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Ruby user's guide | Nuts and bolts |
This chapter addresses a few practical issues.
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.
Ruby follows a common scripting convention, which is to use a pound
sign (#
) 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. Writing comments into
code is a very good practice, and one that is neglected by too many
programmers. Although well written code tends to be
self-documenting, it never hurts to scribble in the margins, and it's
almost always a mistake to believe that others will be able to look at
your code and immediately see it the way you do. (For practical
purposes, you yourself are "another 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? Make it easy on yourself; keep some notes.)
To facilitate large comment blocks, the ruby interpreter also
ignores anything between a line starting with "=begin
"
and another line starting with "=end
".
#!/usr/local/bin/ruby =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 print "Hello, world.\n"
The ruby interpreter processes code as it reads it. There is nothing like a compilation phase; if something hasn't been read yet, it is simply undefined.
# this results in an "undefined method" error: print successor(3),"\n" def successor(x) x + 1 end
However, this does not 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:
# Conversion of fahrenheit to celsius, broken # 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. You already know that
very long and complicated source files are undesirable in any
language; ruby provides load
and require
,
allowing you to break code up into clear, readable, and logically
related chunks. A nice consequence of this is that your program
can execute from near the top of a file while avoiding any problems
with forward references.
load 'f_to_c.rb' printf "%.1f is a bit warm for me.\n", f_to_c( 117 )
# f_to_c.rb # This file can be arranged in any way you like, since the interpreter # will read all of it before the main program needs any of it. def f_to_c(f) ... end
There are several topics that are not yet addressed in this tutorial, including:
initialize
(object constructor) methodto_s
and inspect
methodsBut by now you may be ready to dig into the reference manual to learn about ruby in more depth. The FAQ and Library reference are also important resources.
Good luck, and happy coding!
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