What decides the order of keys when I print a Perl hash?

Question

activePerl 5.8 based

#!C:\\Perl\\bin\\perl.exe
use strict;
use warnings;

# declare a new hash
my %some_hash;

%some_hash = (\"foo\", 35, \"bar\", 12.4, 2.5,         


        

Answer 1

I went over your code and made some notes that I think you will find helpful.

use strict;
use warnings;

# declare a new hash and initialize it at the same time
my %some_hash = (
    foo   => 35,       # use the fat-comma or '=>' operator, it quotes the left side
    bar   => 12.4,     
    2.5   => "hello",
    wilma => 1.72e30,
    betty => "bye",    # perl ignores trailing commas, 
                       # the final comma makes adding items to the end of the list less bug prone.
);

my @any_array = %some_hash; # Hash is expanded into a list of key/value pairs.

print "$_ => $some_hash{$_}\n" 
    for keys %some_hash;

print "\n\n",              # You can print multiple newlines in one string.
      "@any_array\n\n";    # print takes a list of things to print.

# In print @foo; @foo is expanded into a list of items to print.  
# There is no separator between the members of @foo in the output.

# However print "@foo"; interpolates @foo into a string. 
# It inserts spaces between the members of the arrays.


# This is the block form of 'for'
for my $k (sort keys %some_hash)
{
    # Interpolating the variables into a string makes it easier to read the output.
    print "$k => $some_hash{$k}\n";
}

Hashes provide unordered, access to data by a string key.

Arrays provide access to ordered data. Random access is available by using a numerical index.

If you need to preserve the order of a group of values, use an array. If you need to look up members of the group by an associated name, use a hash.

If you need to do both, you can use both structures together:

# Keep an array of sorted hash keys.
my @sorted_items = qw( first second third fourth );

# Store the actual data in the hash.
my %item;
@item{ @sorted_items } = 1..4;  # This is called a hash slice.  
                                # It allows you to access a list of hash elements.
                                # This can be a very powerful way to work with hashes.

# random access
print "third => $item{third}\n";


# When you need to access the data in order, iterate over
# the array of sorted hash keys.  Use the keys to access the
# data in the hash.

# ordered access
for my $name ( @sorted_items ) {
    print "$name => $item{$name}\n";
}

Looking at your code samples, I see a couple of things you might want to work on.

• how looping structures like for and while can be used to reduce repeated code.
• how to use variable interpolation

BTW, I am glad to see you working on basics and improving your code quality. This investment of time will pay off. Keep up the good work.