Topic 5: Hashes CSE2395/CSE3395 Perl Programming Learning Perl 3rd edition chapter 5, pages 73-85...

Topic 5: HashesTopic 5: Hashes

CSE2395/CSE3395Perl Programming

CSE2395/CSE3395Perl Programming

Learning Perl 3rd edition chapter 5, pages 73-85

Programming Perl 3rd edition pages 76-78, 697-700, 703-704, 733-734

perldata manpage

2Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University

In this topicIn this topic

Hashes► aka associative arrays

Hash variables Functions which use hashes Uses of hashes Accessing Perl’s environment

Hashes► aka associative arrays

Hash variables Functions which use hashes Uses of hashes Accessing Perl’s environment


ArraysArrays

Arrays are► ordered► indexed by a number (integer)► dense

– if element n exists, so do elements 0 to n-1

Arrays are► ordered► indexed by a number (integer)► dense

– if element n exists, so do elements 0 to n-1

0 1 2 3 4 5

@array

indices

42 "dog" -0.2 undef 42 0


ArraysArrays

Arrays aren’t always best data structure Imagine array of students’ marks

► indexed by 8-digit student ID number

Arrays aren’t always best data structure Imagine array of students’ marks

► indexed by 8-digit student ID number

@marks

12345678 12345679 12345680 12345681

89 43 undef 70

0

Ten million empty elements in here!


ArraysArrays

Student ID numbers aren’t really numbers anyway► can’t do arithmetic on them► order of two student IDs not really important► really just strings that happen to contain digits

Want some data structure where indices are strings► usually called associative arrays

– or dictionary– or (lookup) table– or hash table

Student ID numbers aren’t really numbers anyway► can’t do arithmetic on them► order of two student IDs not really important► really just strings that happen to contain digits

Want some data structure where indices are strings► usually called associative arrays

– or dictionary– or (lookup) table– or hash table


Associative arraysAssociative arrays

Associative array is an array where► can locate an array element’s value given index► indices are strings► indices are unique► indices are unordered

For example, to look up capital cities of countries

Associative array is an array where► can locate an array element’s value given index► indices are strings► indices are unique► indices are unordered

For example, to look up capital cities of countries

Peru Japan UK Russia Canada Egypt

Lima Tokyo London Moscow Ottawa Cairo

In Perl, associative arrays are called “hashes” (because they’re implemented using hash tables)


Hashes in PerlHashes in Perl

Indices called keys► strings► must be unique► e.g., country names

Contents called values► any scalar► may be duplicated► e.g., capital city names

Can look up value given key, but not vice versa► What’s the capital of Egypt? (easy)► What country is Monrovia the capital of? (hard)

Unordered► You can’t sort a hash!► Perl stores elements in an order optimized for fast lookup

Indices called keys► strings► must be unique► e.g., country names

Contents called values► any scalar► may be duplicated► e.g., capital city names

Can look up value given key, but not vice versa► What’s the capital of Egypt? (easy)► What country is Monrovia the capital of? (hard)

Unordered► You can’t sort a hash!► Perl stores elements in an order optimized for fast lookup

Llama3 pages 73-74; Camel3 pages 51, 76-77; perldata manpage


Hash elementsHash elements

Hash key written inside { curly braces }► contrast with normal arrays using [ square brackets ]► $capital{"Egypt"} # Equal to "Cairo"► $capital{$nation} # Depends on $nation

Can assign to a hash element► overwrites the old value, if there was one

– or creates a new element, if there wasn’t► doesn’t change any other element► $capital{"Australia"} = "Canberra";

Using nonexistent key returns undef► $capital{"Atlantis"} # No such country

Hash key written inside { curly braces }► contrast with normal arrays using [ square brackets ]► $capital{"Egypt"} # Equal to "Cairo"► $capital{$nation} # Depends on $nation

Can assign to a hash element► overwrites the old value, if there was one

– or creates a new element, if there wasn’t► doesn’t change any other element► $capital{"Australia"} = "Canberra";

Using nonexistent key returns undef► $capital{"Atlantis"} # No such country

Llama3 pages 76-78; Camel3 page 67


Testing hash elementsTesting hash elements

Can determine if hash key exists using exists function► exists $capital{"Canada"} # True► exists $capital{"Atlantis"} # False

Not same as using defined► key can exist, but value can be undefined► exists $capital{"Vatican City"} # True► defined $capital{"Vatican City"} # False

Can determine if hash key exists using exists function► exists $capital{"Canada"} # True► exists $capital{"Atlantis"} # False

Not same as using defined► key can exist, but value can be undefined► exists $capital{"Vatican City"} # True► defined $capital{"Vatican City"} # False

Llama3 page 83; Camel3 pages 697-698, 710-711; perlfunc manpage


Deleting hash elementsDeleting hash elements

To remove an entry from a hash, use delete function► delete $capital{"Czechoslovakia"};► exists will now return false for that key

To clear a hash, assign empty list to entire hash► %capital = (); # World anarchy

To remove an entry from a hash, use delete function► delete $capital{"Czechoslovakia"};► exists will now return false for that key

To clear a hash, assign empty list to entire hash► %capital = (); # World anarchy

Llama3 pages 76-77, 83-84; Camel3 pages 699-700; perlfunc manpage


Entire hashesEntire hashes

To refer to an entire hash, use %hash► % instead of $► no curly braces

Can copy hashes► %clone = %hash;

Can initialize hash with many elements by assigning list to it

► for each element, write key followed by value► order of key/value pairs not important► %capital = ("Peru", "Lima", "Japan", "Tokyo", "UK", "London", "Russia", "Moscow", "Canada", "Ottawa", "Egypt", "Cairo");

Hashes flatten back into lists when used in list context► e.g., when passed to a subroutine

To refer to an entire hash, use %hash► % instead of $► no curly braces

Can copy hashes► %clone = %hash;

Can initialize hash with many elements by assigning list to it

► for each element, write key followed by value► order of key/value pairs not important► %capital = ("Peru", "Lima", "Japan", "Tokyo", "UK", "London", "Russia", "Moscow", "Canada", "Ottawa", "Egypt", "Cairo");

Hashes flatten back into lists when used in list context► e.g., when passed to a subroutine

Llama3 pages 78-79; Camel3 pages 76-78


Hash elementsHash elements

Hashes, subroutines, arrays and scalars occupy different namespaces► %x, $x{...} refer to hash %x► @x, $x[...] refer to array @x► &x, x(...) refer to subroutine &x► $x refers to scalar $x

Hash elements interpolate into double-quoted strings► print "The capital of $nation is $capital{$nation}\n";

Entire hashes don’t interpolate at all.► print "%capital"; # Prints "%capital"

Hashes, subroutines, arrays and scalars occupy different namespaces► %x, $x{...} refer to hash %x► @x, $x[...] refer to array @x► &x, x(...) refer to subroutine &x► $x refers to scalar $x

Hash elements interpolate into double-quoted strings► print "The capital of $nation is $capital{$nation}\n";

Entire hashes don’t interpolate at all.► print "%capital"; # Prints "%capital"


Functions that use hashesFunctions that use hashes

How do you print out the contents of a hash?► need to know what keys a hash has

– from each key, can get value with $hash{key}

keys function returns a list of all keys in a hash► order is indeterminate, but same every time► every key is unique

– by definition of hash► keys %capital # Returns list ("Canada", "UK", "Egypt", "Japan", "Peru", "Russia") (maybe)

values function returns a list of all values in a hash► order is same as from keys function► values may be duplicated

– values may be any scalar► values %capital # Returns list ("Ottawa", "London", "Cairo", "Tokyo", "Lima", "Moscow")

How do you print out the contents of a hash?► need to know what keys a hash has

– from each key, can get value with $hash{key}

keys function returns a list of all keys in a hash► order is indeterminate, but same every time► every key is unique

– by definition of hash► keys %capital # Returns list ("Canada", "UK", "Egypt", "Japan", "Peru", "Russia") (maybe)

values function returns a list of all values in a hash► order is same as from keys function► values may be duplicated

– values may be any scalar► values %capital # Returns list ("Ottawa", "London", "Cairo", "Tokyo", "Lima", "Moscow")

Llama3 pages 80-81; Camel3 pages 733-734, 824; perlfunc manpage


TimeoutTimeout

# Printing an entire hash using keys function.

# Initialize the hash.# The => notation is just a pretty-looking# synonym for the , (comma) operator that also quotes# the the word on the left side. Great for hashes.%capital = (Peru => "Lima", Japan => "Tokyo", UK => "London", Russia => "Moscow", Canada => "Ottawa", Egypt => "Cairo");

# Iterate over the hash, once per nation.# Order is indeterminate.foreach $nation (keys %capital){ print "Capital of $nation is $capital{$nation}\n";}

# Printing an entire hash using keys function.

# Initialize the hash.# The => notation is just a pretty-looking# synonym for the , (comma) operator that also quotes# the the word on the left side. Great for hashes.%capital = (Peru => "Lima", Japan => "Tokyo", UK => "London", Russia => "Moscow", Canada => "Ottawa", Egypt => "Cairo");

# Iterate over the hash, once per nation.# Order is indeterminate.foreach $nation (keys %capital){ print "Capital of $nation is $capital{$nation}\n";}


TimeoutTimeout

# Printing an entire hash, sorted by country.

# Initialize the hash.%capital = (Peru => "Lima", Japan => "Tokyo", UK => "London", Russia => "Moscow", Canada => "Ottawa", Egypt => "Cairo");

# Iterate over the hash, once per nation.# Note that this isn't sorting the hash,# nor even iterating over the hash, but# iterating over a sorted list of the hash's keys.foreach $nation (sort keys %capital){ print "Capital of $nation is $capital{$nation}\n";}

# Printing an entire hash, sorted by country.


# Iterate over the hash, once per nation.# Note that this isn't sorting the hash,# nor even iterating over the hash, but# iterating over a sorted list of the hash's keys.foreach $nation (sort keys %capital){ print "Capital of $nation is $capital{$nation}\n";}


Functions that use hashesFunctions that use hashes

keys may return a very large list► perhaps inefficient if you need only one hash element at a time

each function iterates over a hash► one element at a time► on first call, returns a two-element list containing one key/value

pair► subsequent calls return other key/value pairs

– order indeterminate, but guaranteed not to repeat any pairs► when all key/value pairs have been returned once, returns empty

list► state is kept by Perl with hidden attribute on hash variable► much more space-efficient than using keys► typical use

– while (($key, $value) = each %hash) { ... }

keys may return a very large list► perhaps inefficient if you need only one hash element at a time

each function iterates over a hash► one element at a time► on first call, returns a two-element list containing one key/value

pair► subsequent calls return other key/value pairs

– order indeterminate, but guaranteed not to repeat any pairs► when all key/value pairs have been returned once, returns empty

list► state is kept by Perl with hidden attribute on hash variable► much more space-efficient than using keys► typical use

– while (($key, $value) = each %hash) { ... }

Llama3 pages 81-82; Camel3 pages 703-704; perlfunc manpage


TimeoutTimeout

# Printing an entire hash, using each function.


# Iterate over the hash, once per nation.# No provision for sorting the output here,# because order returned by each function# is indeterminate.while (($nation, $city) = each %capital){ print "Capital of $nation is $city\n";}

# Printing an entire hash, using each function.


# Iterate over the hash, once per nation.# No provision for sorting the output here,# because order returned by each function# is indeterminate.while (($nation, $city) = each %capital){ print "Capital of $nation is $city\n";}


Uses of hashesUses of hashes

Hashes useful for► implementing sparse arrays► implementing lookup tables/databases► counting strings► removing duplicates from a list► passing named parameters to subroutines

Hashes useful for► implementing sparse arrays► implementing lookup tables/databases► counting strings► removing duplicates from a list► passing named parameters to subroutines

Llama3 pages 75-76


Hashes: sparse arraysHashes: sparse arrays

Normal arrays are dense► creating $a[10000] creates @a[0..9999] too.

Hash keys are independent► creating $h{"10000"} creates no other other

elements– only elements that exist need to take up memory

► just have to pretend that keys (really strings) are integers

– like student ID numbers► may have to write some code to fake “order” of

elements– foreach $element (sort {$a <=> $b} keys %h)

Normal arrays are dense► creating $a[10000] creates @a[0..9999] too.

Hash keys are independent► creating $h{"10000"} creates no other other

elements– only elements that exist need to take up memory

► just have to pretend that keys (really strings) are integers

– like student ID numbers► may have to write some code to fake “order” of

elements– foreach $element (sort {$a <=> $b} keys %h)


Hashes: lookup tableHashes: lookup table

Using hash, can look up string (value) given string (key)► look up the capital of a country

– capital of Malaysia is Kuala Lumpur► look up a word in a dictionary

– definition of dog is “domestic canine”► look up the IP address of machine

– slashdot.org’s IP address is 66.35.250.150► look up the value of a variable in an interpreter

– value of variable x is 5► look up the title of a book

– book with ISBN 0-596-00027-8 is “Programming Perl”► look up the real name of a student

– student 11111111 is Bart Simpson

Any relationship with a one-to-many relationship is perfect for a hash

Using hash, can look up string (value) given string (key)► look up the capital of a country

– capital of Malaysia is Kuala Lumpur► look up a word in a dictionary

– definition of dog is “domestic canine”► look up the IP address of machine

– slashdot.org’s IP address is 66.35.250.150► look up the value of a variable in an interpreter

– value of variable x is 5► look up the title of a book

– book with ISBN 0-596-00027-8 is “Programming Perl”► look up the real name of a student

– student 11111111 is Bart Simpson

Any relationship with a one-to-many relationship is perfect for a hash


TimeoutTimeout

# Using the program's environment# All processes have a set of names and values which# they inherit from their parents. These can be# set in the shell by typing NAME=VALUE.

print "Your home directory is $ENV{'HOME'}\n";

if ($ENV{'SHELL'} eq "/bin/csh"){ # Commiserate with user. print "Your shell is csh. Yuck!";}

print "Commands are looked for in these dirs:\n";print " $_\n" foreach (split /:/, $ENV{'PATH'}) # split: Topic 7

# Using the program's environment# All processes have a set of names and values which# they inherit from their parents. These can be# set in the shell by typing NAME=VALUE.

print "Your home directory is $ENV{'HOME'}\n";

if ($ENV{'SHELL'} eq "/bin/csh"){ # Commiserate with user. print "Your shell is csh. Yuck!";}

print "Commands are looked for in these dirs:\n";print " $_\n" foreach (split /:/, $ENV{'PATH'}) # split: Topic 7


Hashes: counting stringsHashes: counting strings

Use hash to count frequency of strings► key is the string (“dog”)► value (integer) is the count (has been seen 3 times so

far)► increment the value every time a key is read

Can be used to find intersection (common elements) between two arrays► iterate over first array: count elements found► iterate over second array: include element in result

only if it was seen in the first array► can compute union and difference similarly

Use hash to count frequency of strings► key is the string (“dog”)► value (integer) is the count (has been seen 3 times so

far)► increment the value every time a key is read

Can be used to find intersection (common elements) between two arrays► iterate over first array: count elements found► iterate over second array: include element in result

only if it was seen in the first array► can compute union and difference similarly


TimeoutTimeout

# Counting strings.

%seen = (); # Nothing has been seen so far.

while (<>) # Read words from input.{ chomp; # Increment the counter with line's text as key. $seen{$_}++; print "$_ has been seen $seen{$_} times so far\n";}

# Final report.while (($line, $count) = each %seen){ print "$line was seen $count times overall\n";}

# Counting strings.

%seen = (); # Nothing has been seen so far.

while (<>) # Read words from input.{ chomp; # Increment the counter with line's text as key. $seen{$_}++; print "$_ has been seen $seen{$_} times so far\n";}

# Final report.while (($line, $count) = each %seen){ print "$line was seen $count times overall\n";}


TimeoutTimeout

# Intersection of two arrays.

%seen = ();@intersection = ();

foreach (@one) # Iterate through first array.{ # Remember which elements have been seen. $seen{$_} = 1; # Any true value will do.}

foreach (@two) # Now iterate through second array.{ # Only add to result if was seen in @one. push @intersection, $_ if $seen{$_};}

# Intersection of two arrays.

%seen = ();@intersection = ();

foreach (@one) # Iterate through first array.{ # Remember which elements have been seen. $seen{$_} = 1; # Any true value will do.}

foreach (@two) # Now iterate through second array.{ # Only add to result if was seen in @one. push @intersection, $_ if $seen{$_};}


Hashes: removing duplicatesHashes: removing duplicates

An extension of counting elements in a list► if this is the first time element seen, include in result► otherwise, skip this element

An extension of counting elements in a list► if this is the first time element seen, include in result► otherwise, skip this element


TimeoutTimeout

# Simple implementation of Unix sort and sort -u

# Was -u (unique) switch given?if ($ARGV[0] eq "-u") { $unique = 1; shift; # Remove -u argument.}

# Read all input lines and sort them.@result = sort <>;

if ($unique){

# Filter out anything already seen. @result = grep { !$seen{$_}++ } @result;}

print @result; # Output remaining lines.

# Simple implementation of Unix sort and sort -u

# Was -u (unique) switch given?if ($ARGV[0] eq "-u") { $unique = 1; shift; # Remove -u argument.}

# Read all input lines and sort them.@result = sort <>;

if ($unique){

# Filter out anything already seen. @result = grep { !$seen{$_}++ } @result;}

print @result; # Output remaining lines.


Hashes: named parametersHashes: named parameters

Calling subroutines with many parameters is messy► printformatted(56, "$", 8, 2, "decimal");

– what did the 8 mean again?► especially when some parameters are optional and have a

reasonable default anyway

Can use hash to identify optional parameters and give them values► printformatted(56, prefix => '$', format => "decimal", precision => 8, places => 2);

– self-documenting code– order of parameters no longer matters

► printformatted(56, format => "hex");– only need to name the parameters with non-default values

► subroutines require a little code to handle this

Calling subroutines with many parameters is messy► printformatted(56, "$", 8, 2, "decimal");

– what did the 8 mean again?► especially when some parameters are optional and have a

reasonable default anyway

Can use hash to identify optional parameters and give them values► printformatted(56, prefix => '$', format => "decimal", precision => 8, places => 2);

– self-documenting code– order of parameters no longer matters

► printformatted(56, format => "hex");– only need to name the parameters with non-default values

► subroutines require a little code to handle this


TimeoutTimeout

# Map formats to printf percent-things.%format = (decimal => "d", hex => "x", octal => "o");

# Print a number with a certain format.sub printformatted{ my $number = shift; # Value to print. my %param = ( format => "decimal", # Defaults. precision => "6", @_ # Rest of sub params. ); printf( # Build up printf format string. ($param{"prefix"} . "%" . $param{"precision"} . "." . $param{"places"} . $format{$param{"format"}}), $number);}

# Map formats to printf percent-things.%format = (decimal => "d", hex => "x", octal => "o");

# Print a number with a certain format.sub printformatted{ my $number = shift; # Value to print. my %param = ( format => "decimal", # Defaults. precision => "6", @_ # Rest of sub params. ); printf( # Build up printf format string. ($param{"prefix"} . "%" . $param{"precision"} . "." . $param{"places"} . $format{$param{"format"}}), $number);}


Covered in this topicCovered in this topic

Hashes Hash variables

► $hash{key}, %hash

Functions which use hashes► keys, values► each

Uses of hashes► data lookup► sparse arrays► counting elements in a list► removing duplicates from a list► accessing a process’ environment► subroutines with optional parameters

Hashes Hash variables

► $hash{key}, %hash

Functions which use hashes► keys, values► each

Uses of hashes► data lookup► sparse arrays► counting elements in a list► removing duplicates from a list► accessing a process’ environment► subroutines with optional parameters


Going furtherGoing further

Tying► treat an external file (or any other object) like an

internal hash (or any other type)► Camel3 pages 363-398

Databases► talking to databases with Perl► Programming the Perl DBI by Alligator Descartes and

Tim Bunce, O’Reilly 2000

Shells► the Unix command-line interface► man sh

Tying► treat an external file (or any other object) like an

internal hash (or any other type)► Camel3 pages 363-398

Databases► talking to databases with Perl► Programming the Perl DBI by Alligator Descartes and

Tim Bunce, O’Reilly 2000

Shells► the Unix command-line interface► man sh


Next topicNext topic

Regular expressions► pattern matching

Regular expressions► pattern matching

Llama3 chapters 7-9, pages 98-127Camel3 pages 139-195perlre manpage

Topic 5: Hashes CSE2395/CSE3395 Perl Programming Learning Perl 3rd edition chapter 5, pages 73-85...

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