This chapter addresses a few practical issues.
Statement delimiters
Some languages require some kind of punctuation, often a semicolon
(;), to end each statement in a program. Ruby
instead follows the convention used in shells like sh and
csh. Multiple statements on one line must be
separated by semicolons, but they are not required at the end of a
line; a linefeed is treated like a semicolon. If a line ends
with a backslash (\), the linefeed following it is
ignored; this allows you to have a single logical line that spans
several lines.
Comments
Why write comments? Although well written code tends to be self-documenting, it is often helpful to scribble in the margins, and it can be a mistake to believe that others will be able to look at your code and immediately see it the way you do. Besides, for practical purposes, you yourself are a different person within a few days anyway; which of us hasn't gone back to fix or enhance a program after the passage of time and said, I know I wrote this, but what in blazes does it mean?
Some experienced programmers will point out, quite correctly, that contradictory or outdated comments can be worse than none at all. Certainly, comments shouldn't be a substitute for readable code; if your code is unclear, it's probably also buggy. You may find that you need to comment more while you are learning ruby, and then less as you become better at expressing your ideas in simple, elegant, readable code.
Ruby follows a common scripting convention, which is to use a pound
symbol (#) to denote the start of a comment. Anything
following an unquoted #, to the end of the line on which it
appears, is ignored by the interpreter.
Also, to facilitate large comment blocks, the ruby interpreter also
ignores anything between a line starting with "=begin" and
another line starting with "=end".
=begin
**********************************************************************
This is a comment block, something you write for the benefit of
human readers (including yourself). The interpreter ignores it.
There is no need for a '#' at the start of every line.
**********************************************************************
=end
Organizing your code
Ruby's unusually high level of dynamism means that classes, modules, and methods exist only after their defining code runs. If you're used to programming in a more static language, this can sometimes lead to surprises.
puts successor(3)
def successor(x)
x + 1
end
Although the interpreter checks over the entire script file for
syntax before executing it, the def successor ... end code
has to actually run in order to create the successor method.
So the order in which you arrange a script can matter.
This does not, as it might seem at first glance, force you to organize your code in a strictly bottom-up fashion. When the interpreter encounters a method definition, it can safely include undefined references, as long as you can be sure they will be defined by the time the method is actually invoked:
# down into two steps.
def f_to_c(f)
scale(f - 32.0) # This is a forward reference, but it's okay.
end
def scale(x)
x * 5.0 / 9.0
end
printf "%.1f is a comfortable temperature.\n", f_to_c(72.3)
So while this may seem less convenient than what you may be used to
in Perl or Java, it is less restrictive than trying to write C without
prototypes (which would require you to always maintain a partial
ordering of what references what). Putting top-level code at the
bottom of a source file always works. And even this is less of an
annoyance than it might at first seem. A sensible and painless way to
enforce the behavior you want is to define a main function at
the top of the file, and call it from the bottom.
def main
# Express the top level logic here...
end
# ... put support code here, organized as you see fit ...
main # ... and start execution here.
It also helps that ruby provides tools for breaking complicated
programs into readable, reusable, logically related chunks. We have
already seen the use of include for accessing modules. You
will also find the load and require facilities useful.
load works as if the file it refers to were copied and pasted
in (something like the #include preprocessor directive in
C). require is somewhat more sophisticated, causing code
to be loaded at most once and only when needed.
That's it...
This tutorial should be enough to get you started writing programs in Ruby. As further questions arise, you can get more help from the user community, and from an always-growing body of printed and online resources.
Good luck, and happy coding!