I don't understand why creating a function that subtracts a-b then sorts the numbers in the array.

Question

Could someone explain this a little more clearly?

Answer 1 · 2014-12-02T16:56:02Z

December 2, 2014 4:56pm

I'm assuming you are referring to JavaScript's Array.prototype.sort() function. Array.sort() is only interested in the output of a function (what a-b equals). In the background, Array.sort() takes the arguments given it, then, one at a time, runs the compareFunction (in this case, a-b) and decides what to do based on the answer given.

If the answer is less than 0, then a moves to a lower index than b. Greater than 0 moves b to a lower index than a. An answer of 0 means to leave the arguments where they are.

Think of it this way:

var array = [1,3,5,2];
// an array of 1, 3, 5 and 2

function compareFunc(a, b) {
  return a-b;
};
// the comparison function we're using

array.sort(compareFunc);
// Calls this:
// 1-3 = -2, so 1 (a) is a lower index than 3 (b)
// 3-5 = -2, so 3 (a) is a lower index than 5 (b)
// 5-2 = 3, so 5 (a) is a higher index than 2 (b)
// The array now looks like this: [1,3,2,5]
// Array.sort() calls compareFunc() over and over again
  // until every return is either a negative number or 0
  // (or if it gets a different result from the same values
  // in which case it returns undefined

Hope that makes sense.

Answer 2 · 2014-12-09T01:30:18Z

December 9, 2014 1:30am

This seems to be what is commonly called a 'bubble sort': http://en.wikipedia.org/wiki/Bubble_sort. The animation on the right of that wiki page clarifies the process, although I think the video example method starts at the other end of the list (array), and works backwards (from the right) in comparison to the wiki example, by swapping adjacent items if the left item of an adjacent pair is higher than the right item.

The video used here is not clear on how this method works, especially as it does not mention that this process is repeated along the array (or list) until no further changes are needed. For those with no knowledge of sorting, really it is just saying "this code will sort an array".

There are many other types of sorting algorithms, and choosing the right one can be important especially when ordering large lists (as the sorting process may take a long time, and the 'right' algorithm may reduce this time significantly).

Answer 3 · 2015-02-01T05:01:39Z

February 1, 2015 5:01am

Thanks for the clarification, this didn't make sense to me either until it was more thoroughly explained!

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Joe Williams

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I don't understand why creating a function that subtracts a-b then sorts the numbers in the array.

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Ryan Boone

Ryan Boone

David Lin

David Lin

Simon Tharby

Simon Tharby

huckleberry

huckleberry

Craig Neering

Craig Neering