I am learning Perl Script from here.
I am having problem creating Hash. The code is here:
print "Hello World!\n";
#days = ("1", "2");
print "There are $#days days\n";
print "1 is $days[0]\n";
%months = ("a" => 1, "b" => 2, "c" => 3);
print "There are $#months keys\n";
print "a is $months[0]\n";
for $i (keys %months)
{ print "$i has value $months[$i].\n"}
Now its working fine with the array.
But for Hash its printing "There are -1 keys".
Also its not printing anything for the variable values in last to print calls.
You are using the array syntax on a hash, which does not do what you think at all. Instead of operating on your hash, you are operating on an array called #months. For example:
print "There are $#months keys\n";
This will look for the array #months, see that it is empty, and happily print -1.
When you do
for $i (keys %months) {
print "$i has value $months[$i].\n"
}
Perl will try to convert the keys a, b and c to numbers, which will be 0. This will issue a warning:
Argument "a" isn't numeric in array element ...
Then it will print the empty array element $month[0]. Which will issue an undefined value warning. You do not get these warnings, because you did not use
use strict;
use warnings;
In your script. strict would have told you that #months has not been declared, and stopped this bug right away.
The syntax you should have used is:
print "There are " . (keys %months) . " keys\n";
...
print "$i has value $months{$i}\n";
In Perl, accessing elements in a hash use a slightly different syntax to arrays. Use curlies for hashes, square brackets for arrays:
print "a is $months{a}\n"; # "a is 1"
And $#months is another way of saying 'last index of #months', when what you really meant was to count the number of keys in %months:
printf "There are %d keys\n", scalar keys %months;
If you insist on print instead of printf:
print "There are $#{[keys %months]} keys\n";
(but maybe it's a few steps ahead of where you want to be at the moment)
$#months and $months[0]refer to an array and not a hash. You access the value of a hash by using curly braces $months{key}.
Also, you should use strict; and initialize variables with my(). If you had done that, you would have gotten a compiler error that #months does not exist.
Related
Sorry, I'm super rusty with Perl. See the following code:
foreach my $hash (keys %greylist)
{
$t = $greylist{$hash};
print $greylist{$hash}[4] . "\n";
print $t[4] . "\n";
}
Why does $t[4] evaluate to a blank string, yet $greylist{$hash}[4] which should be the same thing evaluates to an IP address?
$greylist{$hash} contains an array reference. When you do:
print $greylist{$hash}[4];
Perl automatically treats it as an array reference but when you do:
$t = $greylist{$hash};
print $t[4];
You're assigning the array reference to a scalar variable, $t, then attempting to access the 5th element of another variable, #t. use strict would give you an error in this scenario.
Use the arrow operator, ->, to dereference:
$t = $greylist{$hash};
print $t->[4];
perlreftut has a note about this:
If $aref holds a reference to an array, then $aref->[3] is the fourth element of the array. Don't confuse this with $aref[3] , which is the fourth element of a totally different array, one deceptively named #aref . $aref and #aref are unrelated the same way that $item and #item are.
I have the code below:
#a = ((1,2,3),("test","hello"));
print #a[1]
I was expecting it to print
testhello
But it gives me 2.
Sorry for the newbie question (Perl is a bit unnatural to me) but why does it happen and how can I get the result I want?
The way Perl constructs #a is such that it is equivalent to your writing,
#a = (1,2,3,"test","hello");
And that is why when you ask for the value at index 1 by writing #a[1] (really should be $a[1]), you get 2. To demonstrate this, if you were to do the following,
use strict;
use warnings;
my #a = ((1,2,3), ("test","hello"));
my #b = (1,2,3,"test","hello");
print "#a\n";
print "#b\n";
Both print the same line,
1 2 3 test hello
1 2 3 test hello
What you want is to create anonymous arrays within your array - something like this,
my #c = ([1,2,3], ["test","hello"]);
Then if you write the following,
use Data::Dumper;
print Dumper $c[1];
You will see this printed,
$VAR1 = [
'test',
'hello'
];
Perl lists are one-dimensional only, which means (1,2,(3,4)) is automatically flattened to (1,2,3,4). If you want a multidimensional array, you must use references for any dimension beyond the first (which are stored in scalars).
You can get any anonymous array reference with bracket notation [1,2,3,4] or reference an existing array with a backslash my $ref = \#somearray.
So a construct such as my $aref = [1,2,[3,4]] is an array reference in which the first element of the referenced array is 1, the second element is 2, and the third element is another array reference.
(I find when working with multidimensional arrays, that it's less confusing just to use references even for the first dimension, but my #array = (1,2,[3,4]) is fine too.)
By the way, when you stringify a perl reference, you get some gibberish indicating the type of reference and the memory location, like "ARRAY(0x7f977b02ac58)".
Dereference an array reference to an array with #, or get a specific element of the reference with ->.
Example:
my $ref = ['A','B',['C','D']];
print $ref; # prints ARRAY(0x001)
print join ',', #{$ref}; # prints A,B,ARRAY(0x002)
print join ',', #$ref; # prints A,B,ARRAY(0x002) (shortcut for above)
print $ref->[0]; # prints A
print $ref->[1]; # prints B
print $ref->[2]; # prints ARRAY(0x002)
print $ref->[2]->[0]; # prints C
print $ref->[2][0]; # prints C (shortcut for above)
print $ref->[2][1] # prints D
print join ',', #{$ref->[2]}; # prints C,D
I think you're after an array of arrays. So, you need to create an array of array references by using square brackets, like this:
#a = ([1,2,3],["test","hello"]);
Then you can print the second array as follows:
print #{$a[1]};
Which will give you the output you were expecting: testhello
It's just a matter of wrong syntax:
print $a[1]
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.
These are the ones I'm aware of:
The behaviour of a "my" statement modified with a statement modifier conditional or loop construct (e.g. "my $x if ...").
Modifying a variable twice in the same statement, like $i = $i++;
sort() in scalar context
truncate(), when LENGTH is greater than the length of the file
Using 32-bit integers, "1 << 32" is undefined. Shifting by a negative number of bits is also undefined.
Non-scalar assignment to "state" variables, e.g. state #a = (1..3).
One that is easy to trip over is prematurely breaking out of a loop while iterating through a hash with each.
#!/usr/bin/perl
use strict;
use warnings;
my %name_to_num = ( one => 1, two => 2, three => 3 );
find_name(2); # works the first time
find_name(2); # but fails this time
exit;
sub find_name {
my($target) = #_;
while( my($name, $num) = each %name_to_num ) {
if($num == $target) {
print "The number $target is called '$name'\n";
return;
}
}
print "Unable to find a name for $target\n";
}
Output:
The number 2 is called 'two'
Unable to find a name for 2
This is obviously a silly example, but the point still stands - when iterating through a hash with each you should either never last or return out of the loop; or you should reset the iterator (with keys %hash) before each search.
These are just variations on the theme of modifying a structure that is being iterated over:
map, grep and sort where the code reference modifies the list of items to sort.
Another issue with sort arises where the code reference is not idempotent (in the comp sci sense)--sort_func($a, $b) must always return the same value for any given $a and $b.
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.