# Case when and class

**URL:** <https://www.ruby-forum.com/t/case-when-and-class/190002>\
**Category:** Ruby\
**Created:** [June 11, 2010, 8:45am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002 "2010-06-11T08:45:16Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 8:45am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/1 "2010-06-11T08:45:16Z")

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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)

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**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 9:29am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/2 "2010-06-11T09:29:45Z")

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> when ‘String’ then puts my\_var  
> when ‘Array’ Â 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

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<div class="post-metadata">

**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 9:30am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/3 "2010-06-11T09:30:55Z")

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2010/6/11 Une Bévue [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> 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](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.

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<div class="post-metadata">

**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 9:32am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/4 "2010-06-11T09:32:06Z")

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

> 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](http://ruby-doc.org) – you can somply do this:

case my\_var  
when String then puts my\_var  
when Array …  
end

---

<div class="post-metadata">

**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 9:55am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/5 "2010-06-11T09:55:31Z")

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

> > 
> 
> look it up at [ruby-doc.org](http://ruby-doc.org) – you can somply do this:
> 
> case my\_var  
> when String then puts my\_var  
> when Array …  
> end

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

---

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**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 10:59am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/6 "2010-06-11T10:59:20Z")

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Jesús Gabriel y Galán [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

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

yes of course, clear enough, thanks !

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**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 9:35am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/7 "2010-06-11T09:35:37Z")

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> 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

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**Author:** ![Une\_BSSSSvue](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/b4bc9f/32.png) [@Une\_BSSSSvue](https://www.ruby-forum.com/u/Une_BSSSSvue)\
**Post date:** [June 11, 2010, 10:59am UTC](https://www.ruby-forum.com/t/case-when-and-class/190002/8 "2010-06-11T10:59:55Z")

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Rein H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

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

ok, well undestood, thanks !