FinancialAid Expansion - PeLLGrants UpTo $5,500 FinancialAid Expansion - PeLLGrants UpTo $5,500 Which, in the degenerate case of using only ordinary arrays, gives you multidimensional arrays just like C's: Perl will raise an exception if you try to access nonexistent fieldsTo avoid inconsistencies, always use the fields::phash() function provided by the fields pragma. $g = newprint("Greetings"); $scalarref = \$foo; print "$k => $v\n"; $bar = $$scalarref; The left side of the arrow can be any expression returning a reference, including a previous dereferenceNote that $array[$x] is not the same thing as $array->[$x] here: print exists $phash->[0]{shoes};# false, 'shoes' can't be used $x{ \$a } = $a; and not worry about whether the subscripts are reserved wordsIn the rare event that you do wish to do something like Hard references are smart--they keep track of reference counts for you, automatically freeing the thing referred to when its reference count goes to zero(Reference counts for values in self-referential or cyclic data structures may not go to zero without a little help; see perlobj/"Two-Phased Garbage Collection" for a detailed explanation.) If that thing happens to be an object, the object is destructedSee perlobj for more about objects(In a sense, everything in Perl is an object, but we usually reserve the word for references to objects that have been officially "blessed" into a class package.) $rec = get_rec(*STDIN{IO}); # pass both file and dir handles $array{ shift } $struct->{foo}; # same as $struct->[1], i.e"FOO" sub hashem { return { @_ } } # ok print exists $phash->{pants}; # true, your 'pants' have been touched sub get_rec { print "That yields @{[$n + 5]} widgets\n"; Because of being able to omit the curlies for the simple case of $$x, people often make the mistake of viewing the dereferencing symbols as proper operators, and wonder about their precedenceIf they were, though, you could use parentheses instead of bracesThat's not the caseConsider the difference below; case 0 is a short-hand version of case 1, not case 2: The leading +{ and {; always serve to disambiguate the expression to mean either the HASH reference, or the BLOCK. ${$name} = 2; # Sets $foo } &$name(); # Calls &foo() (as in Perl 4) print "refs 1 and 2 refer to the same thing\n"; print exists $phash->{foo}; # true, 'foo' was set in the declaration Note the semicolonExcept for the code inside not being immediately executed, a sub {} is not so much a declaration as it is an operator, like do{} or eval{}(However, no matter how many times you execute that particular line (unless you're in an eval(".")), $coderef will still have a reference to the same anonymous subroutine.) This is one of the only places where giving a prototype to a closure makes much senseIf you wanted to impose scalar context on the arguments of these functions (probably not a wise idea for this particular example), you could have written it this way instead: The standard Tie::RefHash module provides a convenient workaround to this. $objref = Doggie->new(Tail => 'short', Ears => 'long'); $array{ aaa }{ bbb }{ ccc } $coderef = *handler{CODE}; $$arrayref[0] = "January"; The red() and green() functions would be similarTo create these, we'll assign a closure to a typeglob of the name of the function we're trying to build. In human terms, it's a funny way of passing arguments to a subroutine when you define it as well as when you call itIt's useful for setting up little bits of code to run later, such as callbacksYou can even do object-oriented stuff with it, though Perl already provides a different mechanism to do that--see perlobj. $ioref = *STDIN{IO}; ${ "bareword" }; # Error, symbolic reference $array{ shift() } &$h("world"); Because curly brackets (braces) are used for several other things including BLOCKs, you may occasionally have to disambiguate braces at the beginning of a statement by putting a + or a return in front so that Perl realizes the opening brace isn't starting a BLOCKThe economy and mnemonic value of using curlies is deemed worth this occasional extra hassle. $x{ $r } = $r; A typeglob may be dereferenced the same way a reference can, because the dereference syntax always indicates the type of reference desiredSo ${*foo} and ${\$foo} both indicate the same scalar variable. ${$arrayref}[0] = "January"; $arrayref = *ARGV{ARRAY}; $hashref->{"KEY"} = "VALUE"; # Hash element { Case 2 is also deceptive in that you're accessing a variable called %hashref, not dereferencing through $hashref to the hash it's presumably referencingThat would be case 3. my $x = shift; $array{ shift @_ } A reference to an anonymous hash can be created using curly brackets: 'Adam' => 'Eve', print $fh "her um well a hmmm\n"; For this to work, the array must contain extra informationThe first element of the array has to be a hash reference that maps field names to array indicesHere is an example: *foo{THING} returns undef if that particular THING hasn't been used yet, except in the case of scalars*foo{SCALAR} returns a reference to an anonymous scalar if $foo hasn't been used yetThis might change in a future release. Beginning with release 5.005 of Perl, you may use an array reference in some contexts that would normally require a hash referenceThis allows you to access array elements using symbolic names, as if they were fields in a structure. $scalarref = *foo{SCALAR}; And then at least you can use the values(), which will be real refs, instead of the keys(), which won't. 4. Here's a trick for interpolating a subroutine call into a string: print $$$$refrefref; $array{ +shift } You might also think of closure as a way to write a subroutine template without using eval()Here's a small example of how closures work: } Subroutine calls and lookups of individual array elements arise often enough that it gets cumbersome to use method 2As a form of syntactic sugar, the examples for method 2 may be written: Similarly, because of all the subscripting that is done using single words, we've applied the same rule to any bareword that is used for subscripting a hashSo now, instead of writing This prints and then only hard references will be allowed for the rest of the enclosing blockAn inner block may countermand that with &{ $dispatch{$index} }(1,2,3); # call correct routine return scalar $fh>; no strict 'refs'; # allow symbol table manipulation print ${ push } "over"; if ($ref1 == $ref2) { # cheap numeric compare of references print $globref "output\n"; $hashref = *ENV{HASH}; Making References *foo{IO} is an alternative to the *HANDLE mechanism given in perldata/"Typeglobs and Filehandles" for passing filehandles into or out of subroutines, or storing into larger data structuresIts disadvantage is that it won't create a new filehandle for youIts advantage is that you have less risk of clobbering more than you want to with a typeglob assignment(It still conflates file and directory handles, though.) However, if you assign the incoming value to a scalar instead of a typeglob as we do in the examples below, there's no risk of that happening. Access to lexicals that change over type--like those in the for loop above--only works with closures, not general subroutinesIn the general case, then, named subroutines do not nest properly, although anonymous ones doIf you are accustomed to using nested subroutines in other programming languages with their own private variables, you'll have to work at it a bit in PerlThe intuitive coding of this type of thing incurs mysterious warnings about ``will not stay shared''For example, this won't work: you can write just Well, okay, not entirely like C's arrays, actuallyC doesn't know how to grow its arrays on demandPerl does. $$hashref{"KEY"} = "VALUE"; # CASE 0 All of these are self-explanatory except for *foo{IO}It returns the IO handle, used for file handles (perlfunc/open), sockets (perlfunc/socket and perlfunc/socketpair), and directory handles (perlfunc/opendir)For compatibility with previous versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}, though it is deprecated as of 5.8.0If deprecation warnings are in effect, it will warn of its use. $$name = 1; # Sets $foo $r = \@a; If a reference happens to be a reference to an object, then there are probably methods to access the things referred to, and you should probably stick to those methods unless you're in the class package that defines the object's methodsIn other words, be nice, and don't violate the object's encapsulation without a very good reasonPerl does not enforce encapsulationWe are not totalitarians hereWe do expect some basic civility though. $terminal = Term::Cap->Tgetent( { OSPEED => 9600 }); *$name = sub ($) { "FONT COLOR='$name'>$_[0]/FONT>" }; $coderef = sub { print "Boink!\n" }; Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a simple scalar variable containing a reference of the correct type: $pseudohash = fields::phash(foo => "FOO", bar => "BAR"); print "${push}over"; Using a string or number as a reference produces a symbolic reference, as explained aboveUsing a reference as a number produces an integer representing its storage location in memoryThe only useful thing to be done with this is to compare two references numerically to see whether they refer to the same location. has always meant to print "pop on over", even though push is a reserved wordThis has been generalized to work the same outside of quotes, so that push(@$arrayref, $filename); As explained above, a closure is an anonymous function with access to the lexical variables visible when that function was compiledIt retains access to those variables even though it doesn't get run until later, such as in a signal handler or a Tk callback. ${$hashref{"KEY"}} = "VALUE"; # CASE 2 $name->[0] = 4; # Sets $foo[0] } The use warnings pragma or the -w switch will warn you if it interprets a reserved word as a stringBut it will no longer warn you about using lowercase words, because the string is effectively quoted. } You may not (usefully) use a reference as the key to a hashIt will be converted into a string: 7. and even return $x + inner(); @list = (\$a, \@b, \%c); push(@{$arrayref}, $filename); If you try to dereference the key, it won't do a hard dereference, and you won't accomplish what you're attemptingYou might want to do something more like my $x = $_[0] + 35; splutter(*STDOUT); # pass the whole glob local *inner = sub { return $x * 19 }; By using the backslash operator on a variable, subroutine, or value(This works much like the & (address-of) operator in C.) This typically creates another reference to a variable, because there's already a reference to the variable in the symbol tableBut the symbol table reference might go away, and you'll still have the reference that the backslash returnedHere are some examples: Using References It's important to understand that we are specifically not dereferencing $arrayref[0] or $hashref{"KEY"} thereThe dereference of the scalar variable happens before it does any key lookupsAnything more complicated than a simple scalar variable must use methods 2 or 3 belowHowever, a "simple scalar" includes an identifier that itself uses method 1 recursivelyTherefore, the following prints "howdy". For better performance, Perl can also do the translation from field names to array indices at compile time for typed object referencesSee fields. As a special case, \(@foo) returns a list of references to the contents of @foo, not a reference to @foo itselfLikewise for %foo, except that the key references are to copies (since the keys are just strings rather than full-fledged scalars). $score[$x][$y][$z] += 42; Anonymous hash and array composers like these can be intermixed freely to produce as complicated a structure as you wantThe multidimensional syntax described below works for these tooThe values above are literals, but variables and expressions would work just as well, because assignment operators in Perl (even within local() or my()) are executable statements, not compile-time declarations. References of the appropriate type can spring into existence if you dereference them in a context that assumes they existBecause we haven't talked about dereferencing yet, we can't show you any examples yet $arrayref->[0] = "January"; # Array element Symbolic references are names of variables or other objects, just as a symbolic link in a Unix filesystem contains merely the name of a fileThe *glob notation is something of a symbolic reference(Symbolic references are sometimes called "soft references", but please don't call them that; references are confusing enough without useless synonyms.) 6. Pseudo-hashes: Using an array as a hash Note particularly that $x continues to refer to the value passed into newprint() despite "my $x" having gone out of scope by the time the anonymous subroutine runsThat's what a closure is all about. while (my($k,$v) = each %$struct) { print ${push} "over"; Howdy, world! $hashref = \%ENV; It isn't possible to create a true reference to an IO handle (filehandle or dirhandle) using the backslash operatorThe most you can get is a reference to a typeglob, which is actually a complete symbol table entryBut see the explanation of the *foo{THING} syntax belowHowever, you can still use type globs and globrefs as though they were IO handles. use strict 'refs'; This will still print 10, not 20Remember that local() affects package variables, which are all "global" to the package. sub inner { return $x * 19 } # WRONG &{$coderef}(1,2,3); $hashref = { sub showem { { @_ } } # ambiguous (currently ok, but may change) Using a reference as a string produces both its referent's type, including any package blessing as described in perlobj, as well as the numeric address expressed in hexThe ref() operator returns just the type of thing the reference is pointing to, without the addressSee perlfunc/ref for details and examples of its use. print "Be ", red("careful"), "with that ", green("light"); return sub { my $y = shift; print "$x, $y!\n"; }; 'Clyde' => 'Bonnie', sub outer { ${ bareword }; # Okay, means $bareword. }; } $main = MainWindow->new(); my $fh = shift; ${$hashref}{"KEY"} = "VALUE"; 5. use Tk; $phash->{pants} = undef; $arrayref = [1, 2, ['a', 'b', 'c']]; However, since prototype checking happens at compile time, the assignment above happens too late to be of much useYou could address this by putting the whole loop of assignments within a BEGIN block, forcing it to occur during compilation. A work-around is the following: release 5 of Perl it was difficult to represent complex data structures, because all references had to be symbolic--and even then it was difficult to refer to a variable instead of a symbol table entryPerl now not only makes it easier to use symbolic references to variables, but also lets you have "hard" references to any piece of data or codeAny scalar may hold a hard referenceBecause arrays and hashes contain scalars, you can now easily build arrays of arrays, arrays of hashes, hashes of arrays, arrays of hashes of functions, and so on. A reference can be created by using a special syntax, lovingly known as the *foo{THING} syntax*foo{THING} returns a reference to the THING slot in *foo (which is the symbol table entry which holds everything known as foo). $rec = get_rec(*STDIN); # pass the whole glob @$name = (); # Clears @foo One more thing hereThe arrow is optional between brackets subscripts, so you can shrink the above down to Anonymous subroutines act as closures with respect to my() variables, that is, variables lexically visible within the current scopeClosure is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside the context. $phash = fields::phash([qw(foo bar pants)], ['FOO']); WARNING: This section describes an experimental featureDetails may change without notice in future versions. References are often returned by special subroutines called constructorsPerl objects are just references to a special type of object that happens to know which package it's associated withConstructors are just special subroutines that know how to create that associationThey do so by starting with an ordinary reference, and it remains an ordinary reference even while it's also being an objectConstructors are often named new() and called indirectly: Now inner() can only be called from within outer(), because of the temporary assignments of the closure (anonymous subroutine)But when it does, it has normal access to the lexical variable $x from the scope of outer(). local $value = 10; People frequently expect it to work like thisSo it does. ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval Here we've created a reference to an anonymous array of three elements whose final element is itself a reference to another anonymous array of three elements(The multidimensional syntax described later can be used to access thisFor example, after the above, $arrayref->[2][1] would have the value "b".) On the other hand, if you want the other meaning, you can do this: print exists $phash->{bar}; # false, 'bar' has not been used. $objref = new Doggie (Tail => 'short', Ears => 'long'); $globref->print("output\n"); # iff IO::Handle is loaded A reference to an anonymous subroutine can be created by using sub without a subname: A new feature contributing to readability in perl version 5.001 is that the brackets around a symbolic reference behave more like quotes, just as they always have within a stringThat is, NOTE: The current user-visible implementation of pseudo-hashes (the weird use of the first array element) is deprecated starting from Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be implemented differentlyNot only is the current interface rather ugly, but the current implementation slows down normal array and hash use quite noticeablyThe 'fields' pragma interface will remain available. return $x + inner(); print $phash->{foo}; # runtime exception $array[$x]->{"foo"}->[0] = "January"; my $x = $_[0] + 35; use fields; This is one of the cases we mentioned earlier in which references could spring into existence when in an lvalue contextBefore this statement, $array[$x] may have been undefinedIf so, it's automatically defined with a hash reference so that we can look up {"foo"} in itLikewise $array[$x]->{"foo"} will automatically get defined with an array reference so that we can look up [0] in itThis process is called autovivification. sub showem { {; @_ } } # ok This is powerful, and slightly dangerous, in that it's possible to intend (with the utmost sincerity) to use a hard reference, and accidentally use a symbolic reference insteadTo protect against that, you can say A reference to an anonymous array can be created using square brackets: print delete $phash->[0]{foo}; # now key is gone This has the interesting effect of creating a function local to another function, something not normally supported in Perl. This applies only to lexical variables, by the wayDynamic variables continue to work as they have always workedClosure is not something that most Perl programmers need trouble themselves about to begin with. use fields; 1. my $value = 20; print "My sub returned @{[mysub(1,2,3)]} that time.\n"; } 4. ${$hashref}{"KEY"} = "VALUE"; # CASE 1 Symbolic references no strict 'refs'; 2. The second is to use exists() on the hash reference sitting in the first array elementThis checks to see if the given key is a valid field in the pseudo-hash. sub splutter { @colors = qw(red blue green yellow orange purple violet); delete() on a pseudo-hash element only deletes the value corresponding to the key, not the key itselfTo delete the key, you'll have to explicitly delete it from the first hash element. References can be created in several ways. @list = \($a, @b, %c); # same thing! ${$name x 2} = 3; # Sets $foofoo print exists $phash->[0]{bar}; # true, 'bar' is a valid field Admittedly, it's a little silly to use the curlies in this case, but the BLOCK can contain any arbitrary expression, in particular, subscripted expressions: for my $name (@colors) { } $h = newprint("Howdy"); you can force interpretation as a reserved word by adding anything that makes it more than a bareword: That's it for creating referencesBy now you're probably dying to know how to use references to get back to your long-lost dataThere are several basic methods. The way it works is that when the @{.} is seen in the double-quoted string, it's evaluated as a blockThe block creates a reference to an anonymous array containing the results of the call to mysub(1,2,3)So the whole block returns a reference to an array, which is then dereferenced by @{.} and stuck into the double-quoted stringThis chicanery is also useful for arbitrary expressions: # Time passes. In contrast, hard references are more like hard links in a Unix file system: They are used to access an underlying object without concern for what its (other) name isWhen the word "reference" is used without an adjective, as in the following paragraph, it is usually talking about a hard reference. } -borderwidth => 2) print exists $phash->{foo}; # false sub showem { { return @_ } } # ok $coderef = \&handler; Greetings, earthlings! will have the same effect(This would have been a syntax error in Perl 5.000, though Perl 4 allowed it in the spaceless form.) This construct is not considered to be a symbolic reference when you're using strict refs: The bless() operator may be used to associate the object a reference points to with a package functioning as an object classSee perlobj. $globref = \*foo; ${$hashref->{"KEY"}} = "VALUE"; # CASE 3 my $fh = shift; $bar = ${$scalarref}; 2. $refrefref = \\\"howdy"; &$coderef(1,2,3); values %$struct; # will return ("FOO", "BAR") in same some order splutter(*STDOUT{IO}); # pass both file and dir handles We said that references spring into existence as necessary if they are undefined, but we didn't say what happens if a value used as a reference is already defined, but isn't a hard referenceIf you use it as a reference, it'll be treated as a symbolic referenceThat is, the value of the scalar is taken to be the name of a variable, rather than a direct link to a (possibly) anonymous value. sub outer { $push = "pop on "; $array{ "aaa" }{ "bbb" }{ "ccc" } sub hashem { +{ @_ } } # ok print $$ref; $struct->{bar}; # same as $struct->[2], i.e"BAR" print delete $phash->{foo}; # prints $phash->[1], "FOO" $$hashref{"KEY"} = "VALUE"; $pack = "THAT"; Only package variables (globals, even if localized) are visible to symbolic referencesLexical variables (declared with my()) aren't in a symbol table, and thus are invisible to this mechanismFor example: } But don't have to be: use Term::Cap; $globref = *foo{GLOB}; print exists $phash->[0]{foo}; # true, key still exists sub hashem { { @_ } } # silently wrong $menubar = $main->Frame(-relief => "raised", keys %$struct; # will return ("foo", "bar") in some order References are easy to use in PerlThere is just one overriding principle: Perl does no implicit referencing or dereferencingWhen a scalar is holding a reference, it always behaves as a simple scalarIt doesn't magically start being an array or hash or subroutine; you have to tell it explicitly to do so, by dereferencing it. $name = "foo"; Not-so-symbolic references Taking a reference to an enumerated list is not the same as using square brackets--instead it's the same as creating a list of references! For example, if you wanted a function to make a new hash and return a reference to it, you have these options: There are two ways to check for the existence of a key in a pseudo-hashThe first is to use exists()This checks to see if the given field has ever been setIt acts this way to match the behavior of a regular hashFor instance: *$name = *{uc $name} = sub { "FONT COLOR='$name'>@_/FONT>" }; sub newprint { $struct = [{foo => 1, bar => 2}, "FOO", "BAR"]; $arrayref = \@ARGV; Function Templates 1. Using a closure as a function template allows us to generate many functions that act similarlySuppose you wanted functions named after the colors that generated HTML font changes for the various colors: Now all those different functions appear to exist independentlyYou can call red(), RED(), blue(), BLUE(), green(), etcThis technique saves on both compile time and memory use, and is less error-prone as well, since syntax checks happen at compile timeIt's critical that any variables in the anonymous subroutine be lexicals in order to create a proper closureThat's the reasons for the my on the loop iteration variable. Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct typeIn other words, the previous examples could be written like this: use strict 'refs'; $ref = "value"; &$g("earthlings"); 3. $coderef->(1,2,3); # Subroutine call
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