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| Ruby user's guide | Simple examples |

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:

    def fact(n)
      if n == 0
        1
      else
        n * fact(n-1)
      end
    end

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:

    # Program to find the factorial of a number
    # Save this as fact.rb

    def fact(n)
      if n == 0
        1
      else
        n * fact(n-1)
      end
    end

    print fact(ARGV[0].to_i), "\n"

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

    % ruby fact.rb 1
    1
    % ruby fact.rb 5
    120

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

    % ruby fact.rb 40
    815915283247897734345611269596115894272000000000

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

    % ruby fact.rb 400
    64034522846623895262347970319503005850702583026002959458684
    44594280239716918683143627847864746326467629435057503585681
    08482981628835174352289619886468029979373416541508381624264
    61942352307046244325015114448670890662773914918117331955996
    44070954967134529047702032243491121079759328079510154537266
    72516278778900093497637657103263503315339653498683868313393
    52024373788157786791506311858702618270169819740062983025308
    59129834616227230455833952075961150530223608681043329725519
    48526744322324386699484224042325998055516106359423769613992
    31917134063858996537970147827206606320217379472010321356624
    61380907794230459736069956759583609615871512991382228657857
    95493616176544804532220078258184008484364155912294542753848
    03558374518022675900061399560145595206127211192918105032491
    00800000000000000000000000000000000000000000000000000000000
    0000000000000000000000000000000000000000000

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:

    % ruby
    print "hello world\n"
    print "good-bye world\n"
    ^D
    hello world
    good-bye world

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 the tutorial.

If you have an ANSI-compliant terminal (this is almost certainly true if you are running some flavor of UNIX; under DOS you need to have installed `ANSI.SYS` or `ANSI.COM`), 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:

    % ruby eval.rb
    ruby> print "Hello, world.\n"
    Hello, world.
       nil
    ruby> exit

`hello world` is produced by `print`. 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 `print` 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.

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