X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlreftut.pod;h=82ad80e3a6aa70def7d50dd418d7376fb7b68357;hb=c2cf2042fe2e21eb73dcdef54aa1b0e379ae413d;hp=926d232de5a86c1455c66fd8e50051017bbfcd78;hpb=a29d1a25ab4477ba560380bb2d146b6d2e44cedf;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlreftut.pod b/pod/perlreftut.pod index 926d232..82ad80e 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: @@ -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: @@ -120,13 +120,13 @@ variable first. B C<[ ITEMS ]> makes a new, anonymous array, and returns a reference to -that array. C<{ ITEMS }> makes a new, anonymous hash. and returns a +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 }; # $href now holds a reference to a hash @@ -171,10 +171,10 @@ Arrays: On each line are two expressions that do the same thing. The -left-hand versions operate on the array C<@a>, and the right-hand -versions operate on the array that is referred to by C<$aref>, but -once they find the array they're operating on, they do the same things -to the arrays. +left-hand versions operate on the array C<@a>. The right-hand +versions operate on the array that is referred to by C<$aref>. Once +they find the array they're operating on, both versions do the same +things to the arrays. Using a hash reference is I the same: @@ -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 | | | | +-----------+--------+ @@ -332,6 +332,13 @@ structure will look like this: We'll look at output first. Supposing we already have this structure, how do we print it out? + 8 foreach $country (sort keys %table) { + 9 print "$country: "; + 10 my @cities = @{$table{$country}}; + 11 print join ', ', sort @cities; + 12 print ".\n"; + 13 } + C<%table> is an ordinary hash, and we get a list of keys from it, sort the keys, and loop over the keys as usual. The only use of references is in line 10. @@ -349,7 +356,7 @@ Having gotten the list of cities, we sort it, join it, and print it out as usual. Lines 2-7 are responsible for building the structure in the first -place; here they are again: +place. Here they are again: 2 while (<>) { 3 chomp; @@ -375,7 +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; @@ -420,7 +427,7 @@ other references. =item * -In B, you can omit the curly brackets whenever the thing +In B, you can omit the curly brackets whenever the thing inside them is an atomic scalar variable like C<$aref>. For example, C<@$aref> is the same as C<@{$aref}>, and C<$$aref[1]> is the same as C<${$aref}[1]>. If you're just starting out, you may want to adopt @@ -430,11 +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<< $aref2->[23] >> you'll see the change. To copy the array, use @@ -446,16 +453,16 @@ initialized with the contents of the array referred to by C<$aref1>. Similarly, to copy an anonymous hash, you can use - $href = {%{$href}}; + $href2 = {%{$href1}}; -=item * +=item * -To see if a variable contains a reference, use the `ref' function. It +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 @@ -472,7 +479,9 @@ C<==> instead because it's much faster.) You can use a string as if it were a reference. If you use the string C<"foo"> as an array reference, it's taken to be a reference to the -array C<@foo>. This is called a I or I. +array C<@foo>. This is called a I or I. The declaration C disables this +feature, which can cause all sorts of trouble if you use it by accident. =back @@ -492,10 +501,10 @@ to do with references. =head1 Credits -Author: Mark-Jason Dominus, Plover Systems (C) +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. @@ -503,12 +512,8 @@ The original title was I. Copyright 1998 The Perl Journal. -When included as part of the Standard Version of Perl, or as part of -its complete documentation whether printed or otherwise, this work may -be distributed only under the terms of Perl's Artistic License. Any -distribution of this file or derivatives thereof outside of that -package require that special arrangements be made with copyright -holder. +This documentation is free; you can redistribute it and/or modify it +under the same terms as Perl itself. Irrespective of its distribution, all code examples in these files are hereby placed into the public domain. You are permitted and