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order

Returns a permutation which rearranges input array.

Installation

Install from npm:

``
npm install order-permutation
`
Use as follows:

` javascript
var order = require( 'order-permutation' );
`

order( arr, \[compareFunction\] )

The exported function requires as its first parameter the array for which the indices in specified order should be obtained. The optional parameter compareFunction specifies a function defining the sort order. If not supplied, the returned permutation indices of the array sort its elements in increasing order. To sort more complicated objects than numeric primitives, a custom compareFunction has to be supplied. This function compares elements a and b according to the following rules:
- If compareFunction(a, b) is less than 0, a gets a lower index than b
- If compareFunction(a, b) is greater than 0, b gets a lower index than a.

Therefore, the standard compare function which sorts the elements in ascending order is equivalent to
` javascript
function( a, b ) {
if (a < b) {
return -1;
}
if (a > b) {
return 1;
}
return 0;
};
`

$3

#### Increasing Sequence:

Code:
` javascript
order([1, 2, 3, 4])
`

Output:
` javascript
[0, 1, 2, 3]
`

#### Decreasing Sequence:

Code:
` javascript
order([4, 3, 2, 1])
`

Output:
` javascript
[3, 2, 1, 0]
`

#### Custom Compare Function:

Code:
` javascript
var arr = [{name:"Tom", age: 28}, {name:"Lisa",age:23},{name:"Bill", age: 65}]
// order decreasing with age:
order(arr, function(a,b){
return b.age - a.age;
});
`

Output:
` javascript
[2, 0, 1]
`

$3

To obtain the actual sorted array, one can use the at() function of the lodash library, like so:

` javascript
var arr = [5, 1, 3, 2];
var indices = order(arr);
_.at(arr, indices)
`

Output:
` javascript
[ 1, 2, 3, 5 ]
`

Unit Tests

Run tests via the command npm test`

---

License

MIT license.

[npm-image]: https://badge.fury.io/js/order-permutation.svg
[npm-url]: http://badge.fury.io/js/order-permutation

[travis-image]: https://travis-ci.org/Planeshifter/order.svg
[travis-url]: https://travis-ci.org/Planeshifter/order

[dependencies-image]: http://img.shields.io/david/Planeshifter/order.svg
[dependencies-url]: https://david-dm.org/Planeshifter/order