time-to-botec

README.md (4705B)

---

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 # countBy
 
 > Group values according to an indicator function and return group counts.
 
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 <section class="intro">
 
 </section>
 
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 <section class="usage">
 
 ## Usage
 
 ```javascript
 var countBy = require( '@stdlib/utils/count-by' );
 ```
 
 #### countBy( collection, \[options,] indicator )
 
 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.
 
 ```javascript
 function indicator( v ) {
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var out = countBy( arr, indicator );
 // returns { 'b': 3, 'f': 1 }
 ```
 
 An `indicator` function is provided two arguments:
 
 -   `value`: collection element
 -   `index`: collection index
 
 ```javascript
 function indicator( v, i ) {
     console.log( '%d: %d', i, v );
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var out = countBy( arr, indicator );
 // returns { 'b': 3, 'f': 1 }
 ```
 
 The function accepts the following `options`:
 
 -   `thisArg`: execution context.
 
 To set the `indicator` execution context, provide a `thisArg`.
 
 ```javascript
 function indicator( v ) {
     this.count += 1;
     return v[ 0 ];
 }
 var context = {
     'count': 0
 };
 var opts = {
     'thisArg': context
 };
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var out = countBy( arr, opts, indicator );
 // returns { 'b': 3, 'f': 1 }
 
 console.log( context.count );
 // => 4
 ```
 
 </section>
 
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 <section class="notes">
 
 ## Notes
 
 -   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`).
 
 -   The value returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,
 
     ```javascript
     function indicator( v ) {
         return {};
     }
     var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
     var out = countBy( arr, indicator );
     // returns { '[object Object]': 4 }
     ```
 
     while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`. 
 
 </section>
 
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 <section class="examples">
 
 ## Examples
 
 <!-- eslint no-undef: "error" -->
 
 ```javascript
 var randu = require( '@stdlib/random/base/randu' );
 var floor = require( '@stdlib/math/base/special/floor' );
 var countBy = require( '@stdlib/utils/count-by' );
 
 var vals;
 var arr;
 var out;
 var i;
 var j;
 
 vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];
 
 // Generate a random collection...
 arr = new Array( 100 );
 for ( i = 0; i < arr.length; i++ ) {
     j = floor( randu()*vals.length );
     arr[ i ] = vals[ j ];
 }
 
 function indicator( v ) {
     // Use the first letter of each element to define groups:
     return v[ 0 ];
 }
 
 // Compute the group counts:
 out = countBy( arr, indicator );
 console.log( out );
 ```
 
 </section>
 
 <!-- /.examples -->
 
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 </section>
 
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 [mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
 
 [mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
 
 [mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
 
 </section>
 
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