Stable sort_by?

ruby-talk
1

Hi,

I have to sort a list using a number of criterions (a,b,c,d) where d
is the least important criterion and a is the most important one. All
comparisons should be 'higher number: better position'. This is what I
have tried:

···

--------------------------------------------------
require 'pp'

h= {"Team A"=>{:a=>3, :b=>2, :c=>6, :d=>114 },
    "Team B"=>{:a=>0, :b=>-2, :c=>4, :d=>112 },
    "Team C"=>{:a=>3, :b=>4, :c=>4, :d=>110 },
    "Team D"=>{:a=>3, :b=>4, :c=>4, :d=>108 },
}

tmp=h.to_a

[:d,:c,:b,:a].each do |criterion|
  tmp=tmp.sort_by { |a| a[1][criterion] }.reverse
end

pp tmp

--------------------------------------------------

[["Team D", {:d=>108, :a=>3, :b=>4, :c=>4}],
["Team C", {:d=>110, :a=>3, :b=>4, :c=>4}],
["Team A", {:d=>114, :a=>3, :b=>2, :c=>6}],
["Team B", {:d=>112, :a=>0, :b=>-2, :c=>4}]]

but as far as I can see 'Team C' should be better than 'Team D',
because of criterion d. Is it possible that sort_by is not stable? Or
is there something I did wrong?

Patrick

2

Patrick Gundlach wrote:

Hi,

I have to sort a list using a number of criterions (a,b,c,d) where d
is the least important criterion and a is the most important one. All
comparisons should be 'higher number: better position'. This is what I
have tried:

--------------------------------------------------
require 'pp'

h= {"Team A"=>{:a=>3, :b=>2, :c=>6, :d=>114 },
    "Team B"=>{:a=>0, :b=>-2, :c=>4, :d=>112 },
    "Team C"=>{:a=>3, :b=>4, :c=>4, :d=>110 },
    "Team D"=>{:a=>3, :b=>4, :c=>4, :d=>108 },
}

tmp=h.to_a

[:d,:c,:b,:a].each do |criterion|
  tmp=tmp.sort_by { |a| a[1][criterion] }.reverse
end

pp tmp

--------------------------------------------------

[["Team D", {:d=>108, :a=>3, :b=>4, :c=>4}],
["Team C", {:d=>110, :a=>3, :b=>4, :c=>4}],
["Team A", {:d=>114, :a=>3, :b=>2, :c=>6}],
["Team B", {:d=>112, :a=>0, :b=>-2, :c=>4}]]

but as far as I can see 'Team C' should be better than 'Team D',
because of criterion d. Is it possible that sort_by is not stable? Or
is there something I did wrong?

Wouldn't this be simpler ? :

h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

Best Regards,
Fred

···

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3

A follow-up:

--------------------------------------------------
require 'pp'

h= {"Team A"=>{:a=>3, :b=>2, :c=>6, :d=>114 },
    "Team B"=>{:a=>0, :b=>-2, :c=>4, :d=>112 },
    "Team C"=>{:a=>3, :b=>4, :c=>4, :d=>110 },
    "Team D"=>{:a=>3, :b=>4, :c=>4, :d=>108 },
}

this seems to be stable:

pp h.sort{ |a,b|
  res = a[1][:a] <=> b[1][:a]
  if res == 0
    res = a[1][:b] <=> b[1][:b]
  end
  if res == 0
    res = a[1][:c] <=> b[1][:c]
  end
  if res == 0
    res = a[1][:d] <=> b[1][:d]
  end
  res
}.reverse

But I still would like to know if sort_by is unstable (or if am I
doing something wrong).

Patrick

4

Or using Array instead of Hash:

I don't think sort is stable. You have to specify the sorting criteria,
and sort only once.

require 'test/unit'
class TestSortBy < Test::Unit::TestCase
  def test_all
    a= [["A", 3, 2, 6, 114],
        ["B", 0,-2, 4, 112],
        ["C", 3, 4, 4, 110],
        ["D", 3, 4, 4, 108]]

    assert_equal [["A", 3, 2, 6, 114],
                  ["C", 3, 4, 4, 110],
                  ["D", 3, 4, 4, 108],
                  ["B", 0, -2, 4, 112]],
                  a.sort_by {|name,x,y,z,w| [x, w] }.reverse
  end
end

Christer

···

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5

[...]

Wouldn't this be simpler ? :

h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

Oh, yes, that's so much nicer. Thanks.

Patrick

6

Hi --

···

On Tue, 13 Dec 2005, Frederick Ros wrote:

Wouldn't this be simpler ? :

h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

Or:

   h.sort_by {|k,v| v.values_at(:a,:b,:c,:d) }.reverse

(Possibly a little clearer visually.)

David

--
David A. Black
dblack@wobblini.net

"Ruby for Rails", forthcoming from Manning Publications, April 2006!

7

Frederick Ros wrote:

h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

OK, this or something like it should be in the rdoc for sort_by. I was
doing something similar and wound up writing a similar sort with
multiple if tests like the OP because I didn't remember that Array
implements a sane <=>.

Also, would there be a similar cool way to do this in one step where you
want a mix of ascending and descending sorts on different parts? Say I
had a hash (in YAML):

Bobby:
  age: 11
  lastname: Smith
Suzy:
  age: 13
  lastname: Jones
Ted:
  age 12
  lastname: Smith

And I wanted to sort
  - alphabetically by lastname
  - then age oldest to youngest

So in Perl I'd do something like:

my @sorted_keys = sort { $a->{lastname} cmp $b->{lastname}
                                        >> $b->{age} <=> $a->{age} }
keys %data;

In this case I could use

sorted_keys = data.sort_by { |k,v| [ v["lastname"], -1*v["age"] ] }

But what if I wanted reverse alphabetically by lastname (Z-A)? In perl
I'd swap $a and $b in the first comparison, but I can't think of a
spiffy way to do the same using sort_by.

···

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8

Patrick Gundlach <clr10.10.randomuser@spamgourmet.com> writes:

pp h.sort{ |a,b|
  res = a[1][:a] <=> b[1][:a]
  if res == 0
    res = a[1][:b] <=> b[1][:b]
  end
  if res == 0
    res = a[1][:c] <=> b[1][:c]
  end
  if res == 0
    res = a[1][:d] <=> b[1][:d]
  end
  res
}.reverse

Checkout Numeric#nonzero?.

···

Patrick

--
Christian Neukirchen <chneukirchen@gmail.com> http://chneukirchen.org

9

unknown wrote:

h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

Or:

   h.sort_by {|k,v| v.values_at(:a,:b,:c,:d) }.reverse

(Possibly a little clearer visually.)

Yeah ! This one rocks !

···

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10

class Reverser
  attr_accessor :obj

  def initialize(obj)
    @obj = obj
  end
  
  def <=>(other)
    other.obj <=> self.obj
  end
end

class Object
  def rev
    Reverser.new(self)
  end
end

require 'yaml'

data = YAML.load %{
Bobby:
   age: 11
   lastname: Smith
Suzy:
   age: 13
   lastname: Jones
Ted:
   age: 12
   lastname: Smith
}

p data

sorted_keys_0 = data.sort_by {|k,v| [v["lastname"].rev, v["age"]]}
sorted_keys_1 = data.sort_by {|k,v| [v["lastname"], v["age"].rev]}

p sorted_keys_0
p sorted_keys_1

martin

···

Mike Fletcher <lemurific+rforum@gmail.com> wrote:

In this case I could use

sorted_keys = data.sort_by { |k,v| [ v["lastname"], -1*v["age"] ] }

But what if I wanted reverse alphabetically by lastname (Z-A)? In perl
I'd swap $a and $b in the first comparison, but I can't think of a
spiffy way to do the same using sort_by.

11

pp h.sort{ |a,b|
  res = a[1][:a] <=> b[1][:a]
  if res == 0
    res = a[1][:b] <=> b[1][:b]
  end
  if res == 0
    res = a[1][:c] <=> b[1][:c]
  end
  if res == 0
    res = a[1][:d] <=> b[1][:d]
  end
  res
}.reverse

Checkout Numeric#nonzero?.

.... learning a new thing everyday... thanks.

Patrick

12

   h.sort_by {|k,v| v.values_at(:a,:b,:c,:d) }.reverse

(Possibly a little clearer visually.)

Yeah ! This one rocks !

Right, and it passes all my unittests :). So beautiful!

Patrick

13

Does the principle of 'least surprise' suggest that the default sort_by
algorithm should be modified to be stable?

It seems to me that a 'stable' algorithm is the expected behavior.

···

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