Re: Basic Question: How do you check to see if gets is a number?

On Sunday 11 July 2010, Adam Bourg wrote:
> |I wrote a basic script that asks for your age then converts it into
> |months, days and weeks old. I wanted to add some sort of control so that
> |it would return an error if you didn't enter a number but rather a word
> |or a letter or something.
> |
> |Can you please give me pointers? I've done quite a bit of Googling and
> |am not sure how to do this.  When I enter a number or a letter it
> |returns the else statement. Please help! See attached for example of it
> |being run.
> |
> |Here's my script below:
> |
> |# Basic Program to ask for a persons age and out put that in months
> |# Writen by Adam Bourg on 2010-07-11
> |
> |# Define verables
> |months = 12
> |
> |# Question that asks for your age
> |puts "Hello, in the field below please enter your age:"
> |
> |# Get's is the ruby method that asks for your age, it gets the
> |information from # the terminal when you type
> |age = gets
> |
> |# This checks to see if you input a number, if you do it outputs the
> |results
> |# if you don't it asks you for it in a numerical format eg 21
> |if ( age.to_f.to_s == true )
> |  monthsold = age.to_i * months.to_i
> |  puts 'You are ' + monthsold.to_s + ' months old'
> |else
> |  puts 'Error, please enter a number!'
> |  end
> |

There are a couple of errors in your code. First of all, in ruby you never 
compare something with true (or false) in an if. That's because the only value 
which is == to true is... true (and the only value which is == to false is 
false). This explains why you keep getting the error message: you're comparing 
the string returned by the to_s method with true. These are different, so the 
== operator returns false, which leads to executing the "else" branch of the 
if.

The if statement in ruby works in the following way: it checks what value the 
condition evaluates to. If it's the *false* object or the *nil* object, then 
the "else" branch is executed. All other values cause the "if" branch to be 
executed. So, to test whether a condition is true, you simply write:

if some_expression
  ...
else
  ...
end

where some_expression is any piece of ruby code which doesn't evaluate to 
false or nil.

The first step to fix your code, then, is to remove everything left to the 
to_s inside the condition. You can also remove the brackets, as they're not  
needed around the condition in an if statement. 

This leaves us with:

if age.to_f.to_s

Now, the to_s method of any object will return a string which ruby will always 
consider a true value. This means that now we have the opposite problem we had 
before: the error message is never shown. Removing the to_s would help if 
String#to_f didn't return 0.0 (which is also considered a true value) in case 
the string doesn't represent a number.

The simplest way to tell whether a string represents a number is to use the 
Kernel#Float method, or Kernel#Integer if you want an integer number. Unlike 
String#to_f and String#to_i, which always return a number, these methods raise 
an exception if the string can't be translated to a number. So you can write:

#gets returns the string with a trailing newline, so you have to remove it
age = gets.strip

age_numeric = begin Integer(age)
rescue ArgumentError
  puts "Enter a valid number"
  exit
end
#months is already an integer, so there's no need to call to_i on it
monthsold = age_numeric * months

I hope this helps

Stefano