Question

I’m new to Ruby, so apologies if this sounds really silly.

I can’t seem to figure out how to write a “main” code and have methods in the same file (similar to C). I end up with a “main” file which loads a seperate file that has all the methods. I appreciate any guidance on this.

I spotted the following SO post but I don’t understand it:
Should I define a main method in my ruby scripts?

While it’s not a big deal, it’s just easier being able to see all the relevant code in the same file. Thank you.

[-EDIT-]

Thanks to everyone who responded – turns out you just need to define all the methods above the code. An example is below:

def callTest1
    puts "in test 1"
end

def callTest2
    puts "in test 2" 
end

callTest1
callTest2

I think this makes sense as Ruby needs to know all methods beforehand. This is unlike C where there is a header file which clearly list the available functions and therefore, can define them beneath the main() function

Again, thanks to everyone who responded.

Answer 1

@Hauleth’s answer is correct: there is no main method or structure in Ruby. I just want to provide a slightly different view here along with some explanation.

When you execute ruby somefile.rb, Ruby executes all of the code in somefile.rb. So if you have a very small project and want it to be self-contained in a single file, there’s absolutely nothing wrong with doing something like this:

# somefile.rb

class MyClass
  def say_hello
    puts "Hello World"
  end
end

def another_hello
  puts "Hello World (from a method)"
end

c = MyClass.new
c.say_hello
another_hello

It’s not that the first two blocks aren’t executed, it’s just that you don’t see the effects until you actually use the corresponding class/method.

The if __FILE__ == $0 bit is just a way to block off code that you only want to run if this file is being run directly from the command line. __FILE__
is the name of the current file, $0 is the command that was executed by the shell (though it’s smart enough to drop the ruby), so comparing the two tells you precisely that: is this the file that was executed from the command line? This is sometimes done by coders who want to define a class/module in a file and also provide a command-line utility that uses it. IMHO that’s not very good project structure, but just like anything there are use cases where doing it makes perfect sense.

If you want to be able to execute your code directly, you can add a shebang line

#!/usr/bin/env ruby

# rest of somefile.rb

and make it executable with chmod +x somefile.rb (optionally rename it without the .rb extension). This doesn’t really change your situation. The if __FILE__ == $0 still works and still probably isn’t necessary.

Edit

As @steenslag correctly points out, the top-level scope in Ruby is an Object called main. It has slightly funky behavior, though:

irb
>> self
=> main
>> self.class
=> Object
>> main
NameError: undefined local variable or method `main' for main:Object
from (irb):8

Don’t worry about this until you start to dig much deeper into the language. If you do want to learn lots more about this kind of stuff, Metaprogramming Ruby is a great read 🙂