146 lines
3.3 KiB
Markdown
146 lines
3.3 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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# Object Inverse
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> Invert an object, such that keys become values and values become keys.
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<section class="usage">
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## Usage
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```javascript
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var invert = require( '@stdlib/utils/object-inverse' );
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```
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#### invert( obj\[, options] )
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Inverts an `object`, such that keys become values and values become keys.
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```javascript
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var out = invert({
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'a': 'beep',
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'b': 'boop'
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});
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// returns { 'beep': 'a', 'boop': 'b' }
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```
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The function accepts the following `options`:
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- **duplicates**: `boolean` indicating whether to store keys mapped to duplicate values in `arrays`. Default: `true`.
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By default, keys mapped to duplicate values are stored in `arrays`.
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```javascript
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var out = invert({
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'a': 'beep',
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'b': 'beep'
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});
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// returns { 'beep': [ 'a', 'b' ] }
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```
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To **not** allow duplicates, set the `duplicates` option to `false`. The output `key-value` pair will be the `key` most recently inserted into the input `object`.
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```javascript
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var obj = {};
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obj.a = 'beep';
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obj.b = 'boop';
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obj.c = 'beep'; // inserted after `a`
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var out = invert( obj, {
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'duplicates': false
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});
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// returns { 'beep': 'c', 'boop': 'b' }
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```
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</section>
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<!-- /.usage -->
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<section class="notes">
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## Notes
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- Beware when providing `objects` having values which are themselves `objects`. This implementation relies on native `object` serialization (`#toString`) when converting values to keys.
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```javascript
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var obj = {
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'a': [ 1, 2, 3 ],
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'b': {
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'c': 'd'
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}
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};
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var out = invert( obj );
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// returns { '1,2,3': 'a', '[object Object]': 'b' }
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```
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- Insertion order is not guaranteed, as `object` key enumeration is not specified according to the [ECMAScript specification][ecma-262-for-in]. In practice, however, most engines use insertion order to sort an `object`'s keys, thus allowing for deterministic inversion.
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</section>
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<!-- /.notes -->
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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 round = require( '@stdlib/math/base/special/round' );
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var invert = require( '@stdlib/utils/object-inverse' );
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var keys;
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var arr;
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var out;
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var i;
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// Create an array of random integers...
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arr = new Array( 1000 );
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for ( i = 0; i < arr.length; i++ ) {
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arr[ i ] = round( randu()*100.0 );
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}
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// Invert the array to determine value frequency...
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out = invert( arr );
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keys = Object.keys( out );
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for ( i = 0; i < keys.length; i++ ) {
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if ( out[ i ] ) {
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out[ i ] = out[ i ].length;
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} else {
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out[ i ] = 0;
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}
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}
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console.dir( out );
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```
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</section>
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<!-- /.examples -->
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<section class="links">
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[ecma-262-for-in]: http://www.ecma-international.org/ecma-262/5.1/#sec-12.6.4
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</section>
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<!-- /.links -->
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