X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/49d5cbad7e34aa93704ded3cb611586befa38496..54e0f05ce4bb904f953dde352028f27b07cb1fdf:/pod/perlreftut.pod diff --git a/pod/perlreftut.pod b/pod/perlreftut.pod index bbcc179..7898b6d 100644 --- a/pod/perlreftut.pod +++ b/pod/perlreftut.pod @@ -1,4 +1,3 @@ - =head1 NAME perlreftut - Mark's very short tutorial about references @@ -21,7 +20,7 @@ Fortunately, you only need to know 10% of what's in the main page to get One problem that came up all the time in Perl 4 was how to represent a hash whose values were lists. Perl 4 had hashes, of course, but the -values had to be scalars; they couldn't be lists. +values had to be scalars; they couldn't be lists. Why would you want a hash of lists? Let's take a simple example: You have a file of city and country names, like this: @@ -68,11 +67,11 @@ entire hash (or to just about anything else). Names are one kind of reference that you're already familiar with. Think of the President of the United States: a messy, inconvenient bag of blood and bones. But to talk about him, or to represent him in a computer program, all -you need is the easy, convenient scalar string "George Bush". +you need is the easy, convenient scalar string "Barack Obama". References in Perl are like names for arrays and hashes. They're Perl's private, internal names, so you can be sure they're -unambiguous. Unlike "George Bush", a reference only refers to one +unambiguous. Unlike "Barack Obama", a reference only refers to one thing, and you always know what it refers to. If you have a reference to an array, you can recover the entire array from it. If you have a reference to a hash, you can recover the entire hash. But the @@ -102,6 +101,7 @@ reference to that variable. $aref = \@array; # $aref now holds a reference to @array $href = \%hash; # $href now holds a reference to %hash + $sref = \$scalar; # $sref now holds a reference to $scalar Once the reference is stored in a variable like $aref or $href, you can copy it or store it just the same as any other scalar value: @@ -123,10 +123,10 @@ C<[ ITEMS ]> makes a new, anonymous array, and returns a reference to that array. C<{ ITEMS }> makes a new, anonymous hash, and returns a reference to that hash. - $aref = [ 1, "foo", undef, 13 ]; + $aref = [ 1, "foo", undef, 13 ]; # $aref now holds a reference to an array - $href = { APR => 4, AUG => 8 }; + $href = { APR => 4, AUG => 8 }; # $href now holds a reference to a hash @@ -215,7 +215,7 @@ And then replace the hash name with the reference: =head3 B -B is all you really need, because it tells you how to to +B is all you really need, because it tells you how to do absolutely everything you ever need to do with references. But the most common thing to do with an array or a hash is to extract a single element, and the B notation is cumbersome. So there is an @@ -268,7 +268,7 @@ two-dimensional array; you can write C<< $a[ROW]->[COLUMN] >> to get or set the element in any row and any column of the array. The notation still looks a little cumbersome, so there's one more -abbreviation: +abbreviation: =head2 Arrow Rule @@ -315,7 +315,7 @@ structure will look like this: %table - +-------+---+ + +-------+---+ | | | +-----------+--------+ |Germany| *---->| Frankfurt | Berlin | | | | +-----------+--------+ @@ -382,7 +382,7 @@ C<{$table{$country}}>. The C adds a city name to the end of the referred-to array. There's one fine point I skipped. Line 5 is unnecessary, and we can -get rid of it. +get rid of it. 2 while (<>) { 3 chomp; @@ -437,11 +437,11 @@ the habit of always including the curly brackets. This doesn't copy the underlying array: - $aref2 = $aref1; + $aref2 = $aref1; -You get two references to the same array. If you modify +You get two references to the same array. If you modify C<< $aref1->[23] >> and then look at -C<< $aref2->[23] >> you'll see the change. +C<< $aref2->[23] >> you'll see the change. To copy the array, use @@ -455,14 +455,14 @@ Similarly, to copy an anonymous hash, you can use $href2 = {%{$href1}}; -=item * +=item * To see if a variable contains a reference, use the C function. It returns true if its argument is a reference. Actually it's a little better than that: It returns C for hash references and C for array references. -=item * +=item * If you try to use a reference like a string, you get strings like @@ -504,7 +504,7 @@ to do with references. Author: Mark Jason Dominus, Plover Systems (C) This article originally appeared in I -( http://www.tpj.com/ ) volume 3, #2. Reprinted with permission. +( http://www.tpj.com/ ) volume 3, #2. Reprinted with permission. The original title was I.