Implementation of the object.sort method


#1

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.


#2

On Behalf Of Jorge Domenico Bucaran R.:

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

a <=> b

end

botp@pc4all:~$ qri array#sort
------------------------------------------------------------- Array#sort
array.sort -> an_array
array.sort {| a,b | block } -> an_array

 Returns a new array created by sorting self. Comparisons for the
 sort will be done using the <=> operator or using an optional code
 block. The block implements a comparison between a and b,
 returning -1, 0, or +1. See also Enumerable#sort_by.

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

and

irb(main):023:0> a
=> [“d”, “a”, “e”, “c”, “b”]
irb(main):024:0> a.sort do |x,y|
irb(main):025:1* case
irb(main):026:2* when x == “b”
irb(main):027:2> -1
irb(main):028:2> when x == “c”
irb(main):029:2> 1
irb(main):030:2> else
irb(main):031:2* x <=> y
irb(main):032:2> end
irb(main):033:1> end
=> [“b”, “a”, “d”, “e”, “c”]
irb(main):034:0>

here we are giving special treatments to “b” and “c”, where “b” always
come first in comparison while “c” always come last. The rest of the
elements follow the normal comparison using the <=> op.

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.

pls post problem codes/results so everyone may help.

kind regards -botp


#3

On May 8, 9:54 pm, Jorge Domenico Bucaran R. removed_email_address@domain.invalid
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/Quicksort


#4

On 5/9/07, Jorge Domenico Bucaran R. removed_email_address@domain.invalid wrote:

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


#5

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


#6

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

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-quicksort

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.


#7

On May 9, 2007, at 10:01 AM, Jorge Domenico Bucaran R. 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 W.


#8

On 09.05.2007 16:01, Jorge Domenico Bucaran R. wrote:

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


#9

On 5/9/07, Jorge Domenico Bucaran R. removed_email_address@domain.invalid wrote:

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


#10

On Wed, May 09, 2007 at 11:01:58PM +0900, Jorge Domenico Bucaran R.
wrote:

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.


#11

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