# toRot90
> Return a new ndarray where an ndarray is rotated 90 degrees in a specified plane.
## Usage
```javascript
var toRot90 = require( '@stdlib/ndarray/base/to-rot90' );
```
#### toRot90( x, dims, k )
Returns a new ndarray where an ndarray is rotated 90 degrees in a specified plane.
```javascript
var array = require( '@stdlib/ndarray/array' );
var x = array( [ [ 1, 2 ], [ 3, 4 ] ] );
// returns [ [ 1, 2 ], [ 3, 4 ] ]
var y = toRot90( x, [ 0, 1 ], 1 );
// returns [ [ 2, 4 ], [ 1, 3 ] ]
```
The function accepts the following arguments:
- **x**: input ndarray.
- **dims**: dimension indices defining the plane of rotation. Must contain exactly two unique dimension indices. If a dimension index is provided as an integer less than zero, the dimension index is resolved relative to the last dimension, with the last dimension corresponding to the value `-1`.
- **k**: number of times to rotate by 90 degrees. Positive values rotate counterclockwise. Negative values rotate clockwise.
## Notes
- If `k > 0`, the function rotates the plane from the first specified dimension toward the second specified dimension. This means that, for a two-dimensional ndarray and `dims = [0, 1]`, the function rotates the plane counterclockwise.
- If `k < 0`, the function rotates the plane from the second specified dimension toward the first specified dimension. This means that, for a two-dimensional ndarray and `dims = [0, 1]`, the function rotates the plane clockwise.
- Each provided dimension index must reside on the interval `[-ndims, ndims-1]`.
## Examples
```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var toRot90 = require( '@stdlib/ndarray/base/to-rot90' );
// Create a 2x3 matrix:
var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ] );
// Rotate 90 degrees counterclockwise in the (0,1) plane:
var y = toRot90( x, [ 0, 1 ], 1 );
var arr = ndarray2array( y );
// returns [ [ 3, 6 ], [ 2, 5 ], [ 1, 4 ] ]
// Rotate 180 degrees:
y = toRot90( x, [ 0, 1 ], 2 );
arr = ndarray2array( y );
// returns [ [ 6, 5, 4 ], [ 3, 2, 1 ] ]
// Rotate 270 degrees counterclockwise (equivalent to k=-1):
y = toRot90( x, [ 0, 1 ], 3 );
arr = ndarray2array( y );
// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
// Rotate 360 degrees (equivalent to no rotation):
y = toRot90( x, [ 0, 1 ], 4 );
arr = ndarray2array( y );
// returns [ [ 1, 2, 3 ], [ 4, 5, 6 ] ]
// Rotate 90 degrees clockwise (equivalent to k=3):
y = toRot90( x, [ 0, 1 ], -1 );
arr = ndarray2array( y );
// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
// Reverse the rotation direction by reversing the dimension order:
y = toRot90( x, [ 1, 0 ], 1 );
arr = ndarray2array( y );
// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
```