Test if a string is basically an integer in quotes

Question

I need a function, is_an_integer, where

"12".is_an_integer? returns true
"blah".is_an_integer? returns false

how can i do this in ruby? i would write a regex but im assuming there is a helper for this that i am not aware of

Answer 1

The Best and Simple way is using Float

val = Float "234" rescue nil

Float "234" rescue nil #=> 234.0

Float "abc" rescue nil #=> nil

Float "234abc" rescue nil #=> nil

Float nil rescue nil #=> nil

Float "" rescue nil #=> nil

Integer is also good but it will return 0 for Integer nil

Answer 2

A much simpler way could be

/(\D+)/.match('1221').nil? #=> true
/(\D+)/.match('1a221').nil? #=> false
/(\D+)/.match('01221').nil? #=> true

Answer 3

Might not be suitable for all cases but a simple

"12".to_i   => 12
"blah".to_i => 0

might also do for some.

So if it's a number and not 0 it will return a number. If it returns 0 it's either a word string or 0.

Answer 4

Although explicit use of the case equality operator should be avoided, Regexp#=== looks very clean here:

def integer?(str)
  /\A[+-]?\d+\z/ === str
end

integer? "123"    # true
integer? "-123"   # true
integer? "+123"   # true

integer? "a123"   # false
integer? "123b"   # false
integer? "1\n2"   # false

Answer 5

You can do a one liner:

str = ...
int = Integer(str) rescue nil

if int
  int.times {|i| p i}
end

or even

int = Integer(str) rescue false

Depending on what you are trying to do you can also directly use a begin end block with rescue clause:

begin
  str = ...
  i = Integer(str)

  i.times do |j|
    puts j
  end
rescue ArgumentError
  puts "Not an int, doing something else"
end

Answer 6

Solution:

# /initializers/string.rb
class String
  IntegerRegex = /^(\d)+$/

  def integer?
    !!self.match(IntegerRegex)
  end
end

# any_model_or_controller.rb
'12345'.integer? # true
'asd34'.integer? # false

Explanation:

• /^(\d)+$/is regex expression for finding digits in any string. You can test your regex expressions and results at http://rubular.com/.
• We save it in a constant IntegerRegex to avoid unnecessary memory allocation everytime we use it in the method.
• integer? is an interrogative method which should return true or false.
• match is a method on string which matches the occurrences as per the given regex expression in argument and return the matched values or nil.
• !! converts the result of match method into equivalent boolean.
• And declaring the method in existing String class is monkey patching, which doesn't change anything in existing String functionalities, but just adds another method named integer? on any String object.