Perl Hashes Software Tools

Slide 2 “bill” “cheap” What is a Hash? l A hash (or associative array) is like an array, where the index can be any scalar value (not just small non-negative integers). l A hash index is called a key (keys must be unique). l The elements of a hash have no fixed order, unlike arrays. l 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. l The keys are used to lookup the values “0 C” “32 F” keys: values:

Slide 3 Hash Variables l 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. l 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.

Slide 4 Hash Variables l As with arrays, you create new hash elements: $bill{"Gates"} = "cheap"; $bill{"Clinton"} = "busy"; $bill{234.5} = 456.7; l 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"; l Output: Bill Gates is cheap Bill Clinton is busy Bill is 456.7

Slide 5 Hash Variables l 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.

Slide 6 List Representation of a Hash l 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 l The order of the key-value pairs is random in the list representation and cannot be controlled. (Don’t rely on the ordering.)

Slide 7 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 $

Slide 8 keys 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;

Slide 9 keys 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 (keys(%bill)){ # for each key of %bill print "At $k we have $bill{$k}\n"; } $ key At we have At Gates we have cheap At Clinton we have busy $

Slide 10 Printing Hashes You cannot print the entire hash like you can 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).

Slide 11 keys 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.

Slide 12 values 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 $

Slide 13 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 $

Slide 14 delete l How to remove hash elements? Answer: Use the delete function. l 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 $

Slide 15 Hash Slices l Like an array, a hash can be sliced to access a collection of elements. l 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);

Slide 16 Hash Slices l 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 $

Slide 17 Hash Slices - Merging Hashes l Hash slices can also be used to merge hashes. In the following example, if there are duplicate keys, the values from the %small hash are used: $ 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 $

Slide 18 Hash Slices - Merging Hashes l 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 $