# rotl90
> Rotate a matrix (or a stack of matrices) 90 degrees counterclockwise.
## Usage
```javascript
var rotl90 = require( '@stdlib/ndarray/base/rotl90' );
```
#### rotl90( x, k, writable )
Rotates a matrix (or a stack of matrices) 90 degrees counterclockwise.
```javascript
var array = require( '@stdlib/ndarray/array' );
var x = array( [ [ 1, 2 ], [ 3, 4 ] ] );
// returns [ [ 1, 2 ], [ 3, 4 ] ]
var y = rotl90( x, 1, false );
// returns [ [ 2, 4 ], [ 1, 3 ] ]
```
The function accepts the following arguments:
- **x**: input ndarray.
- **k**: number of times to rotate by 90 degrees. Positive values rotate counterclockwise. Negative values rotate clockwise.
- **writable**: boolean indicating whether a returned ndarray should be writable.
## Notes
- The `writable` parameter **only** applies to ndarray constructors supporting **read-only** instances.
- If `k > 0`, the function rotates the matrix counterclockwise.
- If `k < 0`, the function rotates the matrix clockwise.
- The returned ndarray is a **view** of the input ndarray. Accordingly, writing to the original ndarray will **mutate** the returned ndarray and vice versa.
- If provided an ndarray with fewer than two dimensions, the function does not perform a rotation and simply returns a new view of the input ndarray.
## Examples
```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var rotl90 = require( '@stdlib/ndarray/base/rotl90' );
// Create a 2x3 matrix:
var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ] );
// Rotate 90 degrees counterclockwise:
var y = rotl90( x, 1, false );
var arr = ndarray2array( y );
// returns [ [ 3, 6 ], [ 2, 5 ], [ 1, 4 ] ]
// Rotate 180 degrees:
y = rotl90( x, 2, false );
arr = ndarray2array( y );
// returns [ [ 6, 5, 4 ], [ 3, 2, 1 ] ]
// Rotate 270 degrees counterclockwise (equivalent to 90 degrees clockwise):
y = rotl90( x, 3, false );
arr = ndarray2array( y );
// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
// Rotate 360 degrees (equivalent to no rotation):
y = rotl90( x, 4, false );
arr = ndarray2array( y );
// returns [ [ 1, 2, 3 ], [ 4, 5, 6 ] ]
// Rotate 90 degrees clockwise (equivalent to k=3):
y = rotl90( x, -1, false );
arr = ndarray2array( y );
// returns [ [ 4, 1 ], [ 5, 2 ], [ 6, 3 ] ]
```