Ruby - Simple examples
| | 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. :-)
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.
