Why does numberArray.sort() not sort numbers correctly in JavaScript?

Question

I'm practicing the data visualization library d3.js, and I am using a random data generator identical to the following:

function generateRandomData() {
    var i,
        data = [];
    for (i = 0; i < 100; i += 1) {
        data.push(Math.random() * 100);
    }
    return data;
}

I store the value and try to sort it as shown below:

var data = generateRandomData();
data.sort();

Unfortunately, the sorted dataset is not sorted completely - some of the values are actually incorrect. For example, I would have numbers such as [12, 15, 18, 21, 3, 18 ...]. What is the cause of the sort function's inaccuracy?

Note: I found a proper solution, which solved my problem:

data.sort(function (a, b) { return b - a; });

I simply want to know why sort() is unreliable.

nhahtdh · Accepted Answer · 2012-06-27 03:00:42Z

2

sort() function considers everything in the Array as String if it is called without custom ordering function.

The only exception is undefined, which is from Javascript 1.2 always the "largest" item in the sorted array.

[void(0),342,4.5,"ahahaha",null,"zzz", "nen", "nup", 0/0, "KDSFL", "-sdfh",-3324, "-%",Array.prototype.sort,window,Array.zzz,"[oooooo]", "undefined",0].sort()

Reference:

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/sort

edited Jun 27, 2012 at 3:00

answered Jun 27, 2012 at 2:43

nhahtdh

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6 Comments

user166390 Over a year ago

Unfortunately, while correct (+1), this disagrees with the "results" given in the post...

jackwanders Over a year ago

OP's example result of [12, 15, 18, 21, 3, 18 ...] is likely just an example, as running sort() on that array (minus the ellipses) results in [12,15,18,18,21,3], which is the proper sort order assuming all values are strings

jackwanders Over a year ago

@pst true; i thought you were questioning the validity of the answer

Jim Johnson Over a year ago

@jackwanders Yes, you're right, my "result" was just an example, because I didn't want to copy and paste the long result that I had. So why is it that it matters that they're strings and not numbers?

jackwanders Over a year ago

If you don't provide sort() with a sorting function, it converts all values to strings and runs a alphabetic comparison. Therefore, 12 < 2. If you want to sort numerically, you have to provide a sorting function

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