=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 1.23 $, $Date: 2005/04/07 21:39:34 $)
+perlfaq7 - General Perl Language Issues ($Revision: 9620 $)
=head1 DESCRIPTION
They are type specifiers, as detailed in L<perldata>:
- $ for scalar values (number, string or reference)
- @ for arrays
- % for hashes (associative arrays)
- & for subroutines (aka functions, procedures, methods)
- * for all types of that symbol name. In version 4 you used them like
- pointers, but in modern perls you can just use references.
+ $ for scalar values (number, string or reference)
+ @ for arrays
+ % for hashes (associative arrays)
+ & for subroutines (aka functions, procedures, methods)
+ * for all types of that symbol name. In version 4 you used them like
+ pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to encounter that aren't
really type specifiers:
- <> are used for inputting a record from a filehandle.
- \ takes a reference to something.
+ <> are used for inputting a record from a filehandle.
+ \ takes a reference to something.
Note that <FILE> is I<neither> the type specifier for files
nor the name of the handle. It is the C<< <> >> operator applied
subroutine) and the left-hand operand to the C<< => >> operator both
count as though they were quoted:
- This is like this
- ------------ ---------------
- $foo{line} $foo{'line'}
- bar => stuff 'bar' => stuff
+ This is like this
+ ------------ ---------------
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
list. Good style (see L<perlstyle>) says to put them in except for
one-liners:
- if ($whoops) { exit 1 }
- @nums = (1, 2, 3);
+ if ($whoops) { exit 1 }
+ @nums = (1, 2, 3);
+
+ if ($whoops) {
+ exit 1;
+ }
- if ($whoops) {
- exit 1;
- }
- @lines = (
+ @lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
- );
+ );
=head2 How do I skip some return values?
One way is to treat the return values as a list and index into it:
- $dir = (getpwnam($user))[7];
+ $dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements that
you need:
allows fine control of what warning are produced.
See L<perllexwarn> for more details.
- {
+ {
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
- }
+ }
Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
If you have an older version of Perl, the C<$^W> variable (documented
in L<perlvar>) controls runtime warnings for a block:
- {
+ {
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
- }
+ }
Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().
A common mistake is to write:
- unlink $file || die "snafu";
+ unlink $file || die "snafu";
This gets interpreted as:
- unlink ($file || die "snafu");
+ unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the
super low precedence C<or> operator:
- (unlink $file) || die "snafu";
- unlink $file or die "snafu";
+ (unlink $file) || die "snafu";
+ unlink $file or die "snafu";
The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
deliberately have precedence lower than that of list operators for
produces an lvalue. This assigns $x to either $a or $b, depending
on the trueness of $maybe:
- ($maybe ? $a : $b) = $x;
+ ($maybe ? $a : $b) = $x;
=head2 How do I declare/create a structure?
anonymous) hash reference. See L<perlref> and L<perldsc> for details.
Here's an example:
- $person = {}; # new anonymous hash
- $person->{AGE} = 24; # set field AGE to 24
- $person->{NAME} = "Nat"; # set field NAME to "Nat"
-
+ $person = {}; # new anonymous hash
+ $person->{AGE} = 24; # set field AGE to 24
+ $person->{NAME} = "Nat"; # set field NAME to "Nat"
+
If you're looking for something a bit more rigorous, try L<perltoot>.
=head2 How do I create a module?
(contributed by brian d foy)
L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
-in all the gory details. L<perlnewmod> gives a a brief
+in all the gory details. L<perlnewmod> gives a brief
overview of the process along with a couple of suggestions
about style.
Closures are documented in L<perlref>.
I<Closure> is a computer science term with a precise but
-hard-to-explain meaning. Closures are implemented in Perl as anonymous
-subroutines with lasting references to lexical variables outside their
-own scopes. These lexicals magically refer to the variables that were
-around when the subroutine was defined (deep binding).
-
-Closures make sense in any programming language where you can have the
-return value of a function be itself a function, as you can in Perl.
-Note that some languages provide anonymous functions but are not
-capable of providing proper closures: the Python language, for
+hard-to-explain meaning. Usually, closures are implemented in Perl as
+anonymous subroutines with lasting references to lexical variables
+outside their own scopes. These lexicals magically refer to the
+variables that were around when the subroutine was defined (deep
+binding).
+
+Closures are most often used in programming languages where you can
+have the return value of a function be itself a function, as you can
+in Perl. Note that some languages provide anonymous functions but are
+not capable of providing proper closures: the Python language, for
example. For more information on closures, check out any textbook on
functional programming. Scheme is a language that not only supports
but encourages closures.
-Here's a classic function-generating function:
+Here's a classic non-closure function-generating function:
- sub add_function_generator {
- return sub { shift() + shift() };
- }
+ sub add_function_generator {
+ return sub { shift() + shift() };
+ }
- $add_sub = add_function_generator();
- $sum = $add_sub->(4,5); # $sum is 9 now.
+ $add_sub = add_function_generator();
+ $sum = $add_sub->(4,5); # $sum is 9 now.
-The closure works as a I<function template> with some customization
-slots left out to be filled later. The anonymous subroutine returned
-by add_function_generator() isn't technically a closure because it
-refers to no lexicals outside its own scope.
+The anonymous subroutine returned by add_function_generator() isn't
+technically a closure because it refers to no lexicals outside its own
+scope. Using a closure gives you a I<function template> with some
+customization slots left out to be filled later.
Contrast this with the following make_adder() function, in which the
returned anonymous function contains a reference to a lexical variable
that Perl return a proper closure, thus locking in for all time the
value that the lexical had when the function was created.
- sub make_adder {
- my $addpiece = shift;
- return sub { shift() + $addpiece };
- }
-
- $f1 = make_adder(20);
- $f2 = make_adder(555);
+ sub make_adder {
+ my $addpiece = shift;
+ return sub { shift() + $addpiece };
+ }
+
+ $f1 = make_adder(20);
+ $f2 = make_adder(555);
Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
Closures are often used for less esoteric purposes. For example, when
you want to pass in a bit of code into a function:
- my $line;
- timeout( 30, sub { $line = <STDIN> } );
+ my $line;
+ timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
C<< '$line = <STDIN>' >>, there would have been no way for the
hypothetical timeout() function to access the lexical variable
$line back in its caller's scope.
+Another use for a closure is to make a variable I<private> to a
+named subroutine, e.g. a counter that gets initialized at creation
+time of the sub and can only be modified from within the sub.
+This is sometimes used with a BEGIN block in package files to make
+sure a variable doesn't get meddled with during the lifetime of the
+package:
+
+ BEGIN {
+ my $id = 0;
+ sub next_id { ++$id }
+ }
+
+This is discussed in more detail in L<perlsub>, see the entry on
+I<Persistent Private Variables>.
+
=head2 What is variable suicide and how can I prevent it?
-Variable suicide is when you (temporarily or permanently) lose the
-value of a variable. It is caused by scoping through my() and local()
-interacting with either closures or aliased foreach() iterator
-variables and subroutine arguments. It used to be easy to
-inadvertently lose a variable's value this way, but now it's much
-harder. Take this code:
+This problem was fixed in perl 5.004_05, so preventing it means upgrading
+your version of perl. ;)
- my $f = "foo";
- sub T {
- while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
- }
- T;
- print "Finally $f\n";
+Variable suicide is when you (temporarily or permanently) lose the value
+of a variable. It is caused by scoping through my() and local()
+interacting with either closures or aliased foreach() iterator variables
+and subroutine arguments. It used to be easy to inadvertently lose a
+variable's value this way, but now it's much harder. Take this code:
+
+ my $f = 'foo';
+ sub T {
+ while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
+ }
+
+ T;
+ print "Finally $f\n";
+
+If you are experiencing variable suicide, that C<my $f> in the subroutine
+doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
+shows that inside the subroutine the value of C<$f> leaks through when it
+shouldn't, as in this output:
+
+ foobar
+ foobarbar
+ foobarbarbar
+ Finally foo
The $f that has "bar" added to it three times should be a new C<$f>
-(C<my $f> should create a new local variable each time through the loop).
-It isn't, however. This was a bug, now fixed in the latest releases
-(tested against 5.004_05, 5.005_03, and 5.005_56).
+C<my $f> should create a new lexical variable each time through the loop.
+The expected output is:
+
+ foobar
+ foobar
+ foobar
+ Finally foo
=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
objects. See L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.
-See ``Passing Regexes'', below, for information on passing regular
-expressions.
+See "Passing Regexes", later in L<perlfaq7>, for information on
+passing regular expressions.
=over 4
Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:
- func( \$some_scalar );
+ func( \$some_scalar );
- func( \@some_array );
- func( [ 1 .. 10 ] );
+ func( \@some_array );
+ func( [ 1 .. 10 ] );
- func( \%some_hash );
- func( { this => 10, that => 20 } );
+ func( \%some_hash );
+ func( { this => 10, that => 20 } );
- func( \&some_func );
- func( sub { $_[0] ** $_[1] } );
+ func( \&some_func );
+ func( sub { $_[0] ** $_[1] } );
=item Passing Filehandles
sub func {
my $passed_fh = shift;
- my $line = <$fh>;
+ my $line = <$passed_fh>;
}
Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
Here's an example of how to pass in a string to be regex compared
using C<qr//>:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = $val1 =~ /$regex/;
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = $val1 =~ /$regex/;
return $retval;
- }
- $match = compare("old McDonald", qr/d.*D/i);
+ }
+ $match = compare("old McDonald", qr/d.*D/i);
Notice how C<qr//> allows flags at the end. That pattern was compiled
at compile time, although it was executed later. The nifty C<qr//>
had to approach this problem much less intuitively. For example, here
it is again if you don't have C<qr//>:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = eval { $val1 =~ /$regex/ };
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
- }
+ }
- $match = compare("old McDonald", q/($?i)d.*D/);
+ $match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
- return eval "\$val =~ /$regex/"; # WRONG
+ return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due to the double
interpolation of the eval and the double-quoted string. For example:
- $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
+ $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
- eval "\$string =~ /$pattern_of_evil/";
+ eval "\$string =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the O'Reilly book,
I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
To pass an object method into a subroutine, you can do this:
- call_a_lot(10, $some_obj, "methname")
- sub call_a_lot {
- my ($count, $widget, $trick) = @_;
- for (my $i = 0; $i < $count; $i++) {
- $widget->$trick();
- }
- }
+ call_a_lot(10, $some_obj, "methname")
+ sub call_a_lot {
+ my ($count, $widget, $trick) = @_;
+ for (my $i = 0; $i < $count; $i++) {
+ $widget->$trick();
+ }
+ }
Or, you can use a closure to bundle up the object, its
method call, and arguments:
- my $whatnot = sub { $some_obj->obfuscate(@args) };
- func($whatnot);
- sub func {
- my $code = shift;
- &$code();
- }
+ my $whatnot = sub { $some_obj->obfuscate(@args) };
+ func($whatnot);
+ sub func {
+ my $code = shift;
+ &$code();
+ }
You could also investigate the can() method in the UNIVERSAL class
(part of the standard perl distribution).
with lexical variables, though.
You can fake a static variable by using a lexical variable which goes
-of scope. In this example, you define the subroutine C<counter>, and
+out of scope. In this example, you define the subroutine C<counter>, and
it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
block, C<$count> is defined at compile-time, but also goes out of
scope at the end of the BEGIN block. The BEGIN block also ensures that
The data in chunk of memory defined by C<$count> is private to
C<counter>.
- BEGIN {
- my $count = 1;
- sub counter { $count++ }
- }
-
- my $start = count();
-
- .... # code that calls count();
-
- my $end = count();
+ BEGIN {
+ my $count = 1;
+ sub counter { $count++ }
+ }
+
+ my $start = counter();
+
+ .... # code that calls counter();
+
+ my $end = counter();
In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
They can both access C<$count>, and since it has gone out of scope,
there is no other way to access it.
- BEGIN {
- my $count = 1;
- sub increment_count { $count++ }
- sub return_count { $count }
- }
+ BEGIN {
+ my $count = 1;
+ sub increment_count { $count++ }
+ sub return_count { $count }
+ }
To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
For instance:
- sub visible {
- print "var has value $var\n";
- }
-
- sub dynamic {
- local $var = 'local'; # new temporary value for the still-global
- visible(); # variable called $var
- }
-
- sub lexical {
- my $var = 'private'; # new private variable, $var
- visible(); # (invisible outside of sub scope)
- }
-
- $var = 'global';
-
- visible(); # prints global
- dynamic(); # prints local
- lexical(); # prints global
+ sub visible {
+ print "var has value $var\n";
+ }
+
+ sub dynamic {
+ local $var = 'local'; # new temporary value for the still-global
+ visible(); # variable called $var
+ }
+
+ sub lexical {
+ my $var = 'private'; # new private variable, $var
+ visible(); # (invisible outside of sub scope)
+ }
+
+ $var = 'global';
+
+ visible(); # prints global
+ dynamic(); # prints local
+ lexical(); # prints global
Notice how at no point does the value "private" get printed. That's
because $var only has that value within the block of the lexical()
print "lexical is $var\n";
{
- our $var;
- print "global is $var\n";
+ our $var;
+ print "global is $var\n";
}
=head2 What's the difference between deep and shallow binding?
To enforce scalar context in this particular case, however, you need
merely omit the parentheses:
- local($foo) = <FILE>; # WRONG
- local($foo) = scalar(<FILE>); # ok
- local $foo = <FILE>; # right
+ local($foo) = <FILE>; # WRONG
+ local($foo) = scalar(<FILE>); # ok
+ local $foo = <FILE>; # right
You should probably be using lexical variables anyway, although the
issue is the same here:
- my($foo) = <FILE>; # WRONG
- my $foo = <FILE>; # right
+ my($foo) = <FILE>; # WRONG
+ my $foo = <FILE>; # right
=head2 How do I redefine a builtin function, operator, or method?
=head2 How do I create a switch or case statement?
-This is explained in more depth in the L<perlsyn>. Briefly, there's
-no official case statement, because of the variety of tests possible
-in Perl (numeric comparison, string comparison, glob comparison,
-regex matching, overloaded comparisons, ...).
-Larry couldn't decide how best to do this, so he left it out, even
-though it's been on the wish list since perl1.
-
-Starting from Perl 5.8 to get switch and case one can use the
-Switch extension and say:
-
- use Switch;
+If one wants to use pure Perl and to be compatible with Perl versions
+prior to 5.10, the general answer is to write a construct like this:
-after which one has switch and case. It is not as fast as it could be
-because it's not really part of the language (it's done using source
-filters) but it is available, and it's very flexible.
-
-But if one wants to use pure Perl, the general answer is to write a
-construct like this:
-
- for ($variable_to_test) {
- if (/pat1/) { } # do something
- elsif (/pat2/) { } # do something else
- elsif (/pat3/) { } # do something else
- else { } # default
- }
+ for ($variable_to_test) {
+ if (/pat1/) { } # do something
+ elsif (/pat2/) { } # do something else
+ elsif (/pat3/) { } # do something else
+ else { } # default
+ }
-Here's a simple example of a switch based on pattern matching, this
-time lined up in a way to make it look more like a switch statement.
+Here's a simple example of a switch based on pattern matching,
+lined up in a way to make it look more like a switch statement.
We'll do a multiway conditional based on the type of reference stored
in $whatchamacallit:
}
-See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other
-examples in this style.
+See L<perlsyn> for other examples in this style.
Sometimes you should change the positions of the constant and the variable.
For example, let's say you wanted to test which of many answers you were
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:
- chomp($answer = <>);
- if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
- elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
- elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
- elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
- elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
+ chomp($answer = <>);
+ if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
+ elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
+ elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
+ elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
+ elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function references.
- my %commands = (
- "happy" => \&joy,
- "sad", => \&sullen,
- "done" => sub { die "See ya!" },
- "mad" => \&angry,
- );
-
- print "How are you? ";
- chomp($string = <STDIN>);
- if ($commands{$string}) {
- $commands{$string}->();
- } else {
- print "No such command: $string\n";
- }
+ my %commands = (
+ "happy" => \&joy,
+ "sad", => \&sullen,
+ "done" => sub { die "See ya!" },
+ "mad" => \&angry,
+ );
+
+ print "How are you? ";
+ chomp($string = <STDIN>);
+ if ($commands{$string}) {
+ $commands{$string}->();
+ } else {
+ print "No such command: $string\n";
+ }
+
+Note that starting from version 5.10, Perl has now a native switch
+statement. See L<perlsyn>.
+
+Starting from Perl 5.8, a source filter module, C<Switch>, can also be
+used to get switch and case. Its use is now discouraged, because it's
+not fully compatible with the native switch of Perl 5.10, and because,
+as it's implemented as a source filter, it doesn't always work as intended
+when complex syntax is involved.
=head2 How can I catch accesses to undefined variables, functions, or methods?
If you're just a random program, you can do this to find
out what the currently compiled package is:
- my $packname = __PACKAGE__;
+ my $packname = __PACKAGE__;
But, if you're a method and you want to print an error message
that includes the kind of object you were called on (which is
not necessarily the same as the one in which you were compiled):
- sub amethod {
- my $self = shift;
- my $class = ref($self) || $self;
- warn "called me from a $class object";
- }
+ sub amethod {
+ my $self = shift;
+ my $class = ref($self) || $self;
+ warn "called me from a $class object";
+ }
=head2 How can I comment out a large block of perl code?
should claim to understand (by policy). Mark the end of the block
with <=end>.
- # program is here
-
- =begin comment
-
- all of this stuff
-
- here will be ignored
- by everyone
-
+ # program is here
+
+ =begin comment
+
+ all of this stuff
+
+ here will be ignored
+ by everyone
+
=end comment
-
- =cut
-
- # program continues
+
+ =cut
+
+ # program continues
The pod directives cannot go just anywhere. You must put a
pod directive where the parser is expecting a new statement,
Use this code, provided by Mark-Jason Dominus:
- sub scrub_package {
- no strict 'refs';
- my $pack = shift;
- die "Shouldn't delete main package"
- if $pack eq "" || $pack eq "main";
- my $stash = *{$pack . '::'}{HASH};
- my $name;
- foreach $name (keys %$stash) {
- my $fullname = $pack . '::' . $name;
- # Get rid of everything with that name.
- undef $$fullname;
- undef @$fullname;
- undef %$fullname;
- undef &$fullname;
- undef *$fullname;
+ sub scrub_package {
+ no strict 'refs';
+ my $pack = shift;
+ die "Shouldn't delete main package"
+ if $pack eq "" || $pack eq "main";
+ my $stash = *{$pack . '::'}{HASH};
+ my $name;
+ foreach $name (keys %$stash) {
+ my $fullname = $pack . '::' . $name;
+ # Get rid of everything with that name.
+ undef $$fullname;
+ undef @$fullname;
+ undef %$fullname;
+ undef &$fullname;
+ undef *$fullname;
+ }
}
- }
Or, if you're using a recent release of Perl, you can
just use the Symbol::delete_package() function instead.
Beginners often think they want to have a variable contain the name
of a variable.
- $fred = 23;
- $varname = "fred";
- ++$$varname; # $fred now 24
+ $fred = 23;
+ $varname = "fred";
+ ++$$varname; # $fred now 24
This works I<sometimes>, but it is a very bad idea for two reasons.
(like C<%main::>) instead of a user-defined hash. The solution is to
use your own hash or a real reference instead.
- $USER_VARS{"fred"} = 23;
- $varname = "fred";
- $USER_VARS{$varname}++; # not $$varname++
+ $USER_VARS{"fred"} = 23;
+ $varname = "fred";
+ $USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variable
reading a string and expanding it to the actual contents of your program's
own variables:
- $str = 'this has a $fred and $barney in it';
- $str =~ s/(\$\w+)/$1/eeg; # need double eval
+ $str = 'this has a $fred and $barney in it';
+ $str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:
- $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
+ $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous approach. Of course,
you don't need to use a dollar sign. You could use your own scheme to
make it less confusing, like bracketed percent symbols, etc.
- $str = 'this has a %fred% and %barney% in it';
- $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
+ $str = 'this has a %fred% and %barney% in it';
+ $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a variable to
contain the name of a variable is because they don't know how to build
wanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.
- $name = "fred";
- $$name{WIFE} = "wilma"; # set %fred
+ $name = "fred";
+ $$name{WIFE} = "wilma"; # set %fred
- $name = "barney";
- $$name{WIFE} = "betty"; # set %barney
+ $name = "barney";
+ $$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the
problems enumerated above. It would be far better to write:
- $folks{"fred"}{WIFE} = "wilma";
- $folks{"barney"}{WIFE} = "betty";
+ $folks{"fred"}{WIFE} = "wilma";
+ $folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
In those cases, you would turn off C<strict 'refs'> temporarily so you
can play around with the symbol table. For example:
- @colors = qw(red blue green yellow orange purple violet);
- for my $name (@colors) {
- no strict 'refs'; # renege for the block
- *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
- }
+ @colors = qw(red blue green yellow orange purple violet);
+ for my $name (@colors) {
+ no strict 'refs'; # renege for the block
+ *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
+ }
All those functions (red(), blue(), green(), etc.) appear to be separate,
but the real code in the closure actually was compiled only once.
line in your perl script (the "shebang" line) does not contain the
right path to perl (or any other program capable of running scripts).
Sometimes this happens when you move the script from one machine to
-another and each machine has a different path to perl---/usr/bin/perl
+another and each machine has a different path to perl--/usr/bin/perl
versus /usr/local/bin/perl for instance. It may also indicate
that the source machine has CRLF line terminators and the
destination machine has LF only: the shell tries to find
your PATH, which might also mean that the location of perl is not
where you expect it so you need to adjust your shebang line.
+=head1 REVISION
+
+Revision: $Revision: 9620 $
+
+Date: $Date: 2007-05-29 19:57:58 +0200 (Tue, 29 May 2007) $
+
+See L<perlfaq> for source control details and availability.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
https://perl5.git.perl.org / perl5.git / blobdiff