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>> 34.is_a?(Numeric)
=> true

>> "rah rah".is_a?(Numeric)
=> false

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>> "34".is_a?(Numeric)
=> false

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>> "34".to_i
=> 34

# All good so far...

>> "34DFDF".to_i
=> 34

# Ah. Fail.

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>> 34.is_numeric?
=> true

>> "34".is_numeric?
=> true

>> "34DFDF".is_numeric?
=> false

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# lib/core_extensions/object.rb

module CoreExtensions::Object
  def is_numeric?
    true if Float(self) rescue false
  end
end

Object.send(:include, CoreExtensions::Object)

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# config/environment.rb

require 'core_extensions/object'

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>> 34.is_numeric?
=> true
>> "I'm not a number".is_numeric?
=> false
>> "34".is_numeric?
=> true
>> "34DFDF".is_numeric?
=> false

Comments

1. 
   dohzya | June 22, 2009 at 12:33 PM

   I think adding class methods to Numeric would be a better idea than changing the Object class... You will be sure every libraries will work with your change
   
   Example, 2 methods, for testing is an object is a number or if it represents a number
   

   1 2	Numeric.number?("42") # => false Numeric.represents_number?("42") # => true

2. 
   http://antonjenkins.myopenid.com | June 22, 2009 at 12:57 PM

   @dohzya, thanks for taking the time to comment.
   
   I can see how your method is safer and cleaner from a sense that numerical based functionality is grouped with the Numeric class.
   
   But from an aesthetics point of view I prefer...
   
   

   1 2 3 4 5

   >> "42".is_a?(Numeric) => false >> "42".is_numeric? => true

   
   compared to
   
   

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   >> Numeric.number?("42") => false >> Numeric.represents_number?("42") => true

   
   Although now I've written the two side by side I think perhaps there's a case for renaming the is_numeric? method to something like represents_number? because I'm not literally checking to see if the object is Numeric, which is what the name implies. It could be misleading.
   
   Food for thought...
   
   Does anyone know of any potential nastiness happening by putting this method in Object? I'm assuming that the rescue would catch anything unexpected? If there is risk involved with having the method in Object then perhaps putting a class method in Numeric is the best answer?

3. 
   Ruben | June 22, 2009 at 1:18 PM

   Just a quick observation: using Float(obj) works if the obj method has a to_f method. Usually, when this method is defined, is because it acts as numeric in some way
   
   

   1 2 3 4 5

   >> Time.now.is_numeric? => true >> Time.now.to_f => 1245676360.47856

   
   but it may not be exactly what you want.

4. 
   http://antonjenkins.myopenid.com | June 22, 2009 at 2:05 PM

   Good point Ruben. Maybe some type checking in the method could eradicate that...
   
   

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   def is_numeric? return true if self.is_a?(Numeric) or (self.is_a?(String) and Float(self)) rescue return false return false end

   
   So now we're constraining the method to only work with Numeric and String classes. If it isn't one of these classes it returns false.
   
   I'm not sure that's the most elegant way of writing the method but it cures the problem with the Time class you've pointed out. Thanks for bringing that one up.

5. 
   http://antonjenkins.myopenid.com | June 22, 2009 at 2:18 PM

   This is a little neater than above...
   
   

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   def represents_number? self.is_a?(Numeric) or (self.is_a?(String) and Float(self)) ? true : false rescue false end

   
   Edit : Changed method name to represents_number? as it's a more accurate name for what the method is doing.

6. 
   Eric Anderson | June 22, 2009 at 2:42 PM

   I have defined a similar method that seems to work well for me but haven't tested it extensively so I welcome any problems anyone sees with this:
   
   

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   class String def is_number? to_i.to_s == self end end

   
   So the basically idea is if it can go to an integer and back to a string and still be the same value then it is a number.

7. 
   Vít Ondruch | June 22, 2009 at 2:53 PM

   Another variant ....
   
   def is_numeric?
   self.is_a?(Numeric) or (self.is_a?(String) and !!Float(self))
   rescue
   false
   end

8. 
   John Schank | June 22, 2009 at 2:55 PM

   @Eric
   
   That doesn't seem to work for floats.
   
   i.e. "0.4".is_number? => false

9. 
   http://antonjenkins.myopenid.com | June 22, 2009 at 3:13 PM

   @Eric, I'm guessing because it uses to_i it isn't going to work with floats, as John points out.
   
   @Vit, nice little addition. I didn't think of using the result of the condition as the return value and it gets rid of that ternary operator. I don't think we'll be able to get it much neater than that!

10. 
    Joe Grossberg | June 22, 2009 at 3:31 PM

    When dealing with integer-like strings, what's wrong with: ... foo.to_i.to_s == foo ... ?
    
    Also, don't monkeypatch Object, for God's sake.

11. 
    http://antonjenkins.myopenid.com | June 22, 2009 at 3:40 PM

    > what's wrong with: ... foo.to_i.to_s == foo ... ?
    
    It's not the most self documenting statement in the world. Unless you comment that well the average noob wouldn't have a clue what's going on. Plus, as pointed out above, it falls over as soon as foo becomes a float which could be a problem depending on your situation.
    
    > Also, don't monkeypatch Object, for God's sake.
    
    @Joe, I'm interested why you think it would be such a bad idea in this case. If you've read the other comments I'm now constraining the method to only work with instances of String or Numeric so we shouldn't have any unexpected behaviour when called on different classes.
    
    I hear a lot of people being wary of patching at high levels such as Object and sometimes I can see their point. But in this instance I'm not sure I see any massive disadvantage and would be really interested in what people see the pitfalls as being. I appreciate the advice not to do it, I'm just curious why not.

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