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# countIf

> Count the number of truthy elements along one or more [`ndarray`][@stdlib/ndarray/ctor] dimensions.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var countIf = require( '@stdlib/ndarray/count-if' );
```

#### countIf( x\[, options], predicate\[, thisArg] )

Counts the number of truthy elements along one or more [`ndarray`][@stdlib/ndarray/ctor] dimensions.

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

function clbk( value ) {
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Perform reduction:
var out = countIf( x, clbk );
// returns <ndarray>[ 5 ]
```

The function accepts the following arguments:

-   **x**: input [`ndarray`][@stdlib/ndarray/ctor].
-   **options**: function options (_optional_).
-   **predicate**: predicate function.
-   **thisArg**: predicate function execution context (_optional_).

The function accepts the following `options`:

-   **dims**: list of dimensions over which to perform a reduction.
-   **keepdims**: boolean indicating whether the reduced dimensions should be included in the returned [`ndarray`][@stdlib/ndarray/ctor] as singleton dimensions. Default: `false`.

By default, the function performs a reduction over all elements in a provided [`ndarray`][@stdlib/ndarray/ctor]. To reduce specific dimensions, set the `dims` option.

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

function clbk( value ) {
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Perform reduction:
var opts = {
    'dims': [ 1, 2 ]
};
var out = countIf( x, opts, clbk );
// returns <ndarray>[ 2, 2, 1 ]
```

By default, the function returns an [`ndarray`][@stdlib/ndarray/ctor] having a shape matching only the non-reduced dimensions of the input [`ndarray`][@stdlib/ndarray/ctor] (i.e., the reduced dimensions are dropped). To include the reduced dimensions as singleton dimensions in the output [`ndarray`][@stdlib/ndarray/ctor], set the `keepdims` option to `true`.

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

function clbk( value ) {
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Perform reduction:
var opts = {
    'dims': [ 1, 2 ],
    'keepdims': true
};
var out = countIf( x, opts, clbk );
// returns <ndarray>[ [ [ 2 ] ], [ [ 2 ] ], [ [ 1 ] ] ]
```

To set the predicate function execution context, provide a `thisArg`.

<!-- eslint-disable no-invalid-this -->

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

function clbk( value ) {
    this.count += 1;
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Create a context object:
var ctx = {
    'count': 0
};

// Perform reduction:
var out = countIf( x, clbk, ctx );
// returns <ndarray>[ 5 ]

var count = ctx.count;
// returns 6
```

#### countIf.assign( x, out\[, options], predicate\[, thisArg] )

Counts the number of truthy elements along one or more [`ndarray`][@stdlib/ndarray/ctor] dimensions and assigns results to a provided output [`ndarray`][@stdlib/ndarray/ctor].

```javascript
var array = require( '@stdlib/ndarray/array' );
var empty = require( '@stdlib/ndarray/empty' );

function clbk( value ) {
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Create an output ndarray:
var y = empty( [], {
    'dtype': 'int32'
});

// Perform reduction:
var out = countIf.assign( x, y, clbk );
// returns <ndarray>[ 5 ]

var bool = ( out === y );
// returns true
```

The function accepts the following arguments:

-   **x**: input [`ndarray`][@stdlib/ndarray/ctor].
-   **out**: output [`ndarray`][@stdlib/ndarray/ctor]. The output [`ndarray`][@stdlib/ndarray/ctor] must have a shape matching the non-reduced dimensions of the input [`ndarray`][@stdlib/ndarray/ctor].
-   **options**: function options (_optional_).
-   **predicate**: predicate function.
-   **thisArg**: predicate function execution context (_optional_).

The function accepts the following `options`:

-   **dims**: list of dimensions over which to perform a reduction.

By default, the function performs a reduction over all elements in a provided [`ndarray`][@stdlib/ndarray/ctor]. To reduce specific dimensions, set the `dims` option.

```javascript
var array = require( '@stdlib/ndarray/array' );
var empty = require( '@stdlib/ndarray/empty' );

function clbk( value ) {
    return value > 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 0.0, 6.0 ] ] ] );
// returns <ndarray>

// Create an output ndarray:
var y = empty( [ 3 ], {
    'dtype': 'int32'
});

// Perform reduction:
var opts = {
    'dims': [ 1, 2 ]
};
var out = countIf.assign( x, y, opts, clbk );
// returns <ndarray>[ 2, 2, 1 ]

var bool = ( out === y );
// returns true
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The predicate function is provided the following arguments:

    -   **value**: current array element.
    -   **indices**: current array element indices.
    -   **arr**: the input ndarray.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var bernoulli = require( '@stdlib/random/base/bernoulli' ).factory;
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var fillBy = require( '@stdlib/ndarray/fill-by' );
var zeros = require( '@stdlib/ndarray/zeros' );
var isPositiveNumber = require( '@stdlib/assert/is-positive-number' ).isPrimitive;
var countIf = require( '@stdlib/ndarray/count-if' );

var x = zeros( [ 2, 4, 5 ], {
    'dtype': 'float64'
});
x = fillBy( x, bernoulli( 0.90 ) );
console.log( ndarray2array( x ) );

var y = countIf( x, isPositiveNumber );
console.log( 'countIf(x[:,:,:]) =' );
console.log( y.get() );

var opts = {
    'dims': [ 0 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[:,j,k]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 1 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[i,:,k]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 2 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[i,j,:]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 0, 1 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[:,:,k]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 0, 2 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[:,j,:]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 1, 2 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[i,:,:]) =' );
console.log( ndarray2array( y ) );

opts = {
    'dims': [ 0, 1, 2 ],
    'keepdims': true
};
y = countIf( x, opts, isPositiveNumber );
console.log( 'countIf(x[:,:,:]) =' );
console.log( ndarray2array( y ) );
```

</section>

<!-- /.examples -->

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

<section class="related">

</section>

<!-- /.related -->

<section class="links">

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

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

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

</section>

<!-- /.links -->
