Intermediate Perl Learning Notes-chapter 5

Exercise 2:

#! /usr/bin/perl -w
use strict;

my %total_bytes;
my %bytes_sources;
while(<>)
{
    next if /^#/;
    my ($source,$dest,$bytes) = split;
    $total_bytes{$source}{$dest} += $bytes;
}

#### print the hash according to the bytes of sources
foreach my $source(keys %total_bytes)
{
    foreach my $dest (keys %{$total_bytes{$source}})
    {
        $bytes_sources{$source} += $total_bytes{$source}{$dest};
    }
}
my %inverse_hash = reverse %bytes_sources;
foreach my $bytes(reverse sort {$a <=> $b} keys %inverse_hash)
{
    my $source = $inverse_hash{$bytes};
    printf "Source:%-30ssource_total_bytes:%s\n",${source},$bytes;
    my %inverse_dest = reverse (%{$total_bytes{$source}});
    foreach my $destnum(reverse sort {$a <=> $b}keys %inverse_dest)
    {
        printf "\t\tDest:%-20s\tdest_bytes_from_this_source:%s\n",$inverse_dest{$destnum},$destnum;
    }
}
print "\n";

总结:

1.根据原hash的key=>value对建立反向hash的方法:my %inverse_hash = reverse %hash;

2.默认的排序程序根据ascii码顺序来排序列表元素,可以使用subroutine来进行指定高级排序的顺序.如按值排序的方法:

 

sort { $a <=> $b} @array; ######code block can be placed directly within brackets

#####sort using subroutine

sub my_sort_routine

{

...

}

sort my_sort_routine @array;

######sort from most to least

reverse sort {$a <=> $b} @array;