19

I want to specify a custom block method to sort an object array by evaluating two properties. However, after many searches, I didn't find to any example without the <=> operator.

I want to compare a to b:

if a.x less than b.x return -1
if a.x greater than b.x return 1
if a.x equals b.x, then compare by another property , like a.y vs b.y

This is my code and it doesn't work:

ar.sort! do |a,b|
   if a.x < b.y return -1
   elseif a.x > b.x return 1
   else return a.y <=> b.y
end

This block is within a function return is exiting the function and returning -1.

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5
  • why don't you use sort method? Commented Mar 14, 2013 at 4:19
  • @Gashner I'm using it, but I'm intended to customize the evaluation. Commented Mar 14, 2013 at 5:32
  • Why is <=> ruled out? Because you want to do it the hard way? Commented Mar 14, 2013 at 7:35
  • The question is already answered but I want to point out that the problem with your code is due to wrong if syntax. You can simply change it to ``` if a.x < b.x return -1 elsif a.x > b.x return 1 else return a.x <=> b.x end ``` Make sure to end the if block. Also you are sometimes using b.y instead of b.x. Commented Dec 12, 2018 at 0:32
  • can someone explain me shortly what is (a.x, a.y, b.x and b.y). what is the use of that in .sort method . thank you for your response . Commented Oct 12, 2022 at 9:25

4 Answers 4

Reset to default
33

This will give you ascending order for x then for y:

points.sort_by{ |p| [p.x, p.y] }
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3 Comments

+1 for sort_by and Array#<=> which has exactly the semantics OP desires.
This is very elegant! Well done - should be the accepted answer, IMHO
sort_by will be slower for small arrays. Toss a big array or more complex accesses to the values that are the sort keys, and it can quickly outrun regular sort routines.
24

The best answer is provided by @AJcodez below: 

points.sort_by{ |p| [p.x, p.y] }

The "correct" answer I originally provided, while it technically works, is not code I would recommend writing. I recall composing my response to fit the question's use of if/else rather than stopping to think or research whether Ruby had a more expressive, succinct way, which, of course, it does.

With a case statement:

ar.sort do |a, b|
  case
  when a.x < b.x
    -1
  when a.x > b.x
    1
  else
    a.y <=> b.y
  end
end

With ternary:

ar.sort { |a,b| a.x < b.x ? -1 : (a.x > b.x ? 1 : (a.y <=> b.y)) }
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2 Comments

A multi-level ternary statement in ruby is discouraged for readability/maintenance reasons.
The answer below by AJcodez is better than mine here; use that instead.
0

This seems to work:

# Ascending
ar.sort! do |a,b|
  (a-b).abs() > 0 ? a-b : 0
end

# Descending
ar.sort! do |a,b|
  (a-b).abs() > 0 ? b-a : 0
end

There's no need for the spaceship operator ( <=> ) - it seems to be superfluous in Ruby (in terms of syntax efficiency), because Ruby considers 0 to be true, and for this usage specifically (above), you can just put the explicit 0 in the ternary operator.

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1 Comment

sort! isn't sort and will modify the receiver (ar in this case). While sort! is faster, that behavior is something to be aware of.
0

We can do it with more simple way

ar.sort! do |a,b|
  [a.x, a.y] <=> [b.x, b.y]
end

Or

ar.sort! do |a,b|
  result = a.x <=> b.x
  next if result != 0

  a.y <=> b.y
end
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