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1

The JavaScript sort function which takes a parameter allows one to pass in a function.

For example:

var myarray=[25, 8, 7, 41]
myarray.sort(function(a,b){return a - b}) //Array now becomes [7, 8, 25, 41]

How is it that the code

function(a,b)
{
return a - b
}

is interpreted to be ascending? It's supposed to be divided into three cases, <0 , ==0, and >0 ; but how does this make sense when a and b can be anything?

Thank You!

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4 Answers 4

up vote 3 down vote accepted

The reason answering your question is especially tricky, or at least in detail, is because there is no specification that says which sorting algorithm a browser should implement. So telling you specifically how it works on one browser may differ between browsers, or even change over time.

The gist of it is though, you want to think of "a" and "b" as being any two values. If you are returning the result of "a" - "b", then your sort goes in ascending order. If you do "b" - "a", then it is in descending order.

The neat thing about making your own sort functions, is that you can compare the value of "a" and "b" after processing them in a separate function. So lets say you want to sort by the Celsius values, but your array in only in Fahrenheit. You can do something like:

.sort(function(a,b){ return to_fahrenheit(a) - to_fahrenheit(b);}
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That's really cool , how you can create a custom sort function! Thank you ! –  Coffee Mar 22 '12 at 22:42
    
@sch - Thanks so much, I will remember this well now! You guys are the best! –  Coffee Mar 22 '12 at 22:48
up vote 3 down vote

The function sort will call the compareFunction function multiple times and passes to items a and b to it. This will happen multiple times until the array is sorted.

The compare function should return:

  • 0 if a == b;
  • a positive number if a > b;
  • a negative number if b < a.

Now, let's have a look at the function in your code, we have a - b =:

  • 0 if a == b;
  • a positive number if a > b;
  • a negative number if b < a.

So it returns the expected result and the array will be sorted correctly.

For more information, have a look at the documentation.

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Thank you so much , sch. I get it now, and have also read the reference, very thorough! –  Coffee Mar 22 '12 at 22:41
    
Thanks so much, I will remember this well now! You guys are the best! –  Coffee Mar 22 '12 at 22:48
up vote 2 down vote

Its because if b is bigger than a, it will be less than 0. If a == b, it will return 0. Otherwise it will be a positive number.

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So it goes through every single pair of numbers and checks? –  Coffee Mar 22 '12 at 22:16
1  
it depends on how sort is implemented. –  Daniel A. White Mar 22 '12 at 22:17
    
Hmm I see, OK it's geetting clearer. thanks! –  Coffee Mar 22 '12 at 22:18
up vote 1 down vote

This is easier to understand with an example. Let's study each possible case:

  1. Let a=10 and b=20. So a - b is -10, and by convention we return a negative value if a < b, so we're good.
  2. Let a=20 and b=10. So a - b is 10, and by convention we return a positive value if a > b, so we're still good.
  3. Let a=10 and b=10. So a - b is 0, and by convention we return 0 if a == b, and everything works as expected!

In general: if a < b, a - b will always be negative; if a > b, a - b will always be positive; and if a == b, a - b will always be 0, as long as a and b are integer values.

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Thanks so much , Oscar! it clicked! –  Coffee Mar 22 '12 at 22:41

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