Can not print hash element

lightoverhead has asked for the wisdom of the Perl Monks concerning the following question:

Hi monks, I am studying the references on perlreftul. It showed an example of using references to a hash containing countries and cities(http://perldoc.perl.org/perlreftut.html). I created a test file containing the country city list,

    Chicago, USA
    Frankfurt, Germany
    Berlin, Germany
    Washington, USA
    Helsinki, Finland
    New York, USA
[download]

then test the code as below:

#!/usr/bin/perl -w
use strict;
my %table;
while (<>){
    chomp;
    my ($city,$country)=split/,/;
    $table{$country}=[] unless exists $table{$country};
    push @{$table{$country}}, $city;
    
}
my $a=$table{"USA"};
print $a;  #line 12


foreach my $country (sort keys %table){
    
    if ($country eq "USA"){
    
    print "USA is found\n";
    }
    else{}
}
[download]

Wierd thing happend. first, I can not print the element of $table{"USA"}, giving the error "Use of uninitialized value in print at country_city.pl line 12, <> line 6.". Second, the foreach loop can not find $country equals to "USA", thus, it can not print "USA is found", it just printed nothing. What happened? This really confused me. Thanks.

Comment on Can not print hash element Select or Download Code

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Re: Can not print hash element by dreadpiratepeter (Priest) on Sep 23, 2008 at 01:49 UTC
You have not taken the spaces in your input into account. try: `split /\s,\s/;` [download] that should fix it. You were generating a key of " USA", not "USA". -pete "Worry is like a rocking chair. It gives you something to do, but it doesn't get you anywhere."	[reply] [d/l]
Re^2: Can not print hash element by lightoverhead (Pilgrim) on Sep 23, 2008 at 02:05 UTC
Thank you very much. You have sharp eyes. It's fixed.	[reply]
Re^3: Can not print hash element by tinita (Parson) on Sep 23, 2008 at 18:24 UTC
Another opportunity to promote Data::Dumper =) `use Data::Dumper; local $Data::Dumper::Useqq = 1; # print non-printable chars print Dumper \%table;` [download] So, whenever perl seems to be wrong, first debug yourself what data you have.	[reply] [d/l]
Re: Can not print hash element by lamp (Chaplain) on Sep 23, 2008 at 02:05 UTC
use Data::Dumper module for debugging these kind of issues. The following is the output of '%table' hash using Data::Dumper before removing the space from your input. You can find spaces in front of all the country names. `use Data::Dumper; print Dumper(%table);` [download] output `$VAR1 = ' Germany'; $VAR2 = [ 'Frankfurt', 'Berlin' ]; $VAR3 = ' Finland'; $VAR4 = [ 'Helsinki' ]; $VAR5 = ' USA'; $VAR6 = [ 'Chicago', 'Washington', 'New York' ];` [download]	[reply] [d/l] [select]
Re: Can not print hash element by poolpi (Hermit) on Sep 23, 2008 at 10:43 UTC
Skipping duplicate cities and regexp `#!/usr/bin/perl -w use strict; use Data::Dumper; my $country; # Hash ref ;) my $sep = q{,}; while (<DATA>) { chomp; next if /\A\z/; /\s* (.+) \s* $sep \s* (.+) \s*/msx; next unless defined $1 and defined $2; grep { /$1/i } @{ $country->{$2} } or push @{ $country->{$2} }, $1; } print Dumper $country; __DATA__ Chicago, USA Frankfurt, Germany Berlin, Germany Washington, USA Helsinki, Finland Berlin, Germany New York, USA Freetown, Sierra Leone` [download] hth, PooLpi 'Ebry haffa hoe hab im tik a bush'. Jamaican proverb	[reply] [d/l]