Ranking array elements

Question

I need an algorithm to rank elements of an array in Javascript.

Example : I have an array as follows:

[79, 5, 18, 5, 32, 1, 16, 1, 82, 13] 

I need to rank the entries by value. So 82 should receive rank 1, 79 rank 2 etc. If two entries have the same value they receive the same rank and the rank for a lower value is raised.

So for this array, the new ranking array would be:

[2, 7, 4, 7, 3, 9, 5, 9, 1, 6]  

How can I do this?

Answer 1

var arr = [79, 5, 18, 5, 32, 1, 16, 1, 82, 13];  var sorted = arr.slice().sort(function(a,b){return b-a})  var ranks = arr.map(function(v){ return sorted.indexOf(v)+1 });  console.log(ranks);

Result :

[2, 7, 4, 7, 3, 9, 5, 9, 1, 6] 

If you want to be compatible with old browsers, you may have to define a shim for indexOf and for map (note that if you want to do this very fast for very big arrays, you'd better use for loops and use an object as map instead of indexOf).

Answer 2

This won't work with older browsers because it uses ECMAScript 5 features, but it allows you to quickly and succinctly produce an array of rankings even for very large arrays. (It doesn't use indexOf which does a linear search and thus can be slow for large arrays.)

function cmp_rnum(a,b) {     // comparison function: reverse numeric order     return b-a; } function index_map(acc, item, index) {     // reduction function to produce a map of array items to their index     acc[item] = index;     return acc; } function ranks(v) {     var rankindex = v.slice().sort(cmp_rnum).reduceLeft(index_map, Object.create(null));     // reduceLeft() is used so the lowest rank wins if there are duplicates     // use reduce() if you want the highest rank     return v.map(function(item){ return rankindex[item]+1; }); } 

Example output:

> ranks([79, 5, 18, 5, 32, 1, 16, 1, 82, 13]);   [2, 7, 4, 7, 3, 9, 5, 9, 1, 6]