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.4.4 Print a Specific Entry, Given a Year and CategoryWe can traverse either index, and we choose to traverse the %year_index index, since there aresubstantially fewer categories per year than the number of years for which a category is valid:sub print_entry {my($year, $category) = @_;foreach $rlEntry (@{$year_index{$year}}) {if ($rlEntry->[1] eq $category) {print "$category ($year), ", $rlEntry->[2], "\n";return;}}print "No entry for $category ($year) \n";}Previous: 2.3 Professors, Advanced Perl Next: 2.5Students, Courses Programming Pretty-Printing2.3 Professors, Students, 2.5 Pretty-PrintingBookIndexCourses[ Library Home | Perl in a Nutshell | Learning Perl | Learning Perl on Win32 | Programming Perl | Advanced PerlProgramming | Perl Cookbook ] Chapter 2Previous: 2.4 Pass the Next: 2.6Implementing Complex DataEnvelope ResourcesStructures2.5 Pretty-PrintingIn building complicated data structures, it is always nice to have a pretty-printer handy for debugging.There are at least two options for pretty-printing data structures.The first is the Perl debugger itself.Ituses a function called dumpValue in a file called dumpvar.pl, which can be found in the standardlibrary directory.We can help ourselves to it, with the caveat that it is an unadvertised function and couldchange someday.To pretty-print this structure, for example:@sample = (11.233,{3 => 4, "hello" => [6,7]});we write the following:require 'dumpvar.pl';dumpValue(\@sample); # always pass by referenceThis prints something like this:0 11.2331 HASH(0xb75dc0)3 => 4'hello' => ARRAY(0xc70858)0 61 7We will cover the require statement in Chapter 6, Modules.Meanwhile, just think of it as a fancy#include (which doesn't load the file if it is already loaded).The Data::Dumper module available from CPAN is another viable alternative for pretty-printing.Chapter10, Persistence, covers this module in some detail, so we will not say any more about it here.Bothmodules detect circular references and handle subroutine and glob references.It is fun and instructive to write a pretty-printer ourselves.Example 2.5 illustrates a simple effort, whichaccounts for circular references but doesn't follow typeglobs or subroutine references.This example isused as follows:pretty_print(@sample); # Doesn't need a referenceThis prints11.233 { # HASH(0xb78b00): 3 => 4: hello =>: : [ # ARRAY(0xc70858): : : 6: : : 7: : ]}The following code contains specialized procedures (print_array, print_hash, or print_scalar) that knowhow to print specific data types.print_ref, charged with the task of pretty-printing a reference, simplydispatches control to one of the above procedures depending upon the type of argument given to it.Inturn, these procedures may call print_ref recursively if the data types that they handle contain one ormore references.Whenever a reference is encountered, it is also checked with a hash %already_seen to determine whetherthe reference has been printed before.This prevents the routine from going into an infinite loop in thepresence of circular references.All functions manipulate the global variable $level and callprint_indented, which appropriately indents and prints the string given to it.Example 2.5: Pretty-Printing$level = -1; # Level of indentationsub pretty_print {my $var;foreach $var (@_) {if (ref ($var)) {print_ref($var);} else {print_scalar($var);}}}sub print_scalar {++$level;my $var = shift;print_indented ($var);--$level;}sub print_ref {my $r = shift;if (exists ($already_seen{$r})) {print_indented ("$r (Seen earlier)"); return;} else {$already_seen{$r}=1;}my $ref_type = ref($r);if ($ref_type eq "ARRAY") {print_array($r);} elsif ($ref_type eq "SCALAR") {print "Ref -> $r";print_scalar($$r);} elsif ($ref_type eq "HASH") {print_hash($r);} elsif ($ref_type eq "REF") {++$level;print_indented("Ref -> ($r)");print_ref($$r);--$level;} else {print_indented ("$ref_type (not supported)");}}sub print_array {my ($r_array) = @_;++$level;print_indented ("[ # $r_array");foreach $var (@$r_array) {if (ref ($var)) {print_ref($var);} else {print_scalar($var);}}print_indented ("]");--$level;}sub print_hash {my($r_hash) = @_;my($key, $val);++$level;print_indented ("{ # $r_hash");while (($key, $val) = each %$r_hash) {$val = ($val ? $val : '""');++$level;if (ref ($val)) { print_indented ("$key => ");print_ref($val);} else {print_indented ("$key => $val");}--$level;}print_indented ("}");--$level;}sub print_indented {$spaces = ": " x $level;print "${spaces}$_[0]\n";}print_ref simply prints its argument (a reference) and returns if it has already seen this reference.If youwere to read the output produced by this code, you would find it hard to imagine which reference pointsto which structure
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