Forum: Ruby Checking if input == Integer/String?

88b42adf4d7381dadbdbf9de331217d4?d=identicon&s=25 Ulrich Holtzhausen (ollie)
on 2009-01-11 23:15
Hi,

I'm all new to programming and all new to Ruby - hope I haven't made a
mistake by choosing this language.

I'm trying to teach myself by creating tiny (mostly useless) little
applications and then extend on them as I learn.

Today I started out and this is what I got:

#!/usr/bin/env ruby

#Favourite number
puts "What is your favourite number?"
num = gets.to_i
if num != Integer
  puts "Please type a number"
else
  better = num + 1
  puts "Hah! #{num} is pathetic! #{better} is a much better number!!!"
end

Somewhere along the line there's a mistake however, but I take it you
guru's can see what I'm trying to do here?

How can I check if the variable 'num' is an integer and if not, ask the
user to re-input with an integer?

Thanks in advance!
7a561ec0875fcbbe3066ea8fe288ec77?d=identicon&s=25 Sebastian Hungerecker (Guest)
on 2009-01-11 23:36
(Received via mailing list)
Ulrich Holtzhausen wrote:
> guru's can see what I'm trying to do here?
>
> How can I check if the variable 'num' is an integer and if not, ask the
> user to re-input with an integer?

to_i always returns an Integer (if the string isn't a valid number, it
just
returns 0). If you don't want that you can use Integer(str) which raises
an
exeption if the string is not a valid number.
Also note that 4 == Integer is false. Only Integer is == Integer.
num.is_a? Integer would work, but as I said, to_i always returns an
Integer,
so that would always be true.

HTH,
Sebastian
3afd3e5e05dc9310c89aa5762cc8dd1d?d=identicon&s=25 Tim Hunter (Guest)
on 2009-01-11 23:38
(Received via mailing list)
Ulrich Holtzhausen wrote:
> #!/usr/bin/env ruby
>
> Somewhere along the line there's a mistake however, but I take it you
> guru's can see what I'm trying to do here?
>
> How can I check if the variable 'num' is an integer and if not, ask the
> user to re-input with an integer?
>
> Thanks in advance!

The mistake is that Integer is a class in Ruby, so your test "num !=
Integer" is testing to see if num is not equal to the Integer class. It
never will be equal, of course.

The gets method always returns a string. You are calling .to_i on the
string to convert it to an integer, which will work the way you expect
as long as the string is sequence of digits (with an optional leading
sign, of course.) However, .to_i happily "converts" non-integer strings
as well. Bring up irb and try it yourself:

irb(main):001:0> "20lbs. of fertilizer".to_i
=> 20
irb(main):002:0> "x".to_i
=> 0

Oops! It ignored the non-digit characters after "20" and converted "x"
to 0. Not what you wanted to happen, but it's working as documented:

~$ ri String.to_i
------------------------------------------------------------ String#to_i
      str.to_i(base=10)   => integer
------------------------------------------------------------------------
      Returns the result of interpreting leading characters in str as an
      integer base base (2, 8, 10, or 16). Extraneous characters past
      the end of a valid number are ignored. If there is not a valid
      number at the start of str, 0 is returned. This method never
      raises an exception.

What you want is the Integer() method, which will raise an exception
when given an invalid argument. Then you catch the exception.

begin
    n = Integer(str)
rescue ArgumentError
    puts "Not an integer!"
end

Good luck!
88b42adf4d7381dadbdbf9de331217d4?d=identicon&s=25 Ulrich Holtzhausen (ollie)
on 2009-01-11 23:53
Thanks to both of you, I will try Integer(str) (just a bit unsure of how
to implement it but I'll give it a go and return here if I wasn't
successful.
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