Ruby comes with a comprehensive I/O library. However, in most of the
examples in this book we'll stick to a few simple methods. We've
already come across two methods that do output.
puts writes each
of its arguments, adding a newline after each.
print also writes
its arguments, but with no newline. Both can be used to write to any
I/O object, but by default they write to the console.
Another output method we use a lot is
printf, which prints
its arguments under the control of a format string (just like
printf in C or Perl).
printf "Number: %5.2f, String: %s", 1.23, "hello"
|
produces:
Number: 1.23, String: hello
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In this example, the format string
"Number: %5.2f, String: %s"
tells
printf to substitute in a floating point number
(allowing five characters in total, with two after the decimal point)
and a string.
There are many ways to read input into your program. Probably the most
traditional is to use the routine
gets, which returns the next
line from your program's standard input stream.
The
gets routine has a side effect: as well as returning the line
just read, it also stores it into the global variable
$_. This variable is special, in that it is used as the
default argument in many circumstances. If you call
print with no
argument, it prints the contents of
$_. If you write an
if
or
while statement with just a regular expression as the
condition, that expression is matched against
$_. While viewed
by some purists as a rebarbative barbarism, these abbreviations can
help you write some concise programs. For example, the following
program prints all lines in the input stream that contain the word
``Ruby.''
while gets # assigns line to $_
if /Ruby/ # matches against $_
print # prints $_
end
end
|
The ``Ruby way'' to write this would be to use an iterator.
ARGF.each { |line| print line if line =~ /Ruby/ }
|
This uses the predefined object
ARGF, which represents the
input stream that can be read by a program.
