CPTG286K Programming - Perl

Chapter 5 & 6: Hashes & Basic I/O

Hash Variables

• A hash is a collection of scalar data, selected by arbitrary scalars called keys

• Hash elements are ordered internally to facilitate access (don’t try to control this order)

• Hashes resemble Rolodex filing cards whose indexes are similar to hash keys, and the card entries similar to the hash value

Accessing Hashes

• Hash elements are separate scalar variables, accessed by a string index (the key)

• Curly brackets are used to enclose the key:$fred{“aaa”} = “bbb”; # key “aaa”, value “bbb”

$fred{234.5} = 456.7; # key “234.5”, value 456.7

print $fred{“aaa”}; # prints “bbb”

$fred{234.5} += 3; # becomes 459.7

Un/winding Hashes

• Unwinding a hash moves key-value pairs into an array or a copy of a hash

@fred_list = %fred; # unwind %fred into

# array @fred_list

• Winding creates a new hash from an existing array or copy of a hash

%barney = @fred_list; # create %barney from

# array @fred_list

%barney = %fred; # faster winding method

More Hash Windings...

%smooth = (“aaa”,“bbb”,“234.5”,456.7);

# create %smooth from literal values

%backwards = reverse %normal;

# swap keys and values; beware of identical values,

# which after swapping become non-unique keys

Hash Functions

• The following functions are used to process hashes:– Keys– Values– Each– Delete

Keys function

• In list context, keys ($hashname) yields a list of all current keys in a hash %hashname

• In scalar context, keys ($hashname) yields the number of elements (key-value pairs) in the hash

Examples using keys function

$fred{“aaa”} = “bbb”;

$fred{234.5} = 456.7;

@list = keys(%fred); # @list gets (“aaa”, 234.5) # or (234.5, “aaa”) due to

# arbitrary ordering

foreach $key (keys (%fred))# process each $key in list

{ # generated by keys function

print “At $key we have $fred{$key}\n”:

# print key and value pair

}

if (keys(%somehash))

{ # if keys() not zero, %somehash is not empty }

if (%somehash)

{ # if True (not zero), %somehash is not empty }

Values Function

• The values %hashname function yields a list of all current values in %hashname

%lastname = (); # force %lastname empty

$lastname{“fred”} = “flinstone”;

$lastname{“barney”} = “rubble”;

@lastname = values(%lastname); # @lastname may be

# (“flinstone”, “rubble”) or

# (“rubble”,”flinstone”) due

# to arbitrary ordering

Each function

• The each %hashname function returns a key-value pair for each element of %hashname

while (($first, $last) = each(%lastname))

{ # process each %lastname element

# $first is key, $last is value

print “The last name of $first is $last\n”;# print key and value pair

}

Delete function

• The delete %hashname{“key”} function removes the key-value pair from %hashname

%fred = (“aaa”,”bbb”,234.5,34.56);

# %fred has two elements

delete $fred{“aaa”};

# %fred is now (234.5, 34.56)

Hash Slices

• Using slices, two or more hash elements can be accessed at a time

• Hash slices can be used with variable interpolation

• Hash slices can also be used to merge a smaller hash into a larger one

Examples of Hash Slices

# doing this:

$score{“fred”} = 205;

$score{“barney”} = 195;

$score{“dino”} = 30;

# is same as:

($score{“fred”},$score{“barney”},$score{“dino”}) = (205,195,30);

# and same as:

@score{“fred”,”barney”,”dino} = (205,195,30);# slices and variable interpolation

@players = qw(fred barney dino);

print “scores are: @score{@players}\n”;

More hash slice examples

# Merging a smaller hash into a larger one

# Get values of %score, and copy them into the

# %league hash whenever the keys of %score match

# the keys of %league

%league{keys %score} = values %score;

# equivalent to %league = (%league, %score);

Input from <STDIN>

• STDIN can be used to read data in both list and scalar contexts

• In list context, @a = <STDIN>; reads lines until end of file (or CTRL-D)

• In scalar context, $a = <STDIN>; reads input up to newline, or whatever $/ is set to

• <STDIN> also accepts the $_ scalar variable: $_ = <STDIN>;

Example reading STDIN to $_

# The following code…while (defined ($line = <STDIN>))

{chomp $line;print $line;}

# …is same as:while (<STDIN>) # or while ($_ = <STDIN>)

{chomp; # or chomp $_;print; # or print $_;}

Input from the <> operator

• The <> operator behaves like <STDIN>, but takes data from file(s) specified by the @ARGV array

• By default, the element(s) of @ARGV contain command-line arguments (filenames specified at the command-line)

• @ARGV can be redefined in a program

Example of input from <>

#!/usr/bin/perl

# this program (kitty) works like UNIX cat

# invoke with kitty file1 file2 file3

# the following lists the contents of this file:

# kitty kitty.pl

#

while (<>) # read files from command-line

{

print $_; # print output

}

Example redefining @ARGV

#!/usr/bin/perl

# this program (kitty) works like UNIX cat

# and lists contents of files “aaa”, “bbb”,

# and “ccc”

#

@ARGV = (“aaa”,“bbb”,“ccc”);

while (<>) # read files from command-line

{

print $_; # print output

}

Output to STDOUT

• PERL can use print or printf functions to produce output

• The print function returns 1, unless an I/O error occurs

• The printf function allows greater degree of control, and is similar to the printf function in C

STDOUT examples

$a = print(“Hello ”, “world”, “\n”);

# $a is set to 1 when print succeeds

printf “%15s %5d %10.2f\n”, $s, $n, $r

# prints $s as a 15 character string field

# prints a space

# prints $n as a 5 character decimal integer

# prints a space

# prints $r as a 10 character 2 decimal place

# floating point field

# prints a newline