How does sort work?

Ruby
1

I’m confused as to how the sort method works.

If I have:
a = [ “d”, “a”, “e”, “c”, “b” ]

I know that
a.sort {|x,y| x <=> y } (or just .sort)
= [“a”, “b”, “c”, “d”, “e”]

and
a.sort {|x,y| y <=> x }
= [“e”, “d”, “c”, “b”, “a”]

I understand how the <=> operator works, but what exactly is going on in
the block? What are x and y assigned to?

2

Chris S. wrote:

I’m confused as to how the sort method works.

If I have:
a = [ “d”, “a”, “e”, “c”, “b” ]

I know that
a.sort {|x,y| x <=> y } (or just .sort)
= [“a”, “b”, “c”, “d”, “e”]

and
a.sort {|x,y| y <=> x }
= [“e”, “d”, “c”, “b”, “a”]

I understand how the <=> operator works, but what exactly is going on in
the block? What are x and y assigned to?

Sorting arrays is a complex topic that has been given a lot of attention
in computer programming. The goal is to sort the array with the fewest
comparisons, i.e. to sort it as fast as possible. You can google the
topic and read about different sorting methods–you might start by
searching ‘bubble sort’.

The sort method uses an “algorithm” (or methodology) to sort the array.
In order to sort an array you have to compare the elements in the array.
The fewer times you have to compare the elements to arrive at a sorted
array the better. When ruby needs to compare two elements in the
process of sorting an array, ruby assigns the two elements to the
variables x and y. Your block then decides which of the two elements
should come first in the array. Ruby takes that result and proceeds
with the process of sorting the array.

It should be obvious to you that your block has to be called more than
once in order for ruby to sort your array–you can’t sort a 5 element
array by doing one comparison of two of the elements. But what is the
minimum number of comparisons ruby needs to make in order to sort your
array?

3

Chris S. wrote:

I’m confused as to how the sort method works.

If I have:
a = [ “d”, “a”, “e”, “c”, “b” ]

I know that
a.sort {|x,y| x <=> y } (or just .sort)
= [“a”, “b”, “c”, “d”, “e”]

and
a.sort {|x,y| y <=> x }
= [“e”, “d”, “c”, “b”, “a”]

I understand how the <=> operator works, but what exactly is going on in
the block? What are x and y assigned to?

to understand this, you can think of “sort” taking a function as a
parameter. and sort() will pass 2 numbers to this function, and
rearragne the elements according to the return value of -1, 0, or 1.

that’s it. to understand it fully, you can read about “iterators” in
Ruby. where you can take a block as a function, and “yield” arguments
to this function, and get back a return value.

4

to understand this, you can think of “sort” taking a function as a
parameter. and sort() will pass 2 numbers to this function, and
rearragne the elements according to the return value of -1, 0, or 1.

that’s it. to understand it fully, you can read about “iterators” in
Ruby. where you can take a block as a function, and “yield” arguments
to this function, and get back a return value.

I understand sorting and sorting algorithms - I was tripped up by the
function as a parameter thing. Thanks!

5

On Feb 2, 2008 8:02 PM, SpringFlowers AutumnMoon
[email protected] wrote:

and
a.sort {|x,y| y <=> x }
= [“e”, “d”, “c”, “b”, “a”]

I understand how the <=> operator works, but what exactly is going on in
the block? What are x and y assigned to?

to understand this, you can think of “sort” taking a function as a
parameter. and sort() will pass 2 numbers to this function, and
rearragne the elements according to the return value of -1, 0, or 1.

That’s right. Basically, #sort gives you, on each iteration, two
elements out of your list based on it’s own sort algorithm (I think it
uses a quicksort version?). Your code can do whatever it wants to
with these elements, as long as your block returns a -1, 0, or 1 each
time. For example, your sort may be based on a suffix instead of a
beginning character or number (i.e. hexagon vs pentagram when the
order I want is n-grams to come before n-gons). Thus, you can define
the characteristics that must be sorted by, and those characteristics
can be sophisticated if you need them to be.

Todd

6

On 03.02.2008 04:09, Chris S. wrote:

to understand this, you can think of “sort” taking a function as a
parameter. and sort() will pass 2 numbers to this function, and
rearragne the elements according to the return value of -1, 0, or 1.

that’s it. to understand it fully, you can read about “iterators” in
Ruby. where you can take a block as a function, and “yield” arguments
to this function, and get back a return value.

I understand sorting and sorting algorithms - I was tripped up by the
function as a parameter thing. Thanks!

Yeah, a block is an anonymous function.

You can see how it works if you insert a print statement:

irb(main):001:0> a = [ “d”, “a”, “e”, “c”, “b” ]
=> [“d”, “a”, “e”, “c”, “b”]
irb(main):002:0> a.sort {|x,y| p [x,y]; x<=>y}
[“d”, “e”]
[“e”, “b”]
[“d”, “b”]
[“a”, “d”]
[“c”, “d”]
[“b”, “a”]
[“a”, “c”]
[“b”, “c”]
=> [“a”, “b”, “c”, “d”, “e”]
irb(main):003:0>

A nice quiz might be to find out the sorting algorithm based on this
output. :slight_smile:

Kind regards

robert