|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
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
statement. It is unneeded because a ruby function returns the
last thing that was evaluated in it. Use of a
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.to_i), "\n"
ARGV is an array which contains the command line
to_i converts a character string to an
% 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. :-)
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
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
should use this enhanced
that adds visual indenting assistance, warning reports, and color
highlighting. Otherwise, look in the
subdirectory of the ruby distribution for the non-ANSI version that
works on any terminal. Here is a short
% ruby eval.rb ruby> print "Hello, world.\n" Hello, world. nil ruby> exit
hello world is produced by
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,
^D still works too.
Throughout this guide, "
ruby>" denotes the input prompt
for our useful little