Array#sort -block with conditions for <=>'ing

ruby-talk
1

Hello Rubyists,

I want to sort an array of objects of the same class (MyClass). The
array should be sorted by arbitrary attributes of that class, but the
problem is that one attribute can be nil.

The code looks like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute
}

Naturally, if some_attribute is nil, the comparison failed (since nil is
not comparable). I am looking for something like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute unless
(x.some_attribute.nil? or y.some_attribute.nil?)
}

which I have tried without having success (Error was: comparison of
MyClass with MyClass failed).

Does anyone know how to do it? Thanks in advance!

···

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

2

Define <=> on your class:

class MyClass
  def <=>(other)
    if @some_attribute.nil?
      return -1
    elsif @other.some_attribute.nil?
      return 1
    else
      @some_attribute <=> other.some_attribute
    end
  end
end

Farrel

···

On 08/03/07, carp __ <carp@hacksocke.de> wrote:

Hello Rubyists,

I want to sort an array of objects of the same class (MyClass). The
array should be sorted by arbitrary attributes of that class, but the
problem is that one attribute can be nil.

The code looks like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute
}

Naturally, if some_attribute is nil, the comparison failed (since nil is
not comparable). I am looking for something like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute unless
(x.some_attribute.nil? or y.some_attribute.nil?)
}

which I have tried without having success (Error was: comparison of
MyClass with MyClass failed).

Does anyone know how to do it? Thanks in advance!

3

You have to return something from the comparison. Try something like:

my_array.sort { |x,y|
  x1 = x.some_attribute
  y1 = y.some_attribute
  if (not x1.nil?) and (not y1.nil?)
    x1 <=> y1
  elsif x1.nil? and y1.nil?
    0
  elsif x1.nil?
    -1
  else
    1
  end
}

···

On Thu, Mar 08, 2007 at 10:42:44PM +0900, carp __ wrote:

The code looks like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute
}

Naturally, if some_attribute is nil, the comparison failed (since nil is
not comparable). I am looking for something like this:

my_array.sort {
  >x,y| x.some_attribute <=> y.some_attribute unless
(x.some_attribute.nil? or y.some_attribute.nil?)
}

which I have tried without having success (Error was: comparison of
MyClass with MyClass failed).

4

Thank you Brian, the solution you offered solved my problems!

···

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

5

I find your style of mixing imperative and functional return
parameters odd. I would have expected either:

if foo
  -1
elsif bar
  1
else
  x <=> y
end

OR

if foo
  return -1
elsif bar
  return 1
else
  return x <=> y
end

but not the combination you have above. FWIW, I favor the former
style, as it's just a hair faster.

···

On Mar 8, 5:52 am, "Farrel Lifson" <farrel.lif...@gmail.com> wrote:

Define <=> on your class:

class MyClass
  def <=>(other)
    if @some_attribute.nil?
      return -1
    elsif @other.some_attribute.nil?
      return 1
    else
      @some_attribute <=> other.some_attribute
    end
  end
end