Perl Learning - 11 (hash keys, values, each, exists, delete)

Subroutine &keys() and &values() ruturns the keys and values of a hash.　　#!/usr/bin/perl
　　my %hash=("a"=>1,"b"=>2,"c"=>3,);
　　my @k=keys %hash;
　　my @v=values %hash;
　　my $count=keys %hash;
　　print "@k\n@v\n$count\n";
　　if(%hash){
　　print "That's not a empty hash\n";
　　}
　　$ ./key_value.pl
　　c a b
　　3 1 2
　　3
　　That's not a empty hash
　　Although key list and value list are not ordered as we think, they order the same way.
　　Between keys() and values() are called, don't change the hash.
　　&each() returns the key_value pairs of a hash.
　　while(($key,$value)=each %hash){
　　print "$key => $value\n";
　　}
　　The only use of each() in practice is in while() loop.
　　Note while() is a scalar context, when the condition is a list, take the number of elements of list for true/false judgment.
　　If more than 0 element it is true, 0 element is false.
　　We can use foreach to get an ordered key_value pair.
　　foreach $key (sort keys %hash){
　　print "$key => $hash{$key}\n";
　　}
　　&exists() can judge if a key exists in a hash or not.
　　If exists then true, otherwise false.
　　if(exists $hash{"a"}){
　　print "There is a value for a\n";
　　}else{
　　print "There is no value for a\n";
　　}
　　&delete() deletes a pair of value and key from hash.
　　It is not the same as to give undef to a value, with value undef the key_value still exists.
　　The value of hash can be inserted in quotes like value of scalar.
　　#!/usr/bin/perl
　　my %books=(fred=>3,wilma=>1,barney=>0,pebbles=>undef,);
　　foreach $person(sort keys %books){
　　if($books{$person}){
　　print "$person has $books{$person} items\n";
　　}
　　}
　　%ENV stores the environment parameters of the system, we can get the envs:
　　foreach $env(sort keys %ENV){
　　print "The '$env' is '$ENV{$env}'\n";
　　}
　　Every value we exports in shell will be added to %ENV.
　　$ export aa=AAAAAAAA    # set in shell
　　print "The name of 'aa' is '$ENV{aa}'\n";
　　The name of 'aa' is 'AAAAAAAAA'
　　Execsices:
　　1. Get names of some users, print their given names.
　　#!/usr/bin/perl
　　my %names=(
　　fred => "flintstone",
　　barney => "rubble",
　　wilma => "flintstone",
　　);
　　foreach $surname(sort keys %names){
　　print "The name of $surname is $names{$surname}\n";
　　}
　　##################################
　　2. Input some names, each line one name, Print the times of each name appears.
　　#!/usr/bin/perl
　　while(){
　　chomp;
　　$count{$_}++;
　　}
　　foreach $name(sort keys %count){
　　if($count{$name}){
　　print "$name appears $count{$name} times\n";
　　}
　　}
　　##################################
　　3. Print %ENV each line left align, use length() to get the proper length to align.
　　#!/usr/bin/perl
　　my $longest=0;
　　foreach(sort keys %ENV){
　　$longest = length($_) if $longest < length($_);
　　}
　　foreach $env(sort keys %ENV){
　　printf "%-${longest}s: %s\n", $env,$ENV{$env};
　　}