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Determine if a string is a valid float value

Asked 2009-06-23T18:49:14.730
31

Is there a way to simply check if a string value is a valid float value. Calling to_f on a string will convert it to 0.0 if it is not a numeric value. And using Float() raises an exception when it is passed an invalid float string which is closer to what I want, but I don't want to handle catching exceptions. What I really want is a method such as nan? which does exist in the Float class, but that doesn't help because a non-numeric string cannot be converted to a float without being changed to 0.0 (using to_f).

"a".to_f => 0.0

"a".to_f.nan? => false

Float("a") => ArgumentError: invalid value for Float(): "a"

Is there a simple solution for this or do I need to write code to check if a string is a valid float value?

ruby-on-rails ruby
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39

Here's one way:

class String
  def valid_float?
    # The double negation turns this into an actual boolean true - if you're 
    # okay with "truthy" values (like 0.0), you can remove it.
    !!Float(self) rescue false
  end
end

"a".valid_float? #false
"2.4".valid_float? #true

If you want to avoid the monkey-patch of String, you could always make this a class method of some module you control, of course:

module MyUtils
  def self.valid_float?(str)
    !!Float(str) rescue false
  end
end
MyUtils.valid_float?("a") #false
answered 2009-06-23T19:01:49.947

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