192 lines
4.6 KiB
Markdown
192 lines
4.6 KiB
Markdown
<!--
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@license Apache-2.0
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Copyright (c) 2018 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# countBy
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> Group values according to an indicator function and return group counts.
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<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
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<section class="intro">
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</section>
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<!-- /.intro -->
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<!-- Package usage documentation. -->
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<section class="usage">
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## Usage
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```javascript
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var countBy = require( '@stdlib/utils/count-by' );
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```
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#### countBy( collection, \[options,] indicator )
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Groups values according to an `indicator` function, i.e., a function which specifies which group an element in the input `collection` belongs to, and returns group counts.
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```javascript
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function indicator( v ) {
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return v[ 0 ];
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}
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var arr = [ 'beep', 'boop', 'foo', 'bar' ];
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var out = countBy( arr, indicator );
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// returns { 'b': 3, 'f': 1 }
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```
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An `indicator` function is provided two arguments:
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- `value`: collection element
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- `index`: collection index
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```javascript
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function indicator( v, i ) {
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console.log( '%d: %d', i, v );
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return v[ 0 ];
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}
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var arr = [ 'beep', 'boop', 'foo', 'bar' ];
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var out = countBy( arr, indicator );
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// returns { 'b': 3, 'f': 1 }
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```
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The function accepts the following `options`:
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- `thisArg`: execution context.
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To set the `indicator` execution context, provide a `thisArg`.
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```javascript
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function indicator( v ) {
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this.count += 1;
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return v[ 0 ];
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}
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var context = {
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'count': 0
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};
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var opts = {
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'thisArg': context
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};
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var arr = [ 'beep', 'boop', 'foo', 'bar' ];
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var out = countBy( arr, opts, indicator );
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// returns { 'b': 3, 'f': 1 }
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console.log( context.count );
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// => 4
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```
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</section>
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<!-- /.usage -->
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<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="notes">
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## Notes
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- A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).
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- The value returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,
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```javascript
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function indicator( v ) {
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return {};
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}
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var arr = [ 'beep', 'boop', 'foo', 'bar' ];
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var out = countBy( arr, indicator );
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// returns { '[object Object]': 4 }
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```
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while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.
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</section>
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<!-- /.notes -->
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<!-- Package usage examples. -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var randu = require( '@stdlib/random/base/randu' );
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var floor = require( '@stdlib/math/base/special/floor' );
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var countBy = require( '@stdlib/utils/count-by' );
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var vals;
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var arr;
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var out;
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var i;
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var j;
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vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];
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// Generate a random collection...
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arr = new Array( 100 );
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for ( i = 0; i < arr.length; i++ ) {
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j = floor( randu()*vals.length );
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arr[ i ] = vals[ j ];
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}
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function indicator( v ) {
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// Use the first letter of each element to define groups:
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return v[ 0 ];
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}
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// Compute the group counts:
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out = countBy( arr, indicator );
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console.log( out );
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```
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</section>
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<!-- /.examples -->
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<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="references">
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</section>
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<!-- /.references -->
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<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="links">
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[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
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[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
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</section>
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<!-- /.links -->
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