Perl Hashes Learning Objectives: 1. To understand the concept of Hash as a data structure 2. To learn the operations of Hash which are available in Perl 3. To understand the usage & application of hash in Perl

COMP111 Lecture 13 / Slide 2 Perl Hashes Table of Content  What is a Hash ?  Hash Variables  List Representation of a Hash  Hash “reverse”  Retrieving “keys” in a Hash  Printing Hashes  Keys again  Retrieving “values” in a Hash  each  Delete a Hash element with a key  Hash Slices  Hash Slices – Merging Hashes  Command line & Environment  More About Hashes  Use of Hash  Hash of Hashes  Hash of Hashes of Hashes  Array of Arrays

COMP111 Lecture 13 / Slide 3 “bill” “cheap” What is a Hash?  A hash (or associative array) is like an array, where the index can be any scalar value (not just small non-negative integers).  A hash index is called a key (keys must be unique).  The elements of a hash have no fixed order, unlike arrays.  The elements of a hash are like filing cards, where the top half of the card has the key, and the bottom half has the value.  The keys are used to lookup the values “0 C” “32 F” keys: values:

COMP111 Lecture 13 / Slide 4 Hash Variables (1)  Hash variable names begin with the percent sign (%) followed by the usual variable name.  There is no relationship between and %bill, Perl considers them to be separate variables.  Each element of a hash is a separate scalar variable, accessed by the key.  Elements of the hash %bill, are referenced with $bill{$key}, where $key is any scalar expression.

COMP111 Lecture 13 / Slide 5 Hash Variables (2)  As with arrays, you create new hash elements: $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7;  Once created, you can access hash values similar to indexing arrays: print "Bill Gates is ", $bill{"Gates"}, "\n"; $n = "Clinton"; print "Bill $n is $bill{$n}\n"; $n = 234.5; print "Bill $n is $bill{$n}\n";  Output: Bill Gates is cheap Bill Clinton is busy Bill is 456.7

COMP111 Lecture 13 / Slide 6 Hash Variables (3)  Once created, you can change hash values if needed: $bill{234.5} = 456.7;... $bill{234.5} += 3;# makes it  Referencing a hash element that does not exist returns the undef value.

COMP111 Lecture 13 / Slide 7 List Representation of a Hash  You can access the hash as a whole if you need to initialize it or to copy it.  The hash unwinds as a list. Each pair in the list represents the key and its value. $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = = %bill; qw(Gates cheap Clinton busy ) %a # create %a like %bill %a = %bill; # faster way to do the same %b = qw(Gates cheap Clinton busy ); # initialize %b like %bill from list values  The order of the key-value pairs is random in the list representation and cannot be controlled. (Don ’ t rely on the ordering.)

COMP111 Lecture 13 / Slide 8 Hash “reverse”  You can construct a hash with keys and values swapped using the reverse function: %aback = reverse %a;  If %a has two identical values, those will end up as a single element in %aback ( reverse is best used on hashes with unique keys and values). $ cat reverse1 #!/usr/local/bin/perl5 -w $b{"Gates"} = "Bill"; $b{"Clinton"} = "Bill"; %revb = reverse %b; # print out revb key and value... $ reverse1 Bill Gates $

COMP111 Lecture 13 / Slide 9 Retrieving “keys” in a Hash (1)  The keys(%hashname) function returns a list of all the keys currently in the hash. $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = = keys(%bill); gets qw(Gates Clinton 234.5) in some random order  If there are no elements in the hash, then keys() returns an empty list.  As with other Perl functions, the parentheses are = keys %bill;

COMP111 Lecture 13 / Slide 10  The keys function is often used in foreach loops to print or access the elements one-by-one: $ cat key #!/usr/local/bin/perl5 -w $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7; foreach $k (sort(keys(%bill))){ print "At $k we have $bill{$k}\n"; } $ key At we have At Clinton we have busy At Gates we have cheap $ Retrieving “keys” in a Hash (2)

COMP111 Lecture 13 / Slide 11 Printing Hashes  You cannot print the entire hash like you can print arrays: $ cat prhash #!/usr/local/bin/perl5 -w $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7; print "Bill hash: %bill\n"; $ prhash Bill hash: %bill $  Most Perl programmers use a foreach loop to print hashes (as in the previous slide).

COMP111 Lecture 13 / Slide 12 “keys” again  In a scalar context, keys returns the number of elements in the hash: $ cat key1 #!/usr/local/bin/perl5 -w $n = keys(%bill); if($n==0){ print "Bill is empty\n"; exit; } print "Bill has $n elements\n"; $ key1 Bill is empty $  exit is like the C++ exit() function, and ends the program immediately.

COMP111 Lecture 13 / Slide 13 Retrieving “values” in a hash  The values(%hashname) function returns a list of all the values currently in the hash.  The values are in exactly the same order as the keys returned by keys(%hashname). $ cat value #!/usr/local/bin/perl5 -w $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = = = values(%bill); print print $ value Gates Clinton cheap busy $

COMP111 Lecture 13 / Slide 14 each  Another way to print a hash is with the each function.  each returns a key-value pair as a two-element list.  Each time each is called it returns the next key-value pair until all the elements have been accessed.  When no more pairs, each returns an empty list. $ cat each #!/usr/local/bin/perl5 -w $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7; while(($k,$v) = each(%bill)){ print "At $k we have $v\n"; } $ each At we have At Gates we have cheap At Clinton we have busy $

COMP111 Lecture 13 / Slide 15 “delete” a Hash element with a key  How to remove hash elements? Answer: Use the delete function.  The argument is the keyed reference to be deleted: $ cat delete1 #!/usr/local/bin/perl5 -w $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7; delete $bill{"Gates"}; while(($k,$v) = each(%bill)){ print "At $k we have $v\n"; } $ delete1 At we have At Clinton we have busy $

COMP111 Lecture 13 / Slide 16 Hash Slices (1)  Like an array, a hash can be sliced to access a collection of elements.  We can use a hash slice to compact initialization. For example: $b{"Gates"} = "cheap"; $b{"Clinton"} = "busy"; $b{234} = 45; can be shortened to: ($b{"Gates"},$b{"Clinton"},$b{234}) = qw(cheap busy 45); can be hash sliced = qw(cheap busy 45); (Note that it not %b.)  Another "Z"} = (1.. 26);

COMP111 Lecture 13 / Slide 17 Hash Slices (2)  We can also use a hash slice with variable interpolation : $ cat hslice #!/usr/local/bin/perl5 = qw(cheap busy = qw(Gates Clinton); print "The values are: $ hslice The values are: cheap busy $

COMP111 Lecture 13 / Slide 18 Hash Slices - Merging Hashes (1)  Hash slices can also be used to merge hashes.  In the following example, if there are duplicate keys, the values overwritten by the %small hash: $ cat merge #!/usr/local/bin/perl5 = qw(cheap busy = qw(111 %small} = values %small; while(($k,$v) = each(%big)){ print "At $k we have $v\n"; } $ merge At 234 we have 67 At Gates we have cheap At Horner we have 111 At Clinton we have busy $

COMP111 Lecture 13 / Slide 19 Hash Slices - Merging Hashes (2)  A simpler (though slower) way to merge hashes is: $ cat merge1 #!/usr/local/bin/perl5 = qw(cheap busy = qw(111 67); %big = (%big, %small); while(($k,$v) = each(%big)){ print "At $k we have $v\n"; } $ merge1 At 234 we have 67 At Gates we have cheap At Horner we have 111 At Clinton we have busy $

COMP111 Lecture 13 / Slide 20 Command line & Environment (1)  The name of your Perl script is $0  The array for command line parameter  The process ID of your Perl script is $PID  Last shell execution status: $? (backtick ` ` command, or system operator)  The hash containing your current environment: %ENV

COMP111 Lecture 13 / Slide 21 Command line & Environment (2) $ cat p1 #!/usr/local/bin/perl5 -w $argc print "parameter count $argc for $0 (PID: $$)\n"; print "parmamters: $ARGV[0] $ARGV[1]\n"; system "cat non_existing_file > /dev/null 2>&1"; print "Status of last command: $? \n"; system "cd"; print "Status of last command: $? \n"; print "You are $ENV{USER} \n"; $ p1 a b parameter count 2 for p1 (PID: 20489) parmamters: a b Status of last command: 512 Status of last command: 0 You are kwchiu

COMP111 Lecture 13 / Slide 22 More About Hashes  Another way to initialize hashes: $ cat morehash #!/usr/local/bin/perl5 -w %bill = ( "Gates" => "cheap", "Clinton" => "busy"); print "Hello $bill{'Gates'} Bill!\n"; $ morehash Hello cheap Bill!  You may use single quotes for specifying a key (more convenient inside double quotes).

COMP111 Lecture 13 / Slide 23 Use of Hash l Simple hash as simple relations: $Name{94001} = "Peter"; $Name{94002} = "John"; l Simple hash as records: $John{"studno"} = 94002; $John{"CGA"} = 9.1; $John{"SGA"} = 8.7;... l Hash of hashes: (database) tables

COMP111 Lecture 13 / Slide 24 Hash of Hashes (1) Student# (key) Attributes NameCGAGrades 94001John9.1comp111 => A comp211 => B+ … 94002Peter8.5 … …

COMP111 Lecture 13 / Slide 25 Hash of Hashes (2)  Idea: hash element is a hash pointer $ cat hashofhash $student{94001}{'name'} = "John"; $student{94001}{'CGA'} = 9.1; print $student{94001}{'name'}; $student{94002} = {"name" => "Peter", "CGA" => 8.5}; foreach $s (keys(%student)) { print "Student $s: "; print "Name = $student{$s}{'name'}; "; print "CGA = $student{$s}{'CGA'}\n"; } $ hashofhash John Student 94001: Name = John; CGA = 9.1 Student 94002: Name = Peter; CGA = 8.5 Note { } for creating an anonymous hash instead of (); $student{94002} now contains a pointer to that hash Note { } for creating an anonymous hash instead of (); $student{94002} now contains a pointer to that hash

COMP111 Lecture 13 / Slide 26 Hash of Hashes of Hashes l We can form complicated nested data structures with repeated levels of hashes: $ cat hash3 #!/usr/local/bin/perl5 -w$ $student{94001}{'grade'}= {"comp111"=>'A',"comp211"=>'B+'}; print $student{94001}{'grade'}{'comp111'}; $p = $student{94001}{'grade'}; # $p is a ptr foreach $c (keys(%$p)) { # %$p - the hash pointed by p print "$c $$p{$c}\n"; }# $$p{...} - an element of the hash pointed by p $ hash3 comp111 A comp211 B+

COMP111 Lecture 13 / Slide 27 Array of Arrays l We can form 2-dimensional array as = = = (7,8,9); # the backslash is to get the reference of a variable # contains 3 pointers to 3 1-D = print $m[0][1], $m[1][0];# prints 2,4 l Another way is to use the square bracket (address of anonymous = ([1,2,3], [4,5,6], [7,8,9]); l Similarly, we can have hash of arrays and array of hashes, and other nested data structures....