Question

In javascript I have an array as follows:

var foo = [2, 2, 4, 4, 128, 2, 2, 1, 4, 18, 27, 16, 2, 1, 18, 21, 5, 1, 128, 1, 2, 2, 1, 18, 12, 60, 2, 28, 1, 17, 2, 3, 4, 2, 2, 2, 1, 27, 2, 17, 7, 2, 2, 2, 5, 1, 2, 4, 7, 1, 2, 1, 1, 1, 2, 1, 5, 7, 2, 7, 6, 1, 7, 1, 5, 8, 4]; 

And I am interested in finding a way (within one loop, not multiple) to derive a subset array of the highest 10 values, where the previous position of the value is the ‘key’ (so simulating a Map object):

eg:

var fooTopTen = [[4, 128], [18, 128], [25, 60], [27, 28], [10, 27], [37, 27], [15, 21], [9, 18], [14, 18], [23, 18]]; 

Answer 1

My previous answer used a reverse index table, but contained some bugs – which are now fixed – and was harder to understand than the following code.

This is actually the slowest of all solutions given in the answers – for maximum performance, check my other answer.

var foo = [2, 2, 4, 4, 128, 2, 2, 1, 4, 18, 27, 16, 2, 1, 18, 21, 5, 1, 128, 1, 2, 2, 1, 18, 12, 60, 2, 28, 1, 17, 2, 3, 4, 2, 2, 2, 1, 27, 2, 17, 7, 2, 2, 2, 5, 1, 2, 4, 7, 1, 2, 1, 1, 1, 2, 1, 5, 7, 2, 7, 6, 1, 7, 1, 5, 8, 4];  var fooTopTen = [];  // add index to values for(var i = 0, len = foo.length; i < len; ++i)     fooTopTen.push([i, foo[i]]);  // sort first by value (descending order), then by index (ascending order) fooTopTen.sort(function(t1, t2) {     return t2[1] - t1[1] || t1[0] - t2[0]; });  // shorten array to correct size fooTopTen.length = 10;  // output top ten to check result document.writeln('[[' + fooTopTen.join('], [') + ']]'); 

The second part of the comparison function (the one comparing the indices) is not needed, as sort() is stable in most implementations (this isn’t required by ECMA according to MDC). I’ll leave it in as an example to how sorting with multiple requirements can be done…