The only things that need to be declared in Perl are report formats
and subroutines. See the sections below for more information on those
declarations. All uninitialized user-created objects are assumed to
-start with a null or 0 value until they are defined by some explicit
+start with a C<null> or C<0> value until they are defined by some explicit
operation such as assignment. (Though you can get warnings about the
use of undefined values if you like.) The sequence of statements is
executed just once, unlike in B<sed> and B<awk> scripts, where the
implicit loop with either the B<-n> or B<-p> switch. It's just not the
mandatory default like it is in B<sed> and B<awk>.)
-Perl is, for the most part, a free-form language. (The only
-exception to this is format declarations, for obvious reasons.) Comments
-are indicated by the "#" character, and extend to the end of the line. If
-you attempt to use C</* */> C-style comments, it will be interpreted
-either as division or pattern matching, depending on the context, and C++
-C<//> comments just look like a null regular expression, So don't do
-that.
+=head2 Declarations
+
+Perl is, for the most part, a free-form language. (The only exception
+to this is format declarations, for obvious reasons.) Text from a
+C<"#"> character until the end of the line is a comment, and is
+ignored. If you attempt to use C</* */> C-style comments, it will be
+interpreted either as division or pattern matching, depending on the
+context, and C++ C<//> comments just look like a null regular
+expression, so don't do that.
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time. Typically all the declarations are put at
-the beginning or the end of the script.
+the beginning or the end of the script. However, if you're using
+lexically-scoped private variables created with C<my()>, you'll have to make sure
+your format or subroutine definition is within the same block scope
+as the my if you expect to be able to access those private variables.
-As of Perl 5, declaring a subroutine allows a subroutine name to be used
-as if it were a list operator from that point forward in the program. You
-can declare a subroutine without defining it by saying just
+Declaring a subroutine allows a subroutine name to be used as if it were a
+list operator from that point forward in the program. You can declare a
+subroutine without defining it by saying C<sub name>, thus:
sub myname;
$me = myname $0 or die "can't get myname";
-Note that it functions as a list operator though, not a unary
-operator, so be careful to use C<or> instead of C<||> there.
+Note that my() functions as a list operator, not as a unary operator; so
+be careful to use C<or> instead of C<||> in this case. However, if
+you were to declare the subroutine as C<sub myname ($)>, then
+C<myname> would function as a unary operator, so either C<or> or
+C<||> would work.
-Subroutines declarations can also be imported by a C<use> statement.
+Subroutines declarations can also be loaded up with the C<require> statement
+or both loaded and imported into your namespace with a C<use> statement.
+See L<perlmod> for details on this.
-Also as of Perl 5, a statement sequence may contain declarations of
-lexically scoped variables, but apart from declaring a variable name,
-the declaration acts like an ordinary statement, and is elaborated within
-the sequence of statements as if it were an ordinary statement.
+A statement sequence may contain declarations of lexically-scoped
+variables, but apart from declaring a variable name, the declaration acts
+like an ordinary statement, and is elaborated within the sequence of
+statements as if it were an ordinary statement. That means it actually
+has both compile-time and run-time effects.
=head2 Simple statements
side effects. Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional. (A semicolon is still encouraged there if the
-block takes up more than one line, since you may add another line.)
+block takes up more than one line, because you may eventually add another line.)
Note that there are some operators like C<eval {}> and C<do {}> that look
-like compound statements, but aren't (they're just TERMs in an expression),
-and thus need an explicit termination
-if used as the last item in a statement.
+like compound statements, but aren't (they're just TERMs in an expression),
+and thus need an explicit termination if used as the last item in a statement.
Any simple statement may optionally be followed by a I<SINGLE> modifier,
just before the terminating semicolon (or block ending). The possible
unless EXPR
while EXPR
until EXPR
+ foreach EXPR
The C<if> and C<unless> modifiers have the expected semantics,
-presuming you're a speaker of English. The C<while> and C<until>
-modifiers also have the usual "while loop" semantics (conditional
-evaluated first), except when applied to a do-BLOCK (or to the
-now-deprecated do-SUBROUTINE statement), in which case the block
-executes once before the conditional is evaluated. This is so that you
-can write loops like:
+presuming you're a speaker of English. The C<foreach> modifier is an
+iterator: For each value in EXPR, it aliases C<$_> to the value and
+executes the statement. The C<while> and C<until> modifiers have the
+usual "C<while> loop" semantics (conditional evaluated first), except
+when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE
+statement), in which case the block executes once before the
+conditional is evaluated. This is so that you can write loops like:
do {
- $_ = <STDIN>;
+ $line = <STDIN>;
...
- } until $_ eq ".\n";
-
-See L<perlfunc/do>. Note also that the loop control
-statements described later will I<NOT> work in this construct, since
-modifiers don't take loop labels. Sorry. You can always wrap
-another block around it to do that sort of thing.)
+ } until $line eq ".\n";
+
+See L<perlfunc/do>. Note also that the loop control statements described
+later will I<NOT> work in this construct, because modifiers don't take
+loop labels. Sorry. You can always put another block inside of it
+(for C<next>) or around it (for C<last>) to do that sort of thing.
+For C<next>, just double the braces:
+
+ do {{
+ next if $x == $y;
+ # do something here
+ }} until $x++ > $z;
+
+For C<last>, you have to be more elaborate:
+
+ LOOP: {
+ do {
+ last if $x = $y**2;
+ # do something here
+ } while $x++ <= $z;
+ }
=head2 Compound statements
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
- LABEL foreach VAR (ARRAY) BLOCK
+ LABEL foreach VAR (LIST) BLOCK
+ LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
-The C<if> statement is straightforward. Since BLOCKs are always
+The C<if> statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
the sense of the test is reversed.
The C<while> statement executes the block as long as the expression is
-true (does not evaluate to the null string or 0 or "0"). The LABEL is
-optional, and if present, consists of an identifier followed by a
-colon. The LABEL identifies the loop for the loop control statements
-C<next>, C<last>, and C<redo> (see below). If there is a C<continue>
-BLOCK, it is always executed just before the conditional is about to be
-evaluated again, just like the third part of a C<for> loop in C.
-Thus it can be used to increment a loop variable, even when the loop
-has been continued via the C<next> statement (which is similar to the C
-C<continue> statement).
+true (does not evaluate to the null string C<""> or C<0> or C<"0">).
+The LABEL is optional, and if present, consists of an identifier followed
+by a colon. The LABEL identifies the loop for the loop control
+statements C<next>, C<last>, and C<redo>.
+If the LABEL is omitted, the loop control statement
+refers to the innermost enclosing loop. This may include dynamically
+looking back your call-stack at run time to find the LABEL. Such
+desperate behavior triggers a warning if you use the B<-w> flag.
+Unlike a C<foreach> statement, a C<while> statement never implicitly
+localises any variables.
+
+If there is a C<continue> BLOCK, it is always executed just before the
+conditional is about to be evaluated again, just like the third part of a
+C<for> loop in C. Thus it can be used to increment a loop variable, even
+when the loop has been continued via the C<next> statement (which is
+similar to the C C<continue> statement).
+
+=head2 Loop Control
+
+The C<next> command is like the C<continue> statement in C; it starts
+the next iteration of the loop:
+
+ LINE: while (<STDIN>) {
+ next LINE if /^#/; # discard comments
+ ...
+ }
+
+The C<last> command is like the C<break> statement in C (as used in
+loops); it immediately exits the loop in question. The
+C<continue> block, if any, is not executed:
+
+ LINE: while (<STDIN>) {
+ last LINE if /^$/; # exit when done with header
+ ...
+ }
+
+The C<redo> command restarts the loop block without evaluating the
+conditional again. The C<continue> block, if any, is I<not> executed.
+This command is normally used by programs that want to lie to themselves
+about what was just input.
+
+For example, when processing a file like F</etc/termcap>.
+If your input lines might end in backslashes to indicate continuation, you
+want to skip ahead and get the next record.
+
+ while (<>) {
+ chomp;
+ if (s/\\$//) {
+ $_ .= <>;
+ redo unless eof();
+ }
+ # now process $_
+ }
+
+which is Perl short-hand for the more explicitly written version:
+
+ LINE: while (defined($line = <ARGV>)) {
+ chomp($line);
+ if ($line =~ s/\\$//) {
+ $line .= <ARGV>;
+ redo LINE unless eof(); # not eof(ARGV)!
+ }
+ # now process $line
+ }
+
+Note that if there were a C<continue> block on the above code, it would get
+executed even on discarded lines. This is often used to reset line counters
+or C<?pat?> one-time matches.
+
+ # inspired by :1,$g/fred/s//WILMA/
+ while (<>) {
+ ?(fred)? && s//WILMA $1 WILMA/;
+ ?(barney)? && s//BETTY $1 BETTY/;
+ ?(homer)? && s//MARGE $1 MARGE/;
+ } continue {
+ print "$ARGV $.: $_";
+ close ARGV if eof(); # reset $.
+ reset if eof(); # reset ?pat?
+ }
If the word C<while> is replaced by the word C<until>, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
-In either the C<if> or the C<while> statement, you may replace "(EXPR)"
-with a BLOCK, and the conditional is true if the value of the last
-statement in that block is true. (This feature continues to work in Perl
-5 but is deprecated. Please change any occurrences of "if BLOCK" to
-"if (do BLOCK)".)
+The loop control statements don't work in an C<if> or C<unless>, since
+they aren't loops. You can double the braces to make them such, though.
+
+ if (/pattern/) {{
+ next if /fred/;
+ next if /barney/;
+ # so something here
+ }}
+
+The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
+available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
+
+=head2 For Loops
-The C-style C<for> loop works exactly like the corresponding C<while> loop:
+Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
+that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
-is the same as
+is the same as this:
$i = 1;
while ($i < 10) {
$i++;
}
-The foreach loop iterates over a normal list value and sets the
-variable VAR to be each element of the list in turn. The variable is
-implicitly local to the loop (unless declared previously with C<my>),
-and regains its former value upon exiting the loop. The C<foreach>
-keyword is actually a synonym for the C<for> keyword, so you can use
-C<foreach> for readability or C<for> for brevity. If VAR is omitted, $_
-is set to each value. If ARRAY is an actual array (as opposed to an
-expression returning a list value), you can modify each element of the
-array by modifying VAR inside the loop. Examples:
+(There is one minor difference: The first form implies a lexical scope
+for variables declared with C<my> in the initialization expression.)
+
+Besides the normal array index looping, C<for> can lend itself
+to many other interesting applications. Here's one that avoids the
+problem you get into if you explicitly test for end-of-file on
+an interactive file descriptor causing your program to appear to
+hang.
+
+ $on_a_tty = -t STDIN && -t STDOUT;
+ sub prompt { print "yes? " if $on_a_tty }
+ for ( prompt(); <STDIN>; prompt() ) {
+ # do something
+ }
+
+=head2 Foreach Loops
+
+The C<foreach> loop iterates over a normal list value and sets the
+variable VAR to be each element of the list in turn. If the variable
+is preceded with the keyword C<my>, then it is lexically scoped, and
+is therefore visible only within the loop. Otherwise, the variable is
+implicitly local to the loop and regains its former value upon exiting
+the loop. If the variable was previously declared with C<my>, it uses
+that variable instead of the global one, but it's still localized to
+the loop.
+
+The C<foreach> keyword is actually a synonym for the C<for> keyword, so
+you can use C<foreach> for readability or C<for> for brevity. (Or because
+the Bourne shell is more familiar to you than I<csh>, so writing C<for>
+comes more naturally.) If VAR is omitted, C<$_> is set to each value.
+If any element of LIST is an lvalue, you can modify it by modifying VAR
+inside the loop. That's because the C<foreach> loop index variable is
+an implicit alias for each item in the list that you're looping over.
- for (@ary) { s/foo/bar/; }
+If any part of LIST is an array, C<foreach> will get very confused if
+you add or remove elements within the loop body, for example with
+C<splice>. So don't do that.
- foreach $elem (@elements) {
+C<foreach> probably won't do what you expect if VAR is a tied or other
+special variable. Don't do that either.
+
+Examples:
+
+ for (@ary) { s/foo/bar/ }
+
+ foreach my $elem (@elements) {
$elem *= 2;
}
- for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
- print $_, "\n"; sleep(1);
+ for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
+ print $count, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
- foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
+ foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
}
-A BLOCK by itself (labeled or not) is semantically equivalent to a loop
-that executes once. Thus you can use any of the loop control
-statements in it to leave or restart the block. The C<continue> block
-is optional. This construct is particularly nice for doing case
+Here's how a C programmer might code up a particular algorithm in Perl:
+
+ for (my $i = 0; $i < @ary1; $i++) {
+ for (my $j = 0; $j < @ary2; $j++) {
+ if ($ary1[$i] > $ary2[$j]) {
+ last; # can't go to outer :-(
+ }
+ $ary1[$i] += $ary2[$j];
+ }
+ # this is where that last takes me
+ }
+
+Whereas here's how a Perl programmer more comfortable with the idiom might
+do it:
+
+ OUTER: foreach my $wid (@ary1) {
+ INNER: foreach my $jet (@ary2) {
+ next OUTER if $wid > $jet;
+ $wid += $jet;
+ }
+ }
+
+See how much easier this is? It's cleaner, safer, and faster. It's
+cleaner because it's less noisy. It's safer because if code gets added
+between the inner and outer loops later on, the new code won't be
+accidentally executed. The C<next> explicitly iterates the other loop
+rather than merely terminating the inner one. And it's faster because
+Perl executes a C<foreach> statement more rapidly than it would the
+equivalent C<for> loop.
+
+=head2 Basic BLOCKs and Switch Statements
+
+A BLOCK by itself (labeled or not) is semantically equivalent to a
+loop that executes once. Thus you can use any of the loop control
+statements in it to leave or restart the block. (Note that this is
+I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
+C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
+block is optional.
+
+The BLOCK construct is particularly nice for doing case
structures.
SWITCH: {
$nothing = 1;
}
-There is no official switch statement in Perl, because there are
+There is no official C<switch> statement in Perl, because there are
already several ways to write the equivalent. In addition to the
above, you could write
$nothing = 1;
}
-(That's actually not as strange as it looks one you realize that you can
+(That's actually not as strange as it looks once you realize that you can
use loop control "operators" within an expression, That's just the normal
C comma operator.)
$nothing = 1;
}
-or formatted so it stands out more as a "proper" switch statement:
+or formatted so it stands out more as a "proper" C<switch> statement:
SWITCH: {
- /^abc/ && do {
- $abc = 1;
- last SWITCH;
+ /^abc/ && do {
+ $abc = 1;
+ last SWITCH;
};
- /^def/ && do {
- $def = 1;
- last SWITCH;
+ /^def/ && do {
+ $def = 1;
+ last SWITCH;
};
- /^xyz/ && do {
- $xyz = 1;
- last SWITCH;
+ /^xyz/ && do {
+ $xyz = 1;
+ last SWITCH;
};
$nothing = 1;
}
else
{ $nothing = 1 }
+A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
+a temporary assignment to C<$_> for convenient matching:
+
+ SWITCH: for ($where) {
+ /In Card Names/ && do { push @flags, '-e'; last; };
+ /Anywhere/ && do { push @flags, '-h'; last; };
+ /In Rulings/ && do { last; };
+ die "unknown value for form variable where: `$where'";
+ }
+
+Another interesting approach to a switch statement is arrange
+for a C<do> block to return the proper value:
+
+ $amode = do {
+ if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
+ elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
+ elsif ($flag & O_RDWR) {
+ if ($flag & O_CREAT) { "w+" }
+ else { ($flag & O_APPEND) ? "a+" : "r+" }
+ }
+ };
+
+Or
+
+ print do {
+ ($flags & O_WRONLY) ? "write-only" :
+ ($flags & O_RDWR) ? "read-write" :
+ "read-only";
+ };
+
+Or if you are certainly that all the C<&&> clauses are true, you can use
+something like this, which "switches" on the value of the
+C<HTTP_USER_AGENT> envariable.
+
+ #!/usr/bin/perl
+ # pick out jargon file page based on browser
+ $dir = 'http://www.wins.uva.nl/~mes/jargon';
+ for ($ENV{HTTP_USER_AGENT}) {
+ $page = /Mac/ && 'm/Macintrash.html'
+ || /Win(dows )?NT/ && 'e/evilandrude.html'
+ || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
+ || /Linux/ && 'l/Linux.html'
+ || /HP-UX/ && 'h/HP-SUX.html'
+ || /SunOS/ && 's/ScumOS.html'
+ || 'a/AppendixB.html';
+ }
+ print "Location: $dir/$page\015\012\015\012";
+
+That kind of switch statement only works when you know the C<&&> clauses
+will be true. If you don't, the previous C<?:> example should be used.
+
+You might also consider writing a hash of subroutine references
+instead of synthesizing a C<switch> statement.
+
+=head2 Goto
+
+Although not for the faint of heart, Perl does support a C<goto>
+statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
+C<goto>-&NAME. A loop's LABEL is not actually a valid target for
+a C<goto>; it's just the name of the loop.
+
+The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
+execution there. It may not be used to go into any construct that
+requires initialization, such as a subroutine or a C<foreach> loop. It
+also can't be used to go into a construct that is optimized away. It
+can be used to go almost anywhere else within the dynamic scope,
+including out of subroutines, but it's usually better to use some other
+construct such as C<last> or C<die>. The author of Perl has never felt the
+need to use this form of C<goto> (in Perl, that is--C is another matter).
+
+The C<goto>-EXPR form expects a label name, whose scope will be resolved
+dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
+necessarily recommended if you're optimizing for maintainability:
+
+ goto ("FOO", "BAR", "GLARCH")[$i];
+
+The C<goto>-&NAME form is highly magical, and substitutes a call to the
+named subroutine for the currently running subroutine. This is used by
+C<AUTOLOAD()> subroutines that wish to load another subroutine and then
+pretend that the other subroutine had been called in the first place
+(except that any modifications to C<@_> in the current subroutine are
+propagated to the other subroutine.) After the C<goto>, not even C<caller()>
+will be able to tell that this routine was called first.
+
+In almost all cases like this, it's usually a far, far better idea to use the
+structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
+resorting to a C<goto>. For certain applications, the catch and throw pair of
+C<eval{}> and die() for exception processing can also be a prudent approach.
+
+=head2 PODs: Embedded Documentation
+
+Perl has a mechanism for intermixing documentation with source code.
+While it's expecting the beginning of a new statement, if the compiler
+encounters a line that begins with an equal sign and a word, like this
+
+ =head1 Here There Be Pods!
+
+Then that text and all remaining text up through and including a line
+beginning with C<=cut> will be ignored. The format of the intervening
+text is described in L<perlpod>.
+
+This allows you to intermix your source code
+and your documentation text freely, as in
+
+ =item snazzle($)
+
+ The snazzle() function will behave in the most spectacular
+ form that you can possibly imagine, not even excepting
+ cybernetic pyrotechnics.
+
+ =cut back to the compiler, nuff of this pod stuff!
+
+ sub snazzle($) {
+ my $thingie = shift;
+ .........
+ }
+
+Note that pod translators should look at only paragraphs beginning
+with a pod directive (it makes parsing easier), whereas the compiler
+actually knows to look for pod escapes even in the middle of a
+paragraph. This means that the following secret stuff will be
+ignored by both the compiler and the translators.
+
+ $a=3;
+ =secret stuff
+ warn "Neither POD nor CODE!?"
+ =cut back
+ print "got $a\n";
+
+You probably shouldn't rely upon the C<warn()> being podded out forever.
+Not all pod translators are well-behaved in this regard, and perhaps
+the compiler will become pickier.
+
+One may also use pod directives to quickly comment out a section
+of code.
+
+=head2 Plain Old Comments (Not!)
+
+Much like the C preprocessor, Perl can process line directives. Using
+this, one can control Perl's idea of filenames and line numbers in
+error or warning messages (especially for strings that are processed
+with C<eval()>). The syntax for this mechanism is the same as for most
+C preprocessors: it matches the regular expression
+C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
+number for the next line, and C<$2> being the optional filename
+(specified within quotes).
+
+Here are some examples that you should be able to type into your command
+shell:
+
+ % perl
+ # line 200 "bzzzt"
+ # the `#' on the previous line must be the first char on line
+ die 'foo';
+ __END__
+ foo at bzzzt line 201.
+
+ % perl
+ # line 200 "bzzzt"
+ eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
+ __END__
+ foo at - line 2001.
+
+ % perl
+ eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
+ __END__
+ foo at foo bar line 200.
+
+ % perl
+ # line 345 "goop"
+ eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
+ print $@;
+ __END__
+ foo at goop line 345.
+
+=cut
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