Question

What is the easiest way to find out in Rails 3 whether a string str contains a positive float number or not ? (str is not an attribute in an active-record model)

It should work like this:

str = "123"         =>     true
str = "123.456"     =>     true
str = "0"           =>     true
str = ""            =>     false
str = "abcd"        =>     false
str = "-123"        =>     false

Answer 1

Here’s one idea:

class String
  def nonnegative_float?
    Float(self) >= 0
  rescue ArgumentError
    return false
  end
end

However, since you already seem to have a pretty good idea of what a nonnegative float number looks like, you could also match it against a Regexp:

class String
  def nonnegative_float?
    !!match(/\A\+?\d+(?:\.\d+)?\Z/)
  end
end