Related
I know pushing is only passible to array, not hash. But it would be much more convenient to allow pushing key-value pair directly to hash (and I am still surprise it is not possible in perl). I have an example:
#!/usr/bin/perl -w
#superior words begin first, example of that word follow
my #ar = qw[Animals,dog Money,pound Jobs,doctor Food];
my %hash;
my $bool = 1;
sub marine{
my $ar = shift if $bool;
for(#$ar){
my #ar2 = split /,/, $_;
push %hash, ($ar2[0] => $ar2[1]);
}
}
marine(\#ar);
print "$_\n" for keys %hash;
Here I have an array, which has 2 words separately by , comma. I would like to make a hash from it, making the first a key, and the second a value (and if it lacks the value, as does the last Food word, then no value at all -> simply undef. How to make it in perl?
Output:
Possible attempt to separate words with commas at ./a line 4.
Experimental push on scalar is now forbidden at ./a line 12, near ");"
Execution of ./a aborted due to compilation errors.
I might be oversimplyfing things here, but why not simply assign to the hash rather than trying to push into it?
That is, replace this unsupported expression:
push %hash, ($ar2[0] => $ar2[1]);
With:
$hash{$ar2[0]} = $ar2[1];
If I incoporate this in your code, and then dump the resulting hash at the end, I get:
$VAR1 = {
'Food' => undef,
'Money' => 'pound',
'Animals' => 'dog',
'Jobs' => 'doctor'
};
Split inside map and assign directly to a hash like so:
my #ar = qw[Animals,dog Money,pound Jobs,doctor Food];
my %hash_new = map {
my #a = split /,/, $_, 2;
#a == 2 ? #a : (#a, undef)
} #ar;
Note that this can also handle the case with more than one comma delimiter (hence splitting into a max of 2 elements). This can also handle the case with no commas, such as Food - in this case, the list with the single element plus the undef is returned.
If you need to push multiple key/value pairs to (another) hash, or merge hashes, you can assign a list of hashes like so:
%hash = (%hash_old, %hash_new);
Note that the same keys in the old hash will be overwritten by the new hash.
We can assign this array to a hash and perl will automatically look at the values in the array as if they were key-value pairs. The odd elements (first, third, fifth) will become the keys and the even elements (second, fourth, sixth) will become the corresponding values. check url https://perlmaven.com/creating-hash-from-an-array
use strict;
use warnings;
use Data::Dumper qw(Dumper);
my #ar;
my %hash;
#The code in the enclosing block has warnings enabled,
#but the inner block has disabled (misc and qw) related warnings.
{
#You specified an odd number of elements to initialize a hash, which is odd,
#because hashes come in key/value pairs.
no warnings 'misc';
#If your code has use warnings turned on, as it should, then you'll get a warning about
#Possible attempt to separate words with commas
no warnings 'qw';
#ar = qw[Animals,dog Money,pound Jobs,doctor Food];
# join the content of array with comma => Animals,dog,Money,pound,Jobs,doctor,Food
# split the content using comma and assign to hash
# split function returns the list in list context, or the size of the list in scalar context.
%hash = split(",", (join(",", #ar)));
}
print Dumper(\%hash);
Output
$VAR1 = {
'Animals' => 'dog',
'Money' => 'pound',
'Jobs' => 'doctor',
'Food' => undef
};
I wish to fill a hash table successively by applying a function several times. The function takes a hash reference in argument, fills it, and returns it. The hash is taken again in argument by the function.
It seems that the hash is not filled at all.
Here is my code :
Can someone tell me where might be the error please ?
sub extractMLResult {
my (%h1, %h2, %h3, %h4, $param) = #_;
my $h1= shift;
my $h2= shift;
my $h3= shift;
my $h4=shift;
$params= shift;
# read csv file, split it, fill hashes with values
$h1->{$key1}{$key2}{'p'}=$val1;
# ... do the same for the other hashes ...
return (%$h1, %$h2, %$h3, %$h4);
}
my %myhash = ();
my %h1= ();
my %h2= ();
my %h3= ();
my %h4= ();
$myhash{'a'}{'x'}=1;
$myhash{'b'}{'y'}=1;
if (exists $myhash{'a'}){
(%h1, %h2, %h3, %h4) = extractMLResult(\%h1, \%h2, \%h3, \%h4, 'a');
}
if (exists $myhash{'b'}){
(%h1, %h2, %h3, %h4) = extractMLResult(\%h1, \%h2, \%h3, \%h4, 'b');
}
my declares variables in the lexical scope. So the instant you exit your 'if' clause, %$h1 etc. vanishes again.
Also, you're doing some strange things with the assigning, which I don't think will be working thew way you think - you're deferencing your hash-references, and as such returning a list of values.
Those will all be going into %$h1 because of the way list assignments work.
But on the flip side - when you're reading in myfunction your assignment probably isn't doing what you think.
Because you're calling myfunction and passing a list of values, but you're doing a list assignment for %h1. That means all your arguments are 'consumed':
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
sub myfunction {
my (%h1, %h2, %h3, %h4, $param) = #_;
print Dumper \%h1;
print Dumper \%h2;
}
# launch function :
my %h1 = ( a => "test" );
my %h2 = ( b => "wibble" );
myfunction ( \%h1, \%h2 );
As you will see - your arguments are all consumed by the assignment to %h1 and none are left for the rest of your assignments.
More importantly - your code doesn't even compile, because if you do this:
my (%$h1, %$h2, %$h3, %$h4) = myfunction (\%h1, \%h2, \%h3, \%h4, "a");
You get:
Can't declare hash dereference in "my"
So perhaps give us some sample code that actually illustrates the problem - and runs, with some sample data?
Edit: With the more code - the problem is right here:
sub extractMLResult {
my (%h1, %h2, %h3, %h4, $param) = #_;
Because that's not doing what you think it's doing. Because %h1 is a hash, and it's assigned in a list context - all the arguments of #_ are inserted into it. So %h2, %h3, %h4, $param will always be empty/undefined.
You don't indicate whether you're actually using %h1 though, which just means it's confusing nonsense - potentially.
But this bit:
my $h1= shift;
my $h2= shift;
my $h3= shift;
my $h4_parents = shift;
Ok, so you're extracting some hash references here, which is perhaps a little more sane. But naming the same as the hashes is confusing - there's NO relationship between $h1 and %h1, and you'll confuse things in your code if you do that. (Because $h1{key} is from %h1 and nothing to do with $h1 in turn).
But the same problem exists in return:
(%h1, %h2, %h3, %h4) = extractMLResult(\%h1, \%h2, \%h3, \%h4, 'a');
Because your return:
return (%$h1, %$h2, %$h3, %$h4);
This return will give you back an unrolled list containing all the elements in the hashes. But given the way you're packing the hashes, they'll probably be a partially unrolled list, containing hash-references.
But then, they'll all be consumed by %h1 again, in the assignment.
You would need to:
return ( $h1, $h2, $h3, $h4);
And then in your function:
( $h1_ret, $h2_ret, $h3_ret, $h4_ret ) = extractMLResult(\%h1, \%h2, \%h3, \%h4, 'a');
And then unpack:
%h1 = %$h1_ret;
Or just stick with working with references all the way through, which is probably clearer for all concerned.
You are passing hash references into your subroutine. This is a good idea. But inside your subroutine, you are treating your parameters as hashes (not hash references). This is a bad idea.
A hash is initialised from a list. It should be a list with an even number of elements. Each pair of elements in the list will become a key/value pair in the hash.
my %french => ('one', 'un', 'two', 'deux', 'three', 'trois');
We often use the "fat comma" to emphasise the link between keys and values.
my %french => (one => 'un', two => 'deux', three => 'trois');
This means that hash initialisation is a greedy operation. It will use up all of any list that it is given. You cannot initialise two hashes in the same statement:
my (%french, %german) = (one => 'un', two => 'deux',
three => 'drei', four => 'vier');
This doesn't work, as all of the pairs will end up in %french, leaving nothing to populate %german.
This is the same mistake that you are making when extracting the parameters within your subroutine. You have this:
my (%h1, %h2, %h3, %h4, $param) = #_;
Nothing will end up in %h2,%h3,%h4or$paramas the assignment to%his greedy and will take all of the data values from#_` - leaving nothing for the other variables.
But, as you are passing hash references, your code shouldn't look like that. A hash reference is a scalar value (that's pretty much the point of them) so it is stored in a scalar variable.
What you want is this:
# Note, scalars ($), not hashes (%)
my ($h1, $h2, $h3, $h4, $param) = #_;
This should get you started. Note also, that you'll now need to deal with hash references ($h1->{key}) rather than hashes ($h1{key}).
And, please, always include both use strict and use warnings.
I noticed the other day that - while altering values in a hash - that when you dereference a hash in Perl, you actually are making a copy of that hash. To confirm I wrote this quick little script:
#! perl
use warnings;
use strict;
my %h = ();
my $hRef = \%h;
my %h2 = %{$hRef};
my $h2Ref = \%h2;
if($hRef eq $h2Ref) {
print "\n\tThey're the same $hRef $h2Ref";
}
else {
print "\n\tThey're NOT the same $hRef $h2Ref";
}
print "\n\n";
The output:
They're NOT the same HASH(0x10ff6848) HASH(0x10fede18)
This leads me to realize that there could be spots in some of my scripts where they aren't behaving as expected. Why is it even like this in the first place? If you're passing or returning a hash, it would be more natural to assume that dereferencing the hash would allow me to alter the values of the hash being dereferenced. Instead I'm just making copies all over the place without any real need/reason to beyond making syntax a little more obvious.
I realize the fact that I hadn't even noticed this until now shows its probably not that big of a deal (in terms of the need to go fix in all of my scripts - but important going forward). I think its going to be pretty rare to see noticeable performance differences out of this, but that doesn't alter the fact that I'm still confused.
Is this by design in perl? Is there some explicit reason I don't know about for this; or is this just known and you - as the programmer - expected to know and write scripts accordingly?
The problem is that you are making a copy of the hash to work with in this line:
my %h2 = %{$hRef};
And that is understandable, since many posts here on SO use that idiom to make a local name for a hash, without explaining that it is actually making a copy.
In Perl, a hash is a plural value, just like an array. This means that in list context (such as you get when assigning to a hash) the aggregate is taken apart into a list of its contents. This list of pairs is then assembled into a new hash as shown.
What you want to do is work with the reference directly.
for (keys %$hRef) {...}
for (values %$href) {...}
my $x = $href->{some_key};
# or
my $x = $$href{some_key};
$$href{new_key} = 'new_value';
When working with a normal hash, you have the sigil which is either a % when talking about the entire hash, a $ when talking about a single element, and # when talking about a slice. Each of these sigils is then followed by an identifier.
%hash # whole hash
$hash{key} # element
#hash{qw(a b)} # slice
To work with a reference named $href simply replace the string hash in the above code with $href. In other words, $href is the complete name of the identifier:
%$href # whole hash
$$href{key} # element
#$href{qw(a b)} # slice
Each of these could be written in a more verbose form as:
%{$href}
${$href}{key}
#{$href}{qw(a b)}
Which is again a substitution of the string '$href' for 'hash' as the name of the identifier.
%{hash}
${hash}{key}
#{hash}{qw(a b)}
You can also use a dereferencing arrow when working with an element:
$hash->{key} # exactly the same as $$hash{key}
But I prefer the doubled sigil syntax since it is similar to the whole aggregate and slice syntax, as well as the normal non-reference syntax.
So to sum up, any time you write something like this:
my #array = #$array_ref;
my %hash = %$hash_ref;
You will be making a copy of the first level of each aggregate. When using the dereferencing syntax directly, you will be working on the actual values, and not a copy.
If you want a REAL local name for a hash, but want to work on the same hash, you can use the local keyword to create an alias.
sub some_sub {
my $hash_ref = shift;
our %hash; # declare a lexical name for the global %{__PACKAGE__::hash}
local *hash = \%$hash_ref;
# install the hash ref into the glob
# the `\%` bit ensures we have a hash ref
# use %hash here, all changes will be made to $hash_ref
} # local unwinds here, restoring the global to its previous value if any
That is the pure Perl way of aliasing. If you want to use a my variable to hold the alias, you can use the module Data::Alias
You are confusing the actions of dereferencing, which does not inherently create a copy, and using a hash in list context and assigning that list, which does. $hashref->{'a'} is a dereference, but most certainly does affect the original hash. This is true for $#$arrayref or values(%$hashref) also.
Without the assignment, just the list context %$hashref is a mixed beast; the resulting list contains copies of the hash keys but aliases to the actual hash values. You can see this in action:
$ perl -wle'$x={"a".."f"}; for (%$x) { $_=chr(ord($_)+10) }; print %$x'
epcnal
vs.
$ perl -wle'$x={"a".."f"}; %y=%$x; for (%y) { $_=chr(ord($_)+10) }; print %$x; print %y'
efcdab
epcnal
but %$hashref isn't acting any differently than %hash here.
No, dereferencing does not create a copy of the referent. It's my that creates a new variable.
$ perl -E'
my %h1; my $h1 = \%h1;
my %h2; my $h2 = \%h2;
say $h1;
say $h2;
say $h1 == $h2 ?1:0;
'
HASH(0x83b62e0)
HASH(0x83b6340)
0
$ perl -E'
my %h;
my $h1 = \%h;
my $h2 = \%h;
say $h1;
say $h2;
say $h1 == $h2 ?1:0;
'
HASH(0x9eae2d8)
HASH(0x9eae2d8)
1
No, $#{$someArrayHashRef} does not create a new array.
If perl did what you suggest, then variables would get aliased very easily, which would be far more confusing. As it is, you can alias variables with globbing, but you need to do so explicitly.
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, "hello",
"wilma", 1.72e30, "betty", "bye\n");
my #any_array;
#any_array = %some_hash;
print %some_hash;
print "\n";
print #any_array;
print "\n";
print $any_array[0];
print "\n";
print $any_array[1];
print "\n";
print $any_array[2];
print "\n";
print $any_array[3];
print "\n";
print $any_array[4];
print "\n";
print $any_array[5];
print "\n";
print $any_array[6];
print "\n";
print $any_array[7];
print "\n";
print $any_array[8];
print "\n";
print $any_array[9];
Output as this
D:\learning\perl>test.pl
bettybye
bar12.4wilma1.72e+030foo352.5hello
bettybye
bar12.4wilma1.72e+030foo352.5hello
betty
bye
bar
12.4
wilma
1.72e+030
foo
35
2.5
hello
D:\learning\perl>
What decided the elements print order in my sample code?
Any rule to follow when print a mixed(strings, numbers) hash in Perl? Thank you.
bar12.4wilma1.72e+030foo352.5hello
[Updated]
With you guys help, i updated the code as below.
#!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, "hello",
"wilma", 1.72e30, "betty", "bye");
my #any_array;
#any_array = %some_hash;
print %some_hash;
print "\n";
print "\n";
print #any_array;
print "\n";
print "\n";
my #keys;
#keys = keys %some_hash;
for my $k (sort #keys)
{
print $k, $some_hash{$k};
}
output
D:\learning\perl>test.pl
bettybyebar12.4wilma1.72e+030foo352.5hello
bettybyebar12.4wilma1.72e+030foo352.5hello
2.5hellobar12.4bettybyefoo35wilma1.72e+030
D:\learning\perl>
Finially, after called keys and sort functions. The hash keys print followed the rule below
2.5hellobar12.4bettybyefoo35wilma1.72e+030
Elements of a hash are printed out in their internal order, which can not be relied upon and will change as elements are added and removed. If you need all of the elements of a hash in some sort of order, sort the keys, and use that list to index the hash.
If you are looking for a structure that holds its elements in order, either use an array, or use one of the ordered hash's on CPAN.
the only ordering you can rely upon from a list context hash expansion is that key => value pairs will be together.
From perldoc -f keys:
The keys of a hash are returned in an apparently random order. The actual random order is subject to change in future versions of Perl, but it is guaranteed to be the same order as either the values or each function produces (given that the hash has not been modified). Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see Algorithmic Complexity Attacks in perlsec).
...
Perl has never guaranteed any ordering of the hash keys, and the ordering has already changed several times during the lifetime of Perl 5. Also, the ordering of hash keys has always been, and continues to be, affected by the insertion order.
Also note that while the order of the hash elements might be randomised, this "pseudoordering" should not be used for applications like shuffling a list randomly (use List::Util::shuffle() for that, see List::Util, a standard core module since Perl 5.8.0; or the CPAN module Algorithm::Numerical::Shuffle), or for generating permutations (use e.g. the CPAN modules Algorithm::Permute or Algorithm::FastPermute), or for any cryptographic applications.
Note: since you are evaluating a hash in list context, you are at least guaranteed that each key is followed by its corresponding value; e.g. you will never see an output of a 4 b 3 c 2 d 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.
The elements are (almost certainly) printed out in the order they appear (internally) in the hash table itself -- i.e. based on the hash values of their keys.
The general rule to follow is to use something other than a hash table if you care much about the order.
Hashes are not (necessarily) retrieved in a sorted manner. If you want them sorted, you have to do it yourself:
use strict;
use warnings;
my %hash = ("a" => 1, "b" => 2, "c" => 3, "d" => 4);
for my $i (sort keys %hash) {
print "$i -> $hash{$i}\n";
}
You retrieve all the keys from a hash by using keys and you then sort them using sort. Yeah, I know, that crazy Larry Wall guy, who would've ever thought of calling them that? :-)
This outputs:
a -> 1
b -> 2
c -> 3
d -> 4
For most practical purposes, the order in which a hash table (not just Perl hash variables, but hash tables in general) can be considered random.
In reality, depending on the hashing implementation, the order may actually be deterministic. (i.e., If you run the program multiple times putting the same items into the hash table in the same order each time, they'll be stored in the same order each time.) I know that Perl hashes used to have this characteristic, but I'm not sure about current versions. In any case, hash key order is not a reliable source of randomness to use in cases where randomness is desirable.
Short version, then:
Don't use a hash if you care about the order (or lack of order). If you want a fixed order, it will be effectively random and if you want a random order, it will be effectively fixed.
A hash defines no ordering properties. The order in which things come out will be unpredictable.
And if you are crazy and have no duplicate values in your hash, and you need the values sorted, you can call reverse on it.
my %hash = ("a" => 1, "b" => 2, "c" => 3, "d" => 4);
my %reverse_hash = reverse %hash;
print $_ for sort keys %reverse_hash;
Caveat is the unique values part, duplicates will be overwritten and only one value will get in.
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, "hello",
"wilma", 1.72e30, "betty", "bye\n");
my #any_array;
#any_array = %some_hash;
print %some_hash;
print "\n";
print #any_array;
print "\n";
print $any_array[0];
print "\n";
print $any_array[1];
print "\n";
print $any_array[2];
print "\n";
print $any_array[3];
print "\n";
print $any_array[4];
print "\n";
print $any_array[5];
print "\n";
print $any_array[6];
print "\n";
print $any_array[7];
print "\n";
print $any_array[8];
print "\n";
print $any_array[9];
Output as this
D:\learning\perl>test.pl
bettybye
bar12.4wilma1.72e+030foo352.5hello
bettybye
bar12.4wilma1.72e+030foo352.5hello
betty
bye
bar
12.4
wilma
1.72e+030
foo
35
2.5
hello
D:\learning\perl>
What decided the elements print order in my sample code?
Any rule to follow when print a mixed(strings, numbers) hash in Perl? Thank you.
bar12.4wilma1.72e+030foo352.5hello
[Updated]
With you guys help, i updated the code as below.
#!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, "hello",
"wilma", 1.72e30, "betty", "bye");
my #any_array;
#any_array = %some_hash;
print %some_hash;
print "\n";
print "\n";
print #any_array;
print "\n";
print "\n";
my #keys;
#keys = keys %some_hash;
for my $k (sort #keys)
{
print $k, $some_hash{$k};
}
output
D:\learning\perl>test.pl
bettybyebar12.4wilma1.72e+030foo352.5hello
bettybyebar12.4wilma1.72e+030foo352.5hello
2.5hellobar12.4bettybyefoo35wilma1.72e+030
D:\learning\perl>
Finially, after called keys and sort functions. The hash keys print followed the rule below
2.5hellobar12.4bettybyefoo35wilma1.72e+030
Elements of a hash are printed out in their internal order, which can not be relied upon and will change as elements are added and removed. If you need all of the elements of a hash in some sort of order, sort the keys, and use that list to index the hash.
If you are looking for a structure that holds its elements in order, either use an array, or use one of the ordered hash's on CPAN.
the only ordering you can rely upon from a list context hash expansion is that key => value pairs will be together.
From perldoc -f keys:
The keys of a hash are returned in an apparently random order. The actual random order is subject to change in future versions of Perl, but it is guaranteed to be the same order as either the values or each function produces (given that the hash has not been modified). Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see Algorithmic Complexity Attacks in perlsec).
...
Perl has never guaranteed any ordering of the hash keys, and the ordering has already changed several times during the lifetime of Perl 5. Also, the ordering of hash keys has always been, and continues to be, affected by the insertion order.
Also note that while the order of the hash elements might be randomised, this "pseudoordering" should not be used for applications like shuffling a list randomly (use List::Util::shuffle() for that, see List::Util, a standard core module since Perl 5.8.0; or the CPAN module Algorithm::Numerical::Shuffle), or for generating permutations (use e.g. the CPAN modules Algorithm::Permute or Algorithm::FastPermute), or for any cryptographic applications.
Note: since you are evaluating a hash in list context, you are at least guaranteed that each key is followed by its corresponding value; e.g. you will never see an output of a 4 b 3 c 2 d 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.
The elements are (almost certainly) printed out in the order they appear (internally) in the hash table itself -- i.e. based on the hash values of their keys.
The general rule to follow is to use something other than a hash table if you care much about the order.
Hashes are not (necessarily) retrieved in a sorted manner. If you want them sorted, you have to do it yourself:
use strict;
use warnings;
my %hash = ("a" => 1, "b" => 2, "c" => 3, "d" => 4);
for my $i (sort keys %hash) {
print "$i -> $hash{$i}\n";
}
You retrieve all the keys from a hash by using keys and you then sort them using sort. Yeah, I know, that crazy Larry Wall guy, who would've ever thought of calling them that? :-)
This outputs:
a -> 1
b -> 2
c -> 3
d -> 4
For most practical purposes, the order in which a hash table (not just Perl hash variables, but hash tables in general) can be considered random.
In reality, depending on the hashing implementation, the order may actually be deterministic. (i.e., If you run the program multiple times putting the same items into the hash table in the same order each time, they'll be stored in the same order each time.) I know that Perl hashes used to have this characteristic, but I'm not sure about current versions. In any case, hash key order is not a reliable source of randomness to use in cases where randomness is desirable.
Short version, then:
Don't use a hash if you care about the order (or lack of order). If you want a fixed order, it will be effectively random and if you want a random order, it will be effectively fixed.
A hash defines no ordering properties. The order in which things come out will be unpredictable.
And if you are crazy and have no duplicate values in your hash, and you need the values sorted, you can call reverse on it.
my %hash = ("a" => 1, "b" => 2, "c" => 3, "d" => 4);
my %reverse_hash = reverse %hash;
print $_ for sort keys %reverse_hash;
Caveat is the unique values part, duplicates will be overwritten and only one value will get in.