# countIf
> Count the number of elements in an ndarray which pass a test implemented by a predicate function.
## Usage
```javascript
var countIf = require( '@stdlib/ndarray/base/count-if' );
```
#### countIf( arrays, predicate\[, thisArg] )
Counts the number of elements in an ndarray which pass a test implemented by a predicate function.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
function clbk( value ) {
return value > 0.0;
}
// Create a data buffer:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
// Define the shape of the input array:
var shape = [ 3, 1, 2 ];
// Define the array strides:
var sx = [ 4, 4, 1 ];
// Define the index offset:
var ox = 0;
// Create the input ndarray-like object:
var x = {
'dtype': 'float64',
'data': xbuf,
'shape': shape,
'strides': sx,
'offset': ox,
'order': 'row-major'
};
// Perform operation:
var out = countIf( [ x ], clbk );
// returns 5
```
The function accepts the following arguments:
- **arrays**: array-like object containing an input ndarray.
- **predicate**: predicate function.
- **thisArg**: predicate function execution context (_optional_).
The provided ndarray should be an object with the following properties:
- **dtype**: data type.
- **data**: data buffer.
- **shape**: dimensions.
- **strides**: stride lengths.
- **offset**: index offset.
- **order**: specifies whether an ndarray is row-major (C-style) or column major (Fortran-style).
The predicate function is provided the following arguments:
- **value**: current array element.
- **indices**: current array element indices.
- **arr**: the input ndarray.
To set the predicate function execution context, provide a `thisArg`.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
function clbk( value ) {
this.count += 1;
return value > 0.0;
}
// Create a data buffer:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
// Define the shape of the input array:
var shape = [ 3, 1, 2 ];
// Define the array strides:
var sx = [ 4, 4, 1 ];
// Define the index offset:
var ox = 0;
// Create the input ndarray-like object:
var x = {
'dtype': 'float64',
'data': xbuf,
'shape': shape,
'strides': sx,
'offset': ox,
'order': 'row-major'
};
// Create a context object:
var ctx = {
'count': 0
};
// Perform operation:
var out = countIf( [ x ], clbk, ctx );
// returns 5
var count = ctx.count;
// returns 6
```
## Notes
- For very high-dimensional ndarrays which are non-contiguous, one should consider copying the underlying data to contiguous memory before performing the operation in order to achieve better performance.
- If provided an empty ndarray, the function returns `0`.
## Examples
```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray/base/to-array' );
var countIf = require( '@stdlib/ndarray/base/count-if' );
function clbk( value ) {
return value > 0;
}
var x = {
'dtype': 'generic',
'data': discreteUniform( 10, -2, 10, {
'dtype': 'generic'
}),
'shape': [ 5, 2 ],
'strides': [ 2, 1 ],
'offset': 0,
'order': 'row-major'
};
console.log( ndarray2array( x.data, x.shape, x.strides, x.offset, x.order ) );
var out = countIf( [ x ], clbk );
console.log( out );
```