Consecutive sort on Array of Hashes

Ruby
1

Hi,

Here my - strange - problem.

To explain it, let’s take the example of football. I construct an array
of hashes of the results with team_id, total of pts, number of wins,
number of draws and number of defeats such as:

myArray = Array.new
myArray << {:team_id=>1, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>2, :pts=>7, :w=>1, :d=>4, :l=>5}
myArray << {:team_id=>3, :pts=>4, :w=>0, :d=>4, :l=>6}
myArray << {:team_id=>4, :pts=>6, :w=>1, :d=>3, :l=>6}
myArray << {:team_id=>5, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>6, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>8, :pts=>10, :w=>2, :d=>4, :l=>4}
myArray << {:team_id=>9, :pts=>5, :w=>1, :d=>2, :l=>7}
myArray << {:team_id=>10, :pts=>8, :w=>1, :d=>5, :l=>4}
myArray << {:team_id=>11, :pts=>9, :w=>2, :d=>3, :l=>5}
myArray << {:team_id=>12, :pts=>6, :w=>1, :d=>3, :l=>6}
myArray << {:team_id=>13, :pts=>5, :w=>0, :d=>5, :l=>5}

Now, from this array, I want to get the table.
So what I want to do is to sort the array, first by total of pts, then
by number of wins (if 2 teams have the same total of points I put first
the team with more wins) and then by number of draws.

That’s how - logically - I would do it anyway. But that does not seem to
be the Ruby way. Doing it that way gives me a complete mess.

No worries, doing it the other way round works (first sort by draw, then
wins, then pts)! Here is what I run:

puts “Team, Pts, W, D, L”
myArray = myArray.sort { |a,b| b[:d] <=> a[:d]
}.sort { |a,b| b[:w] <=> a[:w]
}.sort { |a,b| b[:pts] <=> a[:pts]
}.each do |row|
puts “#{row[:team_id]}, #{row[:pts]}, #{row[:w]}, #{row[:d]},
#{row[:l]}”
end

Did I say it works? Well almost! Here is my output:

Team, Pts, W, D, L
8, 10, 2, 4, 4
11, 9, 2, 3, 5
10, 8, 1, 5, 4
2, 7, 1, 4, 5
4, 6, 1, 3, 6
12, 6, 1, 3, 6
16, 5, 0, 5, 5
1, 5, 0, 5, 5
9, 5, 1, 2, 7
5, 5, 0, 5, 5
6, 5, 0, 5, 5
3, 4, 0, 4, 6

Good news, we kept the correct team_is with the correct result.
But can anyone explain me what on earth is the line 9, 5, 1, 2, 7 doing
there in the middle?

Thanks for your help!

2

Laurent Colloud wrote:

puts “Team, Pts, W, D, L”
myArray = myArray.sort { |a,b| b[:d] <=> a[:d]
}.sort { |a,b| b[:w] <=> a[:w]
}.sort { |a,b| b[:pts] <=> a[:pts]
}.each do |row|
puts “#{row[:team_id]}, #{row[:pts]}, #{row[:w]}, #{row[:d]},
#{row[:l]}”
end

I suspect the problem is that sort does not keep the order of equal keys
– this is standard depending on which algorithim used.

you need to do one sort with a more complex comparison routine:

{|a,b| if b[:pts] == a[:pts] then
if b[:w] == a[:w] then
b[:d] <=> a[:d]
else
b[:w] <=> a[:w]
end
b[:pts] <=> a[:pts]
end
}

3

On Aug 14, 2006, at 4:04 PM, Laurent Colloud wrote:

myArray << {:team_id=>1, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>13, :pts=>5, :w=>0, :d=>5, :l=>5}

Now, from this array, I want to get the table.
So what I want to do is to sort the array, first by total of pts,
then
by number of wins (if 2 teams have the same total of points I put
first
the team with more wins) and then by number of draws.

Does this do what you were after?

$ irb -r pp

pp myArray.sort_by { |team| [team[:pts], team[:w], team
[:d]] }.reverse
[{:l=>4, :team_id=>8, :pts=>10, :w=>2, :d=>4},
{:l=>5, :team_id=>11, :pts=>9, :w=>2, :d=>3},
{:l=>4, :team_id=>10, :pts=>8, :w=>1, :d=>5},
{:l=>5, :team_id=>2, :pts=>7, :w=>1, :d=>4},
{:l=>6, :team_id=>12, :pts=>6, :w=>1, :d=>3},
{:l=>6, :team_id=>4, :pts=>6, :w=>1, :d=>3},
{:l=>7, :team_id=>9, :pts=>5, :w=>1, :d=>2},
{:l=>5, :team_id=>13, :pts=>5, :w=>0, :d=>5},
{:l=>5, :team_id=>6, :pts=>5, :w=>0, :d=>5},
{:l=>5, :team_id=>5, :pts=>5, :w=>0, :d=>5},
{:l=>5, :team_id=>1, :pts=>5, :w=>0, :d=>5},
{:l=>6, :team_id=>3, :pts=>4, :w=>0, :d=>4}]
=> nil

Hope that helps.

James Edward G. II

4

myArray << {:team_id=>1, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>13, :pts=>5, :w=>0, :d=>5, :l=>5}
be the Ruby way. Doing it that way gives me a complete mess.
puts "#{row[:team_id]}, #{row[:pts]}, #{row[:w]}, #{row[:d]},
4, 6, 1, 3, 6
2, 7 doing
there in the middle?

Thanks for your help!

Each of your sort’s just re-arranges the array by the new criteria,
completely unrelated to previous sorts. Why not simply to create an
appropriate criteria for a single sort:

[ DISCLAIMER: Untested code bellow ]

myArray = myArray.sort { |b, a|
diff = 0
[ :pts, :w, :d ].each do |_criteria|
diff = a[_criteria] - b[_criteria]
break unless diff.zero?
end
diff
}

Or even cooler :wink:

myArray = myArray.sort { |a, b|
[ :pts, :w, :d ].inject(0) { |_m, _c|
_m == 0 ? b[_c] - a[_c] : _m
}
}

Gennady.

5

Hi,

number of draws and number of defeats such as:
[…]

puts “Team, Pts, W, D, L”
myArray = myArray.sort { |a,b| b[:d] <=> a[:d]
}.sort { |a,b| b[:w] <=> a[:w]
}.sort { |a,b| b[:pts] <=> a[:pts]
}.each do |row|
puts “#{row[:team_id]}, #{row[:pts]}, #{row[:w]}, #{row[:d]},
#{row[:l]}”
end

try

myArray.sort {|a,b|
(b[:d] <=> a[:d]).nonzero? ||
(b[:w] <=> a[:w]).nonzero? ||
(b[:pts] <=> a[:pts])}

or (even better):

myArray.sort_by {|a| [a[:d], a[:w], a[:pts]]}

cheers

Simon

6

On Tue, 15 Aug 2006 07:02:52 +0900, Gennady B. wrote:

myArray << {:team_id=>1, :pts=>5, :w=>0, :d=>5, :l=>5}
myArray << {:team_id=>13, :pts=>5, :w=>0, :d=>5, :l=>5}
be the Ruby way. Doing it that way gives me a complete mess.
puts "#{row[:team_id]}, #{row[:pts]}, #{row[:w]}, #{row[:d]},
4, 6, 1, 3, 6
2, 7 doing
there in the middle?

Thanks for your help!

Each of your sort’s just re-arranges the array by the new criteria,
completely unrelated to previous sorts. Why not simply to create an
appropriate criteria for a single sort:

Not of .sort is a stable sort. Then it will preserve the order of items
that are equal according to the current comparison. You would sort from
least significant to most significant exactly as he has done here.

From the results though, I conclude that .sort isn’t a stable sort.

Looking through the C code, I see that .sort is a quicksort.

–Ken

7

Hi guys,

First thank you very much for helping me out on that one.

So here are the results of all your suggestions. And the winner is…

       THEY WORK

By James:

myArray.sort_by { |team| [team[:pts], team[:w], team[:d]] }.reverse

By Gennady (nice one!):

myArray.sort { |b, a|
diff = 0
[ :pts, :w, :d ].each do |_criteria|
diff = a[_criteria] - b[_criteria]
break unless diff.zero?
end
diff
}

By Gennady (nice one again!):

myArray.sort { |a, b|
[ :pts, :w, :d ].inject(0) { |_m, _c|
_m == 0 ? b[_c] - a[_c] : _m
}
}

By Russell (with a few synthax corrections):

myArray.sort {|a,b|
if b[:pts] == a[:pts]
if b[:w] == a[:w]
b[:d] <=> a[:d]
else
b[:w] <=> a[:w]
end
else
b[:pts] <=> a[:pts]
end
}

        THEY DON'T WORK

myArray.sort_by {|a| [a[:d], a[:w], a[:pts]]}

-> Sorted by draws (:d) ASC (and then :w ASC, :pts ASC) but I could
deduct James’solution from the output of this so thanks anyway :).

myArray.sort {|a,b|
(b[:d] <=> a[:d]).nonzero? ||
(b[:w] <=> a[:w]).nonzero? ||
(b[:pts] <=> a[:pts])}

->Sorted by :d DESC (and then :w DESC, :pts DESC)

So thanks again very much.

Pfiouuu, I’ve learned a lot of Ruby in just one night! :slight_smile: