Sorting an number array

Question

When I want to sort an array, using sort() function , it is giving an alphabetically sorted array. Eg.

var a=[9,10,1];
a.sort();

I'm getting a = [1,10,9]

So, as per the suggestions I used another function

function sortfunction(x, y){
    return (x - y) //causes an array to be sorted numerically and ascending
}

and then used

a.sort(sortfunction);

to get the right result.

Can anyone explain in detail, how this works?

Isn't it explained in detail in the Javascript specification? — Barmar, Jul 30 '13 at 02:52
@Barmar—a reference would be handy—[ECMA-262§15.4.4.11](http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.11). — RobG, Jul 30 '13 at 02:54

score 3 · Accepted Answer · edited May 23 '17 at 11:57

3

The first version fails as they're compared like they're strings ("9" is greater than "10"), known as a lexicographic sort.

The custom comparator function is called with a and b being members of the array.

Depending on what is returned, the members are shifted. If 0 is returned, the members are considered equivalent, if a negative number, then a is less than b and the inverse if it's a positive number.

If you want to visualise this, you can always log a and b to the console and observe how they're compared (and note how there is never any redundant comparisons made).

This is all backed by a sorting algorithm, which is left up to the implementation to choose. Chrome, for example, uses different algorithms depending on the type of members.

edited May 23 '17 at 11:57

Community

1
1

answered Jul 30 '13 at 02:52

alex

479,566
201
878
984

How does it work when `x - y` will always be `true` if x and y are not the same? – Derek 朕會功夫 Jul 30 '13 at 02:54
When is `x - y` ever true if they're numbers? – alex Jul 30 '13 at 02:55
3

The comparison function is not boolean. It's tri-state: negative, 0, or positive. – Barmar Jul 30 '13 at 02:56
Oh `.sort` depends on negative or positive values. I thought it receives a Boolean instead. I totally forgot about it and I just read the spec. – Derek 朕會功夫 Jul 30 '13 at 02:58
so , sortfunction return either 0.+ve or -ve. and these are passed as a argument to a.sort(), what will be the first argument to a.sort()? will that be difference of first two elements of the array? – Lonely Jul 30 '13 at 03:01
5-2 is positive, 2-5 is negative. one flips position, one doesn't. – dandavis Jul 30 '13 at 03:01

Sorting an number array

1 Answers1

Related