = require( '@stdlib/ndarray/iter/columns' );
```

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

Returns an iterator which iterates over each column in a matrix (or stack of matrices).

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

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

var iter = nditerColumns( x );

var v = iter.next().value;
// returns <ndarray>

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

v = iter.next().value;
// returns <ndarray>

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

v = iter.next().value;
// returns <ndarray>

arr = ndarray2array( v );
// returns [ 5, 7 ]

// ...
```

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

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

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

var v = iter.next().value;
// returns <ndarray>

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

v.set( 0, 10 );

arr = ndarray2array( v );
// 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.

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## Notes

-   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>

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## Examples

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

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

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

// Create an iterator for iterating over columns:
var it = nditerColumns( x );

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

</section>

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* * *

## See Also

-   <span class="package-name">[`@stdlib/ndarray/iter/column-entries`][@stdlib/ndarray/iter/column-entries]</span><span class="delimiter">: </span><span class="description">create an iterator which returns \[index, column] pairs for each column in a matrix (or stack of matrices).</span>
-   <span class="package-name">[`@stdlib/ndarray/iter/rows`][@stdlib/ndarray/iter/rows]</span><span class="delimiter">: </span><span class="description">create an iterator which iterates over 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>

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

[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray/tree/main/ctor

[@stdlib/ndarray/slice]: https://github.com/stdlib-js/ndarray/tree/main/slice

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[@stdlib/ndarray/iter/column-entries]: https://github.com/stdlib-js/ndarray/tree/main/iter/column-entries

[@stdlib/ndarray/iter/rows]: https://github.com/stdlib-js/ndarray/tree/main/iter/rows

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</section>

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