How does Array#sort work when a block is passed?

Question

I am having a problem understanding how array.sort{ |x,y| block } works exactly, hence how to use it?

An example from Ruby documentation:

   a = [ "d", "a", "e", "c", "b" ]    a.sort                     #=> ["a", "b", "c", "d", "e"]    a.sort { |x,y| y <=> x }   #=> ["e", "d", "c", "b", "a"] 

Answer 1

In your example

a.sort 

is equivalent to

a.sort { |x, y| x <=> y } 

As you know, to sort an array, you need to be able to compare its elements (if you doubt that, just try to implement any sort algorithm without using any comparison, no <, >, <= or >=).

The block you provide is really a function which will be called by the sort algorithm to compare two items. That is x and y will always be some elements of the input array chosen by the sort algorithm during its execution.

The sort algorithm will assume that this comparison function/block will meet the requirements for method <=>:

• return -1 if x < y
• return 0 if x = y
• return 1 if x > y

Failure to provide an adequate comparison function/block will result in array whose order is undefined.

You should now understand why

a.sort { |x, y| x <=> y } 

and

a.sort { |x, y| y <=> x } 

return the same array in opposite orders.

---

To elaborate on what Tate Johnson added, if you implement the comparison function <=> on any of your classes, you gain the following

1. You may include the module Comparable in your class which will automatically define for you the following methods: between?, ==, >=, <, <= and >.
2. Instances of your class can now be sorted using the default (ie without argument) invocation to sort.

Note that the <=> method is already provided wherever it makes sense in ruby's standard library (Bignum, Array, File::Stat, Fixnum, String, Time, etc...).

Answer 2

When you have an array of, let's say, integers to sort, it's pretty straightforward for sort method to order the elements properly - smaller numbers first, bigger at the end. That's when you use ordinary sort, with no block.

But when you are sorting other objects, it may be needed to provide a way to compare (each) two of them. Let's say you have an array of objects of class Person. You probably can't tell if object bob is greater than object mike (i.e. class Person doesn't have method <=> implemented). In that case you'd need to provide some code to explain in which order you want these objects sorted to sort method. That's where the block form kicks in.

people.sort{|p1,p2| p1.age <=> p2.age} people.sort{|p1,p2| p1.children.count <=> p2.children.count} 

etc. In all these cases, sort method sorts them the same way - the same algorithm is used. What is different is comparison logic.