How do I check if user input is an integer in Ruby?

Question

I am trying to loop until user inputs an integer. When user inputs a letter, the following code should print "Think of a number":

print "Think of a number "

while user_input = gets.to_i
  if user_input.is_a? Integer
    puts "your number is #{user_input}"
    break
  else
    print "Think of a number "
  end 
end 

I succeeded with my code when user inputs an integer. However when user inputs a string, the to_i method returns 0, and does not execute the else statement because it is a number.

Answer 1

The main issue with your code is String#to_i method is omnivorous.

"0".to_i #⇒ 0
"0.1".to_i #⇒ 0
"foo".to_i #⇒ 0

That said, user_input in your code is always integer.

What you probably want is to accept digits only (and maybe a leading minus for negatives.) The only concise way to accept a subset of characters is a regular expression.

# chomp to strip out trailing carriage return
user_input = gets.chomp 

if user_input =~ /\A-?\d+\z/
  ...

The regular expression above means nothing save for digits with optional leading minus.

---

Or, even better (credits to @Stefan)

if gets =~ /\A-?\d+\Z/