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comp.lang.ruby

Implementation of the object.sort method.

Jorge Domenico Bucaran Romano

5/9/2007 3:54:00 AM

Hi,

Can you show me an implementation of the Array.sort method? This
implementation would be demonstrative only. This would handle when a
block is passed. The reason is I have trouble understanding why the
following code works as expected:

[3,2,1,4].sort do |a,b|
a <=> b
end

I know it will sort my array but I don't understand the way this method
works because other tests I have made bring none results.

Regards, Jorge.

--
Posted via http://www.ruby-....
10 Answers

Gavin Kistner

5/9/2007 4:55:00 AM

0

On May 8, 9:54 pm, Jorge Domenico Bucaran Romano <jbuca...@gmail.com>
wrote:
> Can you show me an implementation of the Array.sort method?

Slim:~ gkistner$ cd /usr/local/src/ruby-1.8.5-p12/
Slim:/usr/local/src/ruby-1.8.5-p12 gkistner$ cat array.c

[...snip...]

VALUE
rb_ary_sort(ary)
VALUE ary;
{
ary = rb_ary_dup(ary);
rb_ary_sort_bang(ary);
return ary;
}

[...snip...]

VALUE
rb_ary_sort_bang(ary)
VALUE ary;
{
rb_ary_modify(ary);
if (RARRAY(ary)->len > 1) {
FL_SET(ary, ARY_TMPLOCK); /* prohibit modification during sort */
rb_ensure(sort_internal, ary, sort_unlock, ary);
}
return ary;
}

[...snip...]

static VALUE
sort_internal(ary)
VALUE ary;
{
struct ary_sort_data data;

data.ary = ary;
data.ptr = RARRAY(ary)->ptr; data.len = RARRAY(ary)->len;
qsort(RARRAY(ary)->ptr, RARRAY(ary)->len, sizeof(VALUE),
rb_block_given_p()?sort_1:sort_2, &data);
return ary;
}

[...snip...]

static int
sort_1(a, b, data)
VALUE *a, *b;
struct ary_sort_data *data;
{
VALUE retval = rb_yield_values(2, *a, *b);
int n;

n = rb_cmpint(retval, *a, *b);
ary_sort_check(data);
return n;
}



There's your implementation. I suspect that's not what you wanted.
Could you try asking your question again using different words?
Perhaps then I will understand how to help you.

Does it help if you know what the Quicksort[1] algorithm is?

[1] http://en.wikipedia.org/wiki...

Robert Dober

5/9/2007 8:19:00 AM

0

On 5/9/07, Jorge Domenico Bucaran Romano <jbucaran@gmail.com> wrote:
> Hi,
>
> Can you show me an implementation of the Array.sort method? This
> implementation would be demonstrative only. This would handle when a
> block is passed. The reason is I have trouble understanding why the
> following code works as expected:
>
> [3,2,1,4].sort do |a,b|
> a <=> b
> end
>
> I know it will sort my array but I don't understand the way this method
> works because other tests I have made bring none results.
>
> Regards, Jorge.
>

Actually you do not need to know anything about the implementation.
Ruby uses a modified Quicksort as Phrogz pointed out.
The block is just delivering the operation *every* sorting algorithm
must eventually apply, "comparison".
The result of the block applied to two arbitrary elements of the array
(assigned to the parameters a and b) will determine if the LHS is
smaller, equal or greater than the RHS. (depending on the values <0, 0
or >0 respectively).

If you use the variation #sort_by the block will be applied to both
elements first and the results of these applications are compared than
(using <=> if I am not mistaken ).

If you understand the differnce bewteen
a.sort{ rand } and a.sort_by{ rand }
you have grasped the concept.

HTH
Robert


--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

Jorge Domenico Bucaran Romano

5/9/2007 2:02:00 PM

0

Hi,

I want a demonstrative implementation of the sort method to see how the
callback (passed block) is handled, specifically the comparison result
<=>.

For example, in the following code:

puts [4,5,3,2,1].sort do |a,b|
-1
end

puts [4,5,3,2,1].sort do |a,b|
1
end

Both print the array sorted down up. I don't understand how this is
possible so I wanted to see a demonstrative (but factual) implementation
of the sort method to see how this parameters are handled.

Regards, Jorge

--
Posted via http://www.ruby-....

Robert Dober

5/9/2007 2:32:00 PM

0

On 5/9/07, Jorge Domenico Bucaran Romano <jbucaran@gmail.com> wrote:
> Hi,
>
> I want a demonstrative implementation of the sort method to see how the
> callback (passed block) is handled, specifically the comparison result
> <=>.
>
> For example, in the following code:
>
> puts [4,5,3,2,1].sort do |a,b|
> -1
> end
>
> puts [4,5,3,2,1].sort do |a,b|
> 1
> end
>
> Both print the array sorted down up. I don't understand how this is
> possible so I wanted to see a demonstrative (but factual) implementation
> of the sort method to see how this parameters are handled.
What's wrong with the code Phrogz posted?
>
> Regards, Jorge
>
> --
> Posted via http://www.ruby-....
>
>
Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

Robert Klemme

5/9/2007 2:34:00 PM

0

On 09.05.2007 16:01, Jorge Domenico Bucaran Romano wrote:
> Hi,
>
> I want a demonstrative implementation of the sort method to see how the
> callback (passed block) is handled, specifically the comparison result
> <=>.
>
> For example, in the following code:
>
> puts [4,5,3,2,1].sort do |a,b|
> -1
> end
>
> puts [4,5,3,2,1].sort do |a,b|
> 1
> end
>
> Both print the array sorted down up. I don't understand how this is
> possible so I wanted to see a demonstrative (but factual) implementation
> of the sort method to see how this parameters are handled.

Basically since you do not used block parameters in calculating the
block result you cannot expect any particular order. The same code
might yield a different ordering with the next version of Ruby because
the result is just determined in what order the engine passes pairs to
the block.

Kind regards

robert

Ken Bloom

5/9/2007 2:38:00 PM

0

On Wed, 09 May 2007 12:54:26 +0900, Jorge Domenico Bucaran Romano wrote:

> Hi,
>
> Can you show me an implementation of the Array.sort method? This
> implementation would be demonstrative only. This would handle when a
> block is passed. The reason is I have trouble understanding why the
> following code works as expected:
>
> [3,2,1,4].sort do |a,b|
> a <=> b
> end
>
> I know it will sort my array but I don't understand the way this method
> works because other tests I have made bring none results.
>
> Regards, Jorge.

You begin by taking an ordinary sorting algorithm, such as the Quicksort
algorithm given at http://yagni.com/combsort/#ruby-inplace...

You will notice it uses the < operator for comparison. The semantics of
the <=> (spaceship) operator are such that a<b is equivalent to (a<=>b)<0
and that a<=b is equivalent to (a<=>b)<=0.

Once you have made that substitution in the code, you have

def partition(a, first, last)
pivot = a[first]
lastS1 = first
firstUnknown = first + 1
while firstUnknown <= last do
if (a[firstUnknown] <=> pivot) < 0 ##this is the only change
lastS1 += 1
a.swap(firstUnknown, lastS1)
end
firstUnknown += 1
end
a.swap(first, lastS1)
lastS1
end

def quicksort(a, first = 0, last = a.size - 1)
if first < last
pivotIndex = partition(a, first, last)
quicksort(a, first, pivotIndex - 1)
quicksort(a, pivotIndex + 1, last)
end
end

Once you have done this, you can replace the <=> operator with an
equivalent call to a block, as follows:

def partition(a, first, last, &block)
pivot = a[first]
lastS1 = first
firstUnknown = first + 1
while firstUnknown <= last do
if (block.call(a[firstUnknown], pivot)) < 0
lastS1 += 1
a.swap(firstUnknown, lastS1)
end
firstUnknown += 1
end
a.swap(first, lastS1)
lastS1
end

def quicksort(a, first = 0, last = a.size - 1, &block)
return quicksort(a,first,last){|a,b| a<=>b} unless block_given?
if first < last
pivotIndex = partition(a, first, last, &block)
quicksort(a, first, pivotIndex - 1, &block)
quicksort(a, pivotIndex + 1, last, &block)
end
end

I hope this helps.

--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu...

Gary Wright

5/9/2007 2:41:00 PM

0


On May 9, 2007, at 10:01 AM, Jorge Domenico Bucaran Romano wrote:
> I want a demonstrative implementation of the sort method to see how
> the
> callback (passed block) is handled, specifically the comparison result
> <=>.

Sorting is a *big* topic. Ultimately though it comes down to comparing
two elements in the array and then rearranging those elements based on
the result. As the algorithm proceeds it compares different pairs of
items until the list is completely sorted.

The common thread among all sorting algorithms is the need to decide
the order between two elements and this is the purpose of the block
provided to #sort:

[4,1,2,3].sort { |a,b| a <=> b } # => [1, 2, 3, 4]
[4,1,2,3].sort { |a,b| b <=> a } # => [4, 3, 2, 1]

So whenever sort needs to know the ordering of two objects, a and b,
its calls out to the block for the answer. If the block returns -1
then a is considered 'less than' b, 0 means they are equal, and 1
means that a is 'greater than' b. The block will be called *many*
times during the sort operations:

[4,1,2,3].sort { |a,b| puts "#{a}, #{b}"; a <=> b }

output:
4, 2
2, 3
4, 3
1, 3
2, 1

By using a block to compare the two items it allows the sorting
order to be based on whatever criteria is desired--as long as the
block returns -1, 0, or 1 the algorithm can figure the rest out.

It isn't clear if you are interested in the sorting algorithm itself,
the
mechanism by which the block is called and its result used or
something else
entirely but maybe I've provided enough information that you can
clarify your interest.

Gary Wright

Calamitas

5/9/2007 2:45:00 PM

0

On 5/9/07, Jorge Domenico Bucaran Romano <jbucaran@gmail.com> wrote:
> Hi,
>
> I want a demonstrative implementation of the sort method to see how the
> callback (passed block) is handled, specifically the comparison result
> <=>.
>
> For example, in the following code:
>
> puts [4,5,3,2,1].sort do |a,b|
> -1
> end
>
> puts [4,5,3,2,1].sort do |a,b|
> 1
> end
>
> Both print the array sorted down up. I don't understand how this is
> possible so I wanted to see a demonstrative (but factual) implementation
> of the sort method to see how this parameters are handled.

The block binds to the call to puts and not to the call to sort, or in
other words the above is equivalent to:

puts([4,5,3,2,1].sort) do |a,b|
-1
end

You can use parentheses to disambiguate, or use curly braces:

puts( [4,5,3,2,1].sort do
-1
end )

puts [4,5,3,2,1].sort { -1 }

puts( [4,5,3,2,1].sort { -1 } )

Peter

Brian Candler

5/9/2007 3:03:00 PM

0

On Wed, May 09, 2007 at 11:01:58PM +0900, Jorge Domenico Bucaran Romano wrote:
> I want a demonstrative implementation of the sort method to see how the
> callback (passed block) is handled, specifically the comparison result
> <=>.
>
> For example, in the following code:
>
> puts [4,5,3,2,1].sort do |a,b|
> -1
> end
>
> puts [4,5,3,2,1].sort do |a,b|
> 1
> end
>
> Both print the array sorted down up. I don't understand how this is
> possible

It's an artefact of the quicksort algorithm. If you lie to it about how the
elements compare, as you are doing above, then you'll get strange results.

> so I wanted to see a demonstrative (but factual) implementation
> of the sort method to see how this parameters are handled.

# Noddy sort
def mysort(arr, &blk)
blk ||= proc { |a,b| a <=> b } # default if no block passed

(0...arr.size-1).each do |i|
(i+1...arr.size).each do |j|
arr[i],arr[j] = arr[j],arr[i] if blk.call(arr[i],arr[j]) > 0
end
end
arr
end

p mysort([4,5,3,2,1]) { |a,b| puts "Comparing #{a} and #{b}"; a <=> b }

Please don't use this as an example of a good sort algorithm! But it shows
how to do callbacks.

B.

Jorge Domenico Bucaran Romano

5/9/2007 3:52:00 PM

0

Hi,

Thank you all for the replies and useful help. I came up with my demo
code.

class Array
def my_sort
if not block_given?
return self.sort!
end

(0...self.size).each do |i|
(i+1...self.size).each do |k|
self[i], self[k] = self[k], self[i] if
yield(self[i],self[k]) > 0
end
end
self
end
end

puts( [5,9,6,7,8,1,3,2,4,0].my_sort {|a,b| a <=> b})

Now it makes sense.

Regars, Jorge

--
Posted via http://www.ruby-....