Perl 101 (Part 4) - Mind Games - Miscellaneous Stuff (
Page 10 of 10 )
The @ARGV
variable
------------------
Perl allows you to specify additional
parameters to your program on the command line - these parameters are stored in
a special array variable called @ARGV. Thus, the first parameter passed to a
program on the command line would be stored in the variable $ARGV[0], the second
in $ARGV[1], and so on.
Using this information, you could re-write the
very first file-test example above to use @ARGV instead of the <STDIN>
variable for input:
#!/usr/bin/perl
if (-e $ARGV[0])
{
print "File exists!\n";
}
else
{
print "File does not exist!\n";
}
<STDIN> and other file
handles
------------------------------
Perl comes with a few pre-defined
file handles - one of them is <STDIN>, which refers to the standard input
device. Additionally, there's <STDOUT>, which refers to the default output
device [usually the terminal] and <STDERR>, which is the place where all
error messages go [also usually the terminal].
Logical
Operators
-----------------
You've probably seen the || and &&
constructs in the examples we've shown you over the past few lessons. We haven't
explained them yet - and so we're going to rectify that right now.
Both
|| and && are logical operators, commonly used in conditional
expressions. The || operator indicates an OR condition, while the &&
operator indicates an AND condition. For example, consider this:
if (a == 0 || a == 1)
{
do this!
}
In English, this would translate to "if a equals zero OR a
equals one, do this!" But in the case of
if (a == 0 && b == 0)
{
do this!
}
the "do this!" statement would be executed only if a equals
zero AND b equals zero.
Perl also comes with a third logical operator,
the NOT operator - it's usually indicated by a prefixed exclamation mark[!].
Consider the two examples below:
if (a)
{
do this!
}
In English, "if a exists, do this!"
if (!a)
{
do this!
}
In English, "if a does NOT exist, do this!"
And that's
about it for this week. We'll be back in a couple of weeks with more Perl 101.
Till then...stay healthy!
This article copyright Melonfire 2000. All rights reserved.
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