Case when and class

ruby-talk
1

I've found in case / when if the case is about a class :

case my_var.class
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

doesn't work i do have to use :

case my_var.class.to_s
when 'String' then puts my_var
when 'Array' then my_var.each {|v| puts v}
end

why ?

(with ruby 1.9.x)

···

--
« Le verbe aimer est difficile à conjuguer :
  son passé n'est pas simple, son présent n'est qu'indicatif,
  et son futur est toujours conditionnel. »
  (Jean Cocteau)

2

I've found in case / when if the case is about a class :

case my_var.class
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

doesn't work i do have to use :

case my_var.class.to_s
when 'String' then puts my_var
when 'Array' then my_var.each {|v| puts v}
end

why ?

"when" condition in case statement uses === to compare objects. For
strings a = b is true if a and b have the same characters, however
String === String is false:

a = "Hello"

=> "Hello"

b = "Hello"

=> "Hello"

a === b

=> true

String === String

=> false

BTW "===" is a method:

a.===(b)

=> true

so you can define your own rules when objects should be considered equal:

class Marble
  attr_accessor :size, :color

  def initialize(size, color)
    self.size = size
    self.color = color
  end

  def ===(other)
    self.color == other.color
  end

end

m1 = Marble.new(0.5, "red")
m2 = Marble.new(0.5, "green")
m3 = Marble.new(0.7, "red")

case m1
when m2
  puts "Matching marble is m2"
when m3
  puts "Matching marble is m3"
else
  puts "No match"
end

The result is "Matching marble is m3"

Regards,
Rimantas

3

I've found in case / when if the case is about a class :

case my_var.class
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

doesn't work

case calls the method === on the when object passing the case object
as an argument. So the above is calling:

String.=== (my_var.class)

If you check this:

http://ruby-doc.org/core/classes/Module.html#M001666

it says it returns true if the object is *an instance* of this class.
So you don't have to pass the class, but the object itself:

case my_var
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

irb(main):003:0> def test var
irb(main):004:1> case var
irb(main):005:2> when String then puts var
irb(main):006:2> when Array then var.each {|v| puts v}
irb(main):007:2> end
irb(main):008:1> end
=> nil
irb(main):009:0> test "hello"
hello
=> nil
irb(main):010:0> test [1,2,3]
1
2
3
=> [1, 2, 3]

i do have to use :

case my_var.class.to_s
when 'String' then puts my_var
when 'Array' then my_var.each {|v| puts v}
end

You can also do this, but the above is cleaner.

Jesus.

···

2010/6/11 Une Bévue <unbewusst.sein@google.com.invalid>:

4

Because of the behavior of the Module #== operator -- which you can look it up at ruby-doc.org -- you can somply do this:

case my_var
when String then puts my_var
when Array ...
end

···

On 2010-06-10 23:41:05 -0700, Une Bévue said:

I've found in case / when if the case is about a class :

case my_var.class
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

doesn't work i do have to use :

case my_var.class.to_s
when 'String' then puts my_var
when 'Array' then my_var.each {|v| puts v}
end

why ?

(with ruby 1.9.x)

--
Rein Henrichs

http://reinh.com

5

For strings a = b is true if a and b have the same characters

err, I ment a === b . For strings a === b is the same as a == b.
That's mostly true for other objects too (if they don't override ===
),
but not always.

Regards,
Rimantas

6

Woops, that should be Module#=== of course.

···

On 2010-06-11 00:29:24 -0700, Rein Henrichs said:

On 2010-06-10 23:41:05 -0700, Une Bévue said:

I've found in case / when if the case is about a class :

case my_var.class
when String then puts my_var
when Array then my_var.each {|v| puts v}
end

doesn't work i do have to use :

case my_var.class.to_s
when 'String' then puts my_var
when 'Array' then my_var.each {|v| puts v}
end

why ?

(with ruby 1.9.x)

Because of the behavior of the Module #== operator -- which you can look it up at ruby-doc.org -- you can somply do this:

case my_var
when String then puts my_var
when Array ...
end

--
Rein Henrichs

http://reinh.com

7

yes of course, clear enough, thanks !

···

Jesús Gabriel y Galán <jgabrielygalan@gmail.com> wrote:

You can also do this, but the above is cleaner.

--
« Le verbe aimer est difficile à conjuguer :
  son passé n'est pas simple, son présent n'est qu'indicatif,
  et son futur est toujours conditionnel. »
  (Jean Cocteau)

8

ok, well undestood, thanks !

···

Rein Henrichs <reinh@reinh.com> wrote:

Woops, that should be Module#=== of course.

--
« Le verbe aimer est difficile à conjuguer :
  son passé n'est pas simple, son présent n'est qu'indicatif,
  et son futur est toujours conditionnel. »
  (Jean Cocteau)