Let's write a function to compute factorials. The mathematical definition of n factorial is:

n! = 1                (when n==0)
   = n * (n-1)!       (otherwise)

In ruby, this can be written as:

You may notice the repeated occurrence of end. Ruby has been called "Algol-like" because of this. (Actually, the syntax of ruby more closely mimics that of a langage named Eiffel.) You may also notice the lack of a return statement. It is unneeded because a ruby function returns the last thing that was evaluated in it. Use of a return statement here is permissible but unnecessary.

Let's try out our factorial function. Adding one line of code gives us a working program:

Here, ARGV is an array which contains the command line arguments, and to_i converts a character string to an integer.

Does it work with an argument of 40? It would make your calculator overflow...

It does work. Indeed, ruby can deal with any integer which is allowed by your machine's memory. So 400! can be calculated:

We cannot check the correctness at a glance, but it must be right. :-)

The input/evaluation loop

When you invoke ruby with no arguments, it reads commands from standard input and executes them after the end of input:

The ^D above means control-D, a conventional way to signal end-of-input in a Unix context. In DOS/Windows, try pressing F6 or ^Z instead.

Ruby also comes with a program called eval.rb that allows you to enter ruby code from the keyboard in an interactive loop, showing you the results as you go. It will be used extensively through the rest of this guide.

If you have an ANSI-compliant terminal (this is almost certainly true if you are running some flavor of UNIX; under old versions of DOS you need to have installed ANSI.SYS or ANSI.COM; Windows XP, unfortunately, has now made this nearly impossible), you should use this enhanced eval.rb that adds visual indenting assistance, warning reports, and color highlighting. Otherwise, look in the sample subdirectory of the ruby distribution for the non-ANSI version that works on any terminal. Here is a short eval.rb session:

hello world is produced by puts. The next line, in this case nil, reports on whatever was last evaluated; ruby does not distinguish between statements and expressions, so evaluating a piece of code basically means the same thing as executing it. Here, nil indicates that puts does not return a meaningful value. Note that we can leave this interpreter loop by saying exit, although ^D still works too.

Throughout this guide, "ruby>" denotes the input prompt for our useful little eval.rb program.