time-to-botec

README.md (5751B)

---

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 # groupBy
 
 > Group values according to an indicator function.
 
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 <section class="intro">
 
 </section>
 
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 <!-- Package usage documentation. -->
 
 <section class="usage">
 
 ## Usage
 
 ```javascript
 var groupBy = require( '@stdlib/utils/group-by' );
 ```
 
 #### groupBy( collection, \[options,] indicator )
 
 Groups values according to an `indicator` function, which specifies which group an element in the input `collection` belongs to.
 
 ```javascript
 function indicator( v ) {
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var out = groupBy( arr, indicator );
 // returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
 ```
 
 An `indicator` function is provided two arguments:
 
 -   `value`: collection element
 -   `index`: collection index
 
 ```javascript
 function indicator( v, i ) {
     console.log( '%d: %s', i, v );
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var out = groupBy( arr, indicator );
 // returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
 ```
 
 The function accepts the following `options`:
 
 -   `returns`: specifies the output format. If the option equals `'values'`, the function outputs element values. If the option equals `'indices'`, the function outputs element indices. If the option equals `'*'`, the function outputs both element indices and values. Default: `'values'`.
 -   `thisArg`: execution context.
 
 By default, the function returns element values. To return element indices, set the `returns` option to `'indices'`.
 
 ```javascript
 function indicator( v ) {
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var opts = {
     'returns': 'indices'
 };
 var out = groupBy( arr, opts, indicator );
 // returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }
 ```
 
 To return index-element pairs, set the `returns` option to `'*'`.
 
 ```javascript
 function indicator( v ) {
     return v[ 0 ];
 }
 var arr = [ 'beep', 'boop', 'foo', 'bar' ];
 
 var opts = {
     'returns': '*'
 };
 var out = groupBy( arr, opts, indicator );
 // returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }
 ```
 
 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 = groupBy( arr, opts, indicator );
 // returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
 
 console.log( context.count );
 // => 4
 ```
 
 </section>
 
 <!-- /.usage -->
 
 <!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
 
 <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 = groupBy( arr, indicator );
     // returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
     ```
 
     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 groupBy = require( '@stdlib/utils/group-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 groups:
 out = groupBy( arr, indicator );
 console.log( out );
 ```
 
 </section>
 
 <!-- /.examples -->
 
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 <section class="links">
 
 [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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