from the full Perl manual, the table of contents to which can be found
in L<perltoc>.
-Throughout this document you'll see references to other parts of the
+Throughout this document you'll see references to other parts of the
Perl documentation. You can read that documentation using the C<perldoc>
command or whatever method you're using to read this document.
=head2 What is Perl?
-Perl is a general-purpose programming language originally developed for
-text manipulation and now used for a wide range of tasks including
-system administration, web development, network programming, GUI
+Perl is a general-purpose programming language originally developed for
+text manipulation and now used for a wide range of tasks including
+system administration, web development, network programming, GUI
development, and more.
The language is intended to be practical (easy to use, efficient,
processing, and has one of the world's most impressive collections of
third-party modules.
-Different definitions of Perl are given in L<perl>, L<perlfaq1> and
-no doubt other places. From this we can determine that Perl is different
+Different definitions of Perl are given in L<perl>, L<perlfaq1> and
+no doubt other places. From this we can determine that Perl is different
things to different people, but that lots of people think it's at least
worth writing about.
For more information, including instructions for other platforms such as
Windows and Mac OS, read L<perlrun>.
+=head2 Safety net
+
+Perl by default is very forgiving. In order to make it more roboust
+it is recommened to start every program with the following lines:
+
+ #!/usr/bin/perl
+ use strict;
+ use warnings;
+
+The C<use strict;> line imposes some restrictions that will mainly stop
+you from introducing bugs in your code. The C<use warnings;> is more or
+less equivalent to the command line switch B<-w> (see L<perlrun>). This will
+catch various problems in your code and give warnings.
+
=head2 Basic syntax overview
A Perl script or program consists of one or more statements. These
Whitespace is irrelevant:
- print
+ print
"Hello, world"
;
print 42;
You can use parentheses for functions' arguments or omit them
-according to your personal taste. They are only required
+according to your personal taste. They are only required
occasionally to clarify issues of precedence.
print("Hello, world\n");
my $animal = "camel";
my $answer = 42;
-Scalar values can be strings, integers or floating point numbers, and Perl
-will automatically convert between them as required. There is no need
-to pre-declare your variable types.
+Scalar values can be strings, integers or floating point numbers, and Perl
+will automatically convert between them as required. There is no need
+to pre-declare your variable types, but you have to declare them using
+the C<my> keyword the first time you use them. (This is one of the
+requirements of C<use strict;>.)
Scalar values can be used in various ways:
kinds of purposes, and are documented in L<perlvar>. The only one you
need to know about for now is C<$_> which is the "default variable".
It's used as the default argument to a number of functions in Perl, and
-it's set implicitly by certain looping constructs.
+it's set implicitly by certain looping constructs.
print; # prints contents of $_ by default
print $animals[0]; # prints "camel"
print $animals[1]; # prints "llama"
-The special variable C<$#array> tells you the index of the last element
+The special variable C<$#array> tells you the index of the last element
of an array:
print $mixed[$#mixed]; # last element, prints 1.23
-You might be tempted to use C<$#array + 1> to tell you how many items there
+You might be tempted to use C<$#array + 1> to tell you how many items there
are in an array. Don't bother. As it happens, using C<@array> where Perl
expects to find a scalar value ("in scalar context") will give you the number
of elements in the array:
if (@animals < 5) { ... }
-The elements we're getting from the array start with a C<$> because
-we're getting just a single value out of the array -- you ask for a scalar,
+The elements we're getting from the array start with a C<$> because
+we're getting just a single value out of the array -- you ask for a scalar,
you get a scalar.
To get multiple values from an array:
Hashes have no particular internal order, though you can sort the keys
and loop through them.
-Just like special scalars and arrays, there are also special hashes.
+Just like special scalars and arrays, there are also special hashes.
The most well known of these is C<%ENV> which contains environment
variables. Read all about it (and other special variables) in
L<perlvar>.
A reference is a scalar value and can refer to any other Perl data
type. So by storing a reference as the value of an array or hash
-element, you can easily create lists and hashes within lists and
+element, you can easily create lists and hashes within lists and
hashes. The following example shows a 2 level hash of hash
structure using anonymous hash references.
my $variables = {
- scalar => {
+ scalar => {
description => "single item",
sigil => '$',
},
(i.e. a bunch of statements surrounded by curly-braces) in which they
are defined.
- my $a = "foo";
+ my $x = "foo";
+ my $some_condition = 1;
if ($some_condition) {
- my $b = "bar";
- print $a; # prints "foo"
- print $b; # prints "bar"
+ my $y = "bar";
+ print $x; # prints "foo"
+ print $y; # prints "bar"
}
- print $a; # prints "foo"
- print $b; # prints nothing; $b has fallen out of scope
+ print $x; # prints "foo"
+ print $y; # prints nothing; $y has fallen out of scope
Using C<my> in combination with a C<use strict;> at the top of
-your Perl scripts means that the interpreter will pick up certain common
+your Perl scripts means that the interpreter will pick up certain common
programming errors. For instance, in the example above, the final
C<print $b> would cause a compile-time error and prevent you from
running the program. Using C<strict> is highly recommended.
information about modules and CPAN).
The conditions can be any Perl expression. See the list of operators in
-the next section for information on comparison and boolean logic operators,
+the next section for information on comparison and boolean logic operators,
which are commonly used in conditional statements.
=over 4
Perl comes with a wide selection of builtin functions. Some of the ones
we've already seen include C<print>, C<sort> and C<reverse>. A list of
-them is given at the start of L<perlfunc> and you can easily read
+them is given at the start of L<perlfunc> and you can easily read
about any given function by using C<perldoc -f I<functionname>>.
Perl operators are documented in full in L<perlop>, but here are a few
le less than or equal
ge greater than or equal
-(Why do we have separate numeric and string comparisons? Because we don't
-have special variable types, and Perl needs to know whether to sort
+(Why do we have separate numeric and string comparisons? Because we don't
+have special variable types, and Perl needs to know whether to sort
numerically (where 99 is less than 100) or alphabetically (where 100 comes
before 99).
|| or
! not
-(C<and>, C<or> and C<not> aren't just in the above table as descriptions
+(C<and>, C<or> and C<not> aren't just in the above table as descriptions
of the operators -- they're also supported as operators in their own
-right. They're more readable than the C-style operators, but have
-different precedence to C<&&> and friends. Check L<perlop> for more
+right. They're more readable than the C-style operators, but have
+different precedence to C<&&> and friends. Check L<perlop> for more
detail.)
=item Miscellaneous
=head2 Files and I/O
You can open a file for input or output using the C<open()> function.
-It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
+It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
but in short:
open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
The C<< <> >> operator is most often seen in a C<while> loop:
- while (<$in>) { # assigns each line in turn to $_
+ while (<$in>) { # assigns each line in turn to $_
print "Just read in this line: $_";
}
^ start of string
$ end of string
-Quantifiers can be used to specify how many of the previous thing you
-want to match on, where "thing" means either a literal character, one
-of the metacharacters listed above, or a group of characters or
+Quantifiers can be used to specify how many of the previous thing you
+want to match on, where "thing" means either a literal character, one
+of the metacharacters listed above, or a group of characters or
metacharacters in parentheses.
* zero or more of the previous thing
/^\d+/ string starts with one or more digits
/^$/ nothing in the string (start and end are adjacent)
- /(\d\s){3}/ a three digits, each followed by a whitespace
+ /(\d\s){3}/ a three digits, each followed by a whitespace
character (eg "3 4 5 ")
- /(a.)+/ matches a string in which every odd-numbered letter
+ /(a.)+/ matches a string in which every odd-numbered letter
is a (eg "abacadaf")
# This loop reads from STDIN, and prints non-blank lines:
=item Parentheses for capturing
-As well as grouping, parentheses serve a second purpose. They can be
+As well as grouping, parentheses serve a second purpose. They can be
used to capture the results of parts of the regexp match for later use.
The results end up in C<$1>, C<$2> and so on.
to us as a special array called C<@_> (see L<perlvar> for more on that).
The default argument to the C<shift> function just happens to be C<@_>.
So C<my $logmessage = shift;> shifts the first item off the list of
-arguments and assigns it to C<$logmessage>.
+arguments and assigns it to C<$logmessage>.
We can manipulate C<@_> in other ways too:
OO Perl is relatively simple and is implemented using references which
know what sort of object they are based on Perl's concept of packages.
-However, OO Perl is largely beyond the scope of this document.
+However, OO Perl is largely beyond the scope of this document.
Read L<perlboot>, L<perltoot>, L<perltooc> and L<perlobj>.
As a beginning Perl programmer, your most common use of OO Perl will be
projects / p5sagit/p5-mst-13.2.git / commitdiff