User Guide #03 - Ruby - Simple examples

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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:

# The program finds the factorial of the number
# Save this file 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

% 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

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.CON ), you should use this eval.rb , that adds visual indenting assistance, warning reports, and color highlighting. Otherwise, look in the

sample

% 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 Ctrl + D still works too.

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