From: James Harrison <oscartheduck@...>
Date: 2010-08-01T15:16:33+09:00
Subject: Re: Argument Error


On Jul 31, 2010, at 3:46 PM 7/31/10, Hd Pwnz0r wrote:

> Hi.
> 
> I'm learning Ruby and I want to know how to make a script have an
> argument, i.e.
> 
> ruby script.rb 3 2
> 
> So far I have this:
> 
> abort "Correct syntax is ruby [script].rb [number to exponentially
> multiply by] [exponent]" unless ARGV.size == 2
> var = gets.chomp
> var2 = gets.chomp
> puts  (var.to_i ** var2)
> 
> It's a simple exponential calculator.
> 
> I get this error when trying to run it with 2 arguments (2 and 3).
> 
> exponent.rb:2:in `gets': No such file or directory - 2 (Errno::ENOENT)
> from exponent.rb:2:in `gets'        from exponent.rb:2:in `<main>'
> 
> 

gets reads from an IO stream, not the arguments on the command line:

http://ruby-doc.org/core/classes/IO.html#M002272

The arguments on the command line are stored in the ARGV array. That's why you check ARGV's size to verify that there are only 2 arguments.

So the code should read the options out of the ARGV array, not trying to gets things. There are a couple of ways to do this. My favourite looks like:

var = ARGV.shift
var2 = ARGV.shift

A couple of more complicated ways, but more flexible ways, of getting command line arguments are optionparser and getopts. I typically use getopts, with no real rationale behind it than that I learned it that way first.