PERL 5

PERL PARSER DEFINITIONS

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:35:00 +0000

PERL PARSER DEFINITIONS

REVISED: Sunday, March 3, 2013

Perl parser basic concepts.

I. PERL PARSER DEFINITIONS

A. SCANNER

A scanner is the first module in a compiler or interpreter. A scanner's job is to read the source file one character at a time. A scanner can also keep track of which line number and character is currently being read.

B. LEXER

A lexer is a module that serves to break up the source file into pieces called tokens. A lexer calls the scanner to get characters one at a time and organizes them into tokens and token types. The lexer calls the scanner to pass the scanner one character at a time and groups them together and identifies them as tokens for the language parser.

C. PARSER

A parser is the part of a compiler that really understands the syntax of the language. A parser calls the lexer to get tokens and processes the tokens according to the syntax of the language.

Perl is "the language for text analysis." The built-in operators make pattern matching, text searching, and replacing enjoyable.

Lets start by examining the following file, and we will have fun as we go along.

II. PERL OUTPUT EXAMPLE SOURCE CODE

"Copy Paste" the following Perl program into your text editor:

#Unicode
use utf8;

#Provides undefined value warnings.
use warnings;

#enables -w flag.
use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
#The my() function makes the variables local to the main package.
use strict;

#stack trace
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

open STDOUT, ">C:/Strawberry/flower.txt";

print "A bug sat in a silver flower\n";

print "thinking silver thoughts.\n";

print "A bigger bug out for a walk\n";

print "climbed up that silver flower stalk\n";

print "and snapped the small bug down his jaws\n";
print "without a pause\n";
print "without a care\n";
print "for all the bug’s small silver thoughts.\n";
print "It isn’t right it isn’t fair that big bug ate that little bug\n";
print "because that little bug was there.\n";
print "\n";
print "He also ate his underwear.\n";
close STDOUT;

From your text editor, "File Save As" writeFlower.pl using the path to the folder of your Perl download; e.g.:

C:\Strawberry\writeFlower.pl

Depending on your operating system platform, when you open Perl, you will have a Perl prompt similar to the following:

C:\WINDOWS\system32>

From the above Perl prompt type:

C:\WINDOWS\system32>C:\Strawberry\writeFlower.pl

Press Enter.

After you press Enter, Perl will display the following:

C:\WINDOWS\system32>

Instead of writing the poem to your screen, Perl wrote the poem to the file flower.txt using the path to the folder of the Strawberry Perl download. flower.txt will be the first file we will examine as we write our Perl parsers.

III. PERL INPUT EXAMPLE SOURCE CODE

"Copy Paste" the following Perl program into your text editor:

#Unicode
use utf8;

#Provides undefined value warnings.
use warnings;

#enables -w flag.
use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
#The my() function makes the variables local to the main package.
use strict;

#stack trace
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

#Specify the file.
$file = "C:/Strawberry/flower.txt";
#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";
#Read the file into the array @lines.
@lines = <FILE>;
#Print the file to the screen.
print @lines;
#Close the file.
close FILE;

"File Save As" readFlower.pl using the path to the folder of your Perl download; e.g.:

C:\Strawberry\readFlower.pl

From the Perl prompt type the following:

C:\WINDOWS\system32>C:\Strawberry\readFlower.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\readFlower.pl
A bug sat in a silver flower
thinking silver thoughts.
A bigger bug out for a walk
climbed up that silver flower stalk
and snapped the small bug down his jaws
without a pause
without a care
for all the bug’s small silver thoughts.
It isn’t right it isn’t fair that big bug ate that little bug
because that little bug was there.

He also ate his underwear.

C:\WINDOWS\system32>

You have learned some of the basic concepts required for a Perl parser, enjoy!

Elcric Otto Circle

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PERL WARN( ) FUNCTION

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:33:00 +0000

PERL WARN( ) FUNCTION

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl warn( ) function.

I. PERL WARN( ) FUNCTION

The warn( ) function and the die( ) function have the same functionality; however, when the warn( ) function is used your program is not exited.

The warn( ) function is better suited for non-fatal messages like low memory or disk space conditions.

If we try to change to the /examples directory and the change fails, the consequences are not fatal because the files can still be written to the current directory.

chdir('/examples') ||
warn("WARNING: Using current directory instead of /examples: $!\n");

If the /examples directory does not exist, the above code displays the following error message:

WARNING: Using current directory instead of /examples:

II. PERL $!

The $! variable is used in the above error message to have it followed by a display of the appropriate system error message.

You have learned how to use the Perl warn( ) function.

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PERL FOREACH LOOP

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:31:00 +0000

PERL FOREACH LOOP

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl foreach loop.

I. PERL FOREACH LOOP

The basic kinds of loops in Perl are the for loop, the while loop, and the foreach loop. The foreach loop is a form of the for loop.

The foreach loop is used to iterate through the elements of an array.

The foreach loop can be used to:

A. Find the largest element in an array.

B. Print the elements of an array.

C. Determine if a certain value is one of the elements of an array.

II. PERL FOREACH LOOP SYNTAX

The syntax for the foreach loop is as follows:

foreach loopVariable (ARRAY) {
statements;
}

The loopVariable is assigned the value of each array element, until the end of the array is reached.

III. PERL FOREACH LOOP EXAMPLE

In addition to the Perl builtin system functions Perl gives you the ability to make subroutines, which are user-defined functions. The Perl builtin system functions can be combined and incorporated into your user-defined functions.

The following subroutine uses the foreach loop to iterate through the elements of an array:

#Subroutine max definition
#Parameter array @_
sub max {
   my($max) = shift(@_);

   foreach my $temp (@_) {
   $max = $temp if $temp > $max;
   }

   return($max);
}

IV.  PERL FOREACH LOOP SOURCE CODE

A. CREATE TEXT INPUT DATA FILE

First we need to create the text input data file.

A Perl script is just a text file. Perl script files end with the .pl extension.

"Copy Paste" the following Perl program into your text editor:

#Unicode
use utf8;

#Provides undefined value warnings.
use warnings;

#enables -w flag.
#Turn on verbose warnings.
use diagnostics;

#Helps you catch typos.
#Forces you to use my( ) function to declare all variables.
#my variables (lexical variables) are faster than globals.
use strict;

#Stack trace.
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

open STDOUT, ">c:/strawberry/array.txt";

print "0\n";
print "1\n";
print "2\n";
print "3\n";
print "4\n";
print "5\n";
print "6\n";
print "7\n";
print "8\n";
print "9\n";

close STDOUT;

"File Save As" writeArray.pl using the path of the Perl download; e.g.:

c:\strawberry\writeArray.pl

Depending on your platform you will have a Perl prompt as follows:

C:\WINDOWS\system32>

From the above Perl prompt type:

C:\WINDOWS\system32>c:\strawberry\writeArray.pl

Press Enter.

Perl will display the following on your Perl prompt:

C:\WINDOWS\system32>

The Perl prompt screen is blank because the numbers we plan to load into our array were printed to the array.txt file on your hard drive and not your Perl prompt screen.

Now we have a text file named array.txt which contains the numbers we want to read from the text file into an array.

B. PROCESS DATA FILE

"Copy Paste" the following Perl program into your text editor:

#Unicode.
use utf8;

#Provides undefined value warnings.
use warnings;

#enables -w flag
#Turn on verbose warnings.
use diagnostics;

#Helps you catch typos.
#Forces you to use my( ) function to declare all variables.
#my variables (lexical variables) are faster than globals.
use strict;

#Stack trace.
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

#Subroutine max definition
#Parameter array @_
sub max {
my($max) = shift(@_);

foreach my $temp (@_) {
$max = $temp if $temp > $max;
}

return($max);
}

#Specify the file.
my $file = "c:/strawberry/array.txt";

#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";

#Read text data file into array @lines.
my @lines = <FILE>;

#Call (invoke) subroutine and print return.
print &max(@lines);

#Close file handle FILE.
close FILE;

"File Save As" readArray.pl using the path of the Perl download; e.g.:

c:\strawberry\readArray.pl

From the Perl prompt window type the following:

C:\WINDOWS\system32>c:\strawberry\readArray.pl

Press Enter and the following will display on your Perl prompt window:

C:\WINDOWS\system32>c:\strawberry\readArray.pl
9

C:\WINDOWS\system32>

The above example demonstrates the basic steps you will perform when you want to use data file input with your Perl programs.

V. PERL FOREACH LOOP SUMMARY

When you want to solve a problem that includes using an array always think about how the foreach loop can help you. Using the foreach loop we easily found the largest element in our array. All we needed was data to load into the array. The data will be the information you want to process.

You have learned how to use the Perl foreach loop.

Elcric Otto Circle

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PERL FUNCTIONS AND OPERATORS

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:30:00 +0000

PERL FUNCTIONS AND OPERATORS

REVISED: Sunday, March 3, 2013

PERL FUNCTIONS AND OPERATIONS

abs	absolute value
chdir	change current directory
chmod	change permissions of file/directory
chomp	remove terminal newline from string variable
chop	remove last character from string variable
chown	change ownership of file or directory
close	close a file handle
closedir	close a directory handle
cos	cosine
defined	test whether variable is defined
delete	delete a key from a hash
die	exit with an error message
each	iterate through keys and values of a hash
eof	test a filehandle for end of file
eval	evaluate a string as a perl expression
exec	quit Perl and execute a system command
exists	test that a hash key exists
exit	exit from the Perl script
glob	expand a directory listing using shell wildcards
gmtime	current time in GMT
grep	filter an array for entries that meet a criterion
index	find location of a substring inside a larger string
int	throw away the fractional part of a floating point number
join	join an array together into a string
keys	return the keys of a hash
kill	send a signal to one or more processes
last	exit enclosing loop
lc	convert string to lowercase
lcfirst	lowercase first character of string
length	find length of string
local	temporarily replace the value of a global variable
localtime	return time in local timezone
log	natural logarithm
m//	pattern match operation
map	perform on operation on each member of array or list
mkdir	make a new directory
my	create a local variable
next	jump to the top of enclosing loop
open	open a file for reading or writing
opendir	open a directory for listing
pack	pack a list into a compact binary representation
package	create a new namespace for a module
pop	pop the last item off the end of an array
print	print to terminal or a file
printf	formatted print to a terminal or file
push	push a value onto the end of an array
q/STRING/	generalized single-quote operation
qq/STRING/	generalized double-quote operation
qx/STRING/	generalized backtick operation
qw/STRING/	turn a space-delimited string of words into a list
rand	random number generator
read	read binary data from a file
readdir	read the contents of a directory
readline	read a line from a text file
readlink	determine the target of a symbolic link
redo	restart a loop from the top
ref	return the type of a variable reference
rename	rename or move a file
require	load functions defined in a library file
return	return a value from a user-defined subroutine
reverse	reverse a string or list
rewinddir	rewind a directory handle to the beginning
rindex	find a substring in a larger string, from right to left
rmdir	remove a directory
s///	pattern substitution operation
scalar	force an expression to be treated as a scalar
seek	reposition a filehandle to an arbitrary point in a file
select	make a filehandle the default for output
shift	shift a value off the beginning of an array
sin	sine
sleep	put the script to sleep for a while
sort	sort an array or list by user-specified criteria
splice	insert/delete array items
split	split a string into pieces according to a pattern
sprintf	formatted string creation
sqrt	square root
stat	get information about a file
sub	define a subroutine
substr	extract a substring from a string
symlink	create a symbolic link
system	execute an operating system command, then return to Perl
tell	return the position of a filehandle within a file
tie	associate a variable with a database
time	return number of seconds since a given date
tr///	replace characters in a string
truncate	truncate a file makes file smaller
uc	uppercase a string
ucfirst	uppercase first character of a string
umask	change file creation mask
undef	undefine removes a variable
unlink	delete a file
unpack	the reverse of pack
untie	the reverse of tie
unshift	move a value onto the beginning of an array
use	import variables and functions from a library module
values	return the values of a hash variable
wantarray	return true in an array context
warn	print a warning to standard error
write	formatted report generation

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PERL DIE( ) AND EVAL( ) FUNCTIONS

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:29:00 +0000

PERL DIE() AND EVAL() FUNCTIONS

REVISED: Sunday, March 3, 2013

In this tutorial, you will learn how to use the Perl die( ) function and the Perl eval( ) function.

I. PERL DIE( ) FUNCTION

The die( ) function is used to quit your script and display an error message.

The die( ) function syntax is as follows:

die(LIST);

The elements of LIST are printed to STDERR, and then the script will exit, setting the script's return value to $! (errno).

die( ) terminates the Perl program if there is an error and prints out an error message.

The $! is a system generated error message number telling you why the system did not do what you requested.

To use the die( ) function, place it on the right side of the || operator.

chdir('/user/printer') || die( "printerYE2011.pl died changing directories.");

If the /user/printer directory does not exist the following error message will display:

printerYE2011.pl died changing directories.

Always include an error message telling the user what happened. You also want the error messages to be informative and to look professional.

For example, you could use the following code to display the error text associated with errno:

chdir('/user/printer') || die("$!");

The above code could display the appropriate system error message.

II. PERL EVAL( ) FUNCTION

Experiment and look for error messages that communicate best to your users. Especially if you are the user. If you only run a program on rare occasions, error messages will help you remember how to make your program work the way you designed it to work. For example:

$code = "chdir('/user/printer')";

eval($code) || die("PROBLEM WITH LINE: $code\n$!, stopped.");

The above code could display the following error message:

PROBLEM WITH LINE: chdir('/user/printer')
No such file or directory, stopped.

The eval( ) function compiles and executes its arguments at runtime.

First, the Perl code in $code is executed. If an error arises, the Perl code in $code is displayed as text by the die( ) function.

In this tutorial, you have learned how to use the Perl die( ) function and the Perl eval( ) function.

Elcric Otto Circle

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PERL FILEHANDLES

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:28:00 +0000

PERL FILEHANDLES

REVISED: Sunday, March 3, 2013

You will learn the basic fundamentals of Perl filehandles.

I. PERL FILEHANDLE DEFINITION

All input and output with Perl files is achieved through filehandling.

In Perl, files are given a name, a handle, basically another way of saying alias. A filehandle is nothing more than a nickname for the files you intend to use in your Perl scripts and programs.

Like any Perl identifier you could use letters, digits, and underscores; however, you should only use all uppercase letters for your filehandle identifier and do not start the filehandle with a digit.

II. PERL RESERVED FILEHANDLE NAMES

The following six Perl filehandle names have special properties: STDIN, STDOUT, STDERR, DATA, ARGV, and ARGVOUT. Only use these six filehandle names when you want to use their special properties.

The STDIN, standard in, filehandle is normally connected to your keyboard.

The STDOUT, standard out, filehandle is normally connected to your computer monitor.

The STDERR, standard error, filehandle is also normally connected to your computer monitor.

III. PERL FILEHANDLE EXAMPLES

If we have a long file name such as helloworld.txt, that we do not want to type over and over in our program, we could use HELLO as the file nickname or filehandle and the following statement to open the file.

open HELLO, "helloworld.txt";

In the above statement HELLO is the name of our filehandle. The filehandle is followed by a comma. The comma is followed by the name of our file in double quotes. The Perl statement is ended with a semi-colon. Now you can read and write from HELLO.

When you are done you can use the following statement to close helloworld.txt

close HELLO;

In this tutorial, you have learned the basic fundamentals of Perl filehandles.

Elcric Otto Circle

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PERL HASH

noreply@blogger.com (Ron Tinnel) — Wed, 14 Dec 2011 10:27:00 +0000

PERL HASH

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl hash.

I. PERL HASH

A Perl hash is a Perl array that is indexed by a string instead of a number. Hashes are like arrays except they link a key to a value. To define a hash, use the percent (%) symbol before the name.

A. PERL HASH INDEXING

Hashes can be key-value pair indexed using the two scalar variables $key and $value. The table below contains USA currency portrait and dollar amounts. $key is the last name of the person whose portrait is on the bill and $value is the face value of the bill in dollars.

The hash %bills is declared in bill amount order $value.

$key	$value
Washington	1
Jefferson	2
Lincoln	5
Hamilton	10
Jackson	20
Grant	50
Franklin	100
McKinley	500
Cleveland	1000
Madison	5000
Chase	10000
Wilson	100000

II. PERL HASH EXAMPLE SOURCE CODE

Copy paste the following into your text editor.

#Provides undefined value warnings.
use warnings;

#enables perl's -w flag.
use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
#The my() function makes the variables local to the main package.
use strict;

#stack trace
use Carp ( );
local $SIG{__WARN__} = \&Carp::confess;

#Specify the file.
my($file) = "C:/Strawberry/hash.pl";

#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";

#DEFINE HASH %bills

my(%bills) = (
Washington => 1,
Jefferson => 2,
Lincoln => 5,
Hamilton => 10,
Jackson => 20,
Grant => 50,
Franklin => 100,
McKinley => 500,
Cleveland => 1000,
Madison => 5000,
Chase => 10000,
Wilson => 100000,
);

print "\n\tBILLS SORTED BY PORTRAIT NAME\n";
foreach my $key (sort (keys(%bills))) {
if (length($key) <= 6){
printf ("\t%s \t\t%.6s\n", $key, $bills{$key});
}
else{
printf ("\t%s \t%.6s\n", $key, $bills{$key});
}

}

#Close the file.
close FILE;

From your text editor, "File Save As" hash.pl using the path to the folder of your Perl download.

From the Perl prompt type the following:

C:\WINDOWS\system32>C:\Strawberry\hash.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\hash.pl

BILLS SORTED BY PORTRAIT NAME
Chase 10000
Cleveland 1000
Franklin 100
Grant 50
Hamilton 10
Jackson 20
Jefferson 2
Lincoln 5
Madison 5000
McKinley 500
Washington 1
Wilson 100000

C:\WINDOWS\system32>

III. PERL HASH SUMMARY

The hash %bills is sorted by bill portrait last name $key.

$key	$value
Chase	10000
Cleveland	1000
Franklin	100
Grant	50
Hamilton	10
Jackson	20
Jefferson	2
Lincoln	5
Madison	5000
McKinley	500
Washington	1
Wilson	100000

The each( ) function removes the topmost key-value pair from the hash and stores them in the variables $key and $value. Each time the loop iterates, the statement ($key, $value) = each(%bills) executes and the hash pops off the top key-value pair and continues doing so until it has gone through every key-value pair in the hash. When it is done, each( ) returns false and the loop stops running.

You have learned how to use the Perl hash.

Elcric Otto Circle

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PERL SPLIT OPERATOR

noreply@blogger.com (Ron Tinnel) — Mon, 21 Nov 2011 21:57:00 +0000

PERL SPLIT OPERATOR

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl split operator.

I. PERL SPLIT OPERATOR

The Perl split operator is commonly used when parsing data from a file or from another program. The Perl split operator allows you to break up a string into an array or a list, by using a specific pattern.

The split operator is used to split a string into smaller sections. You can split a string on a single character, a group of characters or a regular expression, a pattern. You can also specify how many pieces to split the string into.

The Perl split operator syntax is as follows:

LIST = split(/PATTERN/, EXPR, LIMIT);

LIST is a list, array or hash that is returned by the split operator. If LIST is omitted, the Perl split operator will be called in a scalar context and the EXPR will be split in the @_ array. If LIST is omitted the operator will return the number of fields in which the EXPR will be split. If your call to a split operator is inside the body of a subroutine it alters the value of @_.

PATTERN is normally a regular expression; however, PATTERN can be a single character or a string. If LIMIT is not specified the EXPR is split on every occurrence of the PATTERN. If the PATTERN parameter is omitted, the Perl split operator will split on whitespaces, skipping any leading whitespaces.

EXPR is the string expression that will be split into an array or a list. If EXPR is omitted, the content of the special scalar variable $_ will be split.

LIMIT is the maximum number of fields the EXPR will be split into.

Only the word split is compulsory. You can omit the parenthesis, the LIST that will be returned and any of the third arguments presented above.

If you use one of the following metacharacter inside the pattern you need to escape the metacharacter by using the \ backslash character to indicate that the metacharacter is to be regarded as something to match and not some fancy character:

^ $ ( ) \ | @ [ { ? . + *

For example, if you want to split your string using the pattern /&&/ you need to write it as /\&\&/.

The Perl join operator is the opposite of the Perl split operator. There are a lot of situations where they are used together.

The Perl join operator concatenates; i.e., joins the elements of a list into a single string.

The Perl split operator breaks up a single string into a list of strings and returns that list.

II. PERL SPLIT OPERATOR SOURCE CODE

Copy paste the following into your text editor.

#Provides undefined value warnings.
use warnings;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
use strict;

#Specify the file.
my($file) = "C:/Strawberry/splitOperator.pl";
#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";

#initialize array

my(@fields) = split(/-/, "808-253-4321");
print "The telephone number is: @fields\n\n";

#Close the file.
close FILE;

From your text editor, "File Save As" splitOperator.pl using the path to the folder of your Perl download.

From the Perl prompt type the following:

C:\WINDOWS\system32>C:\Strawberry\splitOperator.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\splitOperator.pl
The telephone number is: 808 253 4321

C:\WINDOWS\system32>

You have learned how to use the Perl split operator.

Elcric Otto Circle

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PERL JOIN OPERATOR

noreply@blogger.com (Ron Tinnel) — Mon, 21 Nov 2011 10:01:00 +0000

PERL JOIN OPERATOR

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl join operator.

I. PERL JOIN OPERATOR INTRODUCTION

The syntax form of Perl join operator is as follows:

$string = join (EXPR, LIST);

The join operator has two parameters:

EXPR may be any string and it represents a separator for the element of the list or array.

LIST represents a list or array whose elements will be merged into a string.

The join operator will return a string that contains the elements of the array or list, connected through a string separator.

II. PERL JOIN OPERATOR EXAMPLE

The following example demonstrates the use of the join operator:

#initialize an array
my @perlFunc = ("Mary","had","a","little","lamb.");
my $perlFunc = join " ", @perlFunc;
print "Perl Functions: $perlFunc\n";

Output: Mary had a little lamb.

We used the space separator " " to glue the elements of the @perlFunc array. You can omit the parentheses when you call the Perl join operator.

You have learned how to use the Perl join operator.

Elcric Otto Circle

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PERL FOR LOOP

noreply@blogger.com (Ron Tinnel) — Fri, 18 Nov 2011 10:23:00 +0000

PERL FOR LOOP

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl for loop.

I. PERL FOR LOOP

The basic kinds of loops in Perl are the for loop, the while loop, and the foreach loop. The Perl for loop is used to run a block of code a specific number of times.

The Perl for loop syntax is as follows:

for(initialization; test; increment){
block of statements;
}

The for loop has three arguments separated by two semicolons.

Step 1. The first argument, the counter, is initialized. Step 1. is executed only once at the beginning of the for statement.

Step 2. The second argument, the test, is evaluated. If the test evaluates to true, the block of statements are executed; otherwise the for loop is terminated.

Step 3. After the block of statements, enclosed by the curly braces, are executed, the counter is incremented by the third argument and "Step 2." is repeated.

Perl loops through "Step 2." and "Step 3." until the test expression is false and the for loop is terminated.

If the test expression is never false, you have an infinite loop which can be stopped by pressing Ctrl+c.

Here is a code snippet to print a message 10 times.

for($counter = 1; $counter <= 10; $counter++){
print "for loop #$counter\n";
}

Step 1. $counter is initialized to 1.

Step 2. Perl checks to see if the $counter is less than or equal to 10. $counter is 1; the test is true therefore, Perl executes the code inside the block.

Step 3. Perl increases $counter by 1.

The process continues looping through "Step 2." and "Step 3." until $counter is equal to 11 at which time the test is false.

$counter is no longer less than or equal to 10; therefore, the loop exits, after the code block has executed 10 times.

II. PERL FOR LOOP SOURCE CODE

You can initialize the counter outside the loop, and increment the control variable inside the block as shown by the following example.

Copy paste the following into your text editor.

#Unicode

use utf8;
#Provides undefined value warnings.
use warnings;

#enables -w flag.

use diagnostics;
#Helps you catch typos.
#Forces you to use my() function to declare all variables.
use strict;

#stack trace
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

#Specify the file.

my($file) = "C:/Strawberry/forLoop.pl";

#Open the file and read data.
#Die with grace if it fails.

open (FILE, "<$file") or die "Can't open $file: $!\n";

my($counter) = 1;

for(; $counter <= 10;){

print "for loop #$counter\n";

$counter++;

}

#Close the file.

close FILE;

From your text editor, "File Save As" forLoop.pl using the path to the folder of your Perl download.

From the Perl prompt type the following:
C:\WINDOWS\system32>C:\Strawberry\forLoop.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\forLoop.pl
for loop #1
for loop #2
for loop #3
for loop #4
for loop #5
for loop #6
for loop #7
for loop #8
for loop #9
for loop #10

C:\WINDOWS\system32>

You have learned how to use the Perl for loop.

Elcric Otto Circle

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PERL IF STATEMENT

noreply@blogger.com (Ron Tinnel) — Fri, 18 Nov 2011 05:07:00 +0000

PERL IF STATEMENT

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl "if, else if, and else" statements to write simple logic in your Perl scripts.

I. PERL QUICK REVIEW

Before we introduce something new, we will have a quick review:

A. CALCULATIONS

Perl uses double-precision floating point values for calculations.

B. STRING

Perl defines string as a sequence of characters. The shortest string is the null string it contains no characters. The longest string is only limited by the available memory of your computer.

C. INTERPOLATION

Interpolation of scalar variables is a mechanism to convert a scalar variable inside a string into string.

D. BLOCK

Perl groups statements into blocks of code. A block is surrounded by a pair of ({ }) curly braces and can be nested within a block.

E. LOGICAL COMPARISON

In Perl, everything is true except the number zero 0, empty string "", zero string "0", and undefined (undef) values.

II. PERL "IF" CONTROL STRUCTURE

Decision statements use the "if" keyword to execute a statement block depending on the evaluation of conditional expressions. Therefore, you can use the Perl "if" control structure to write simple logic in your code.

The "if" control structure can be nested.

The syntax of an "if" control structure is as follows:

if(condition){
statements;
}

If the condition is true the statements inside the block will be executed: for example:

$x = 1;
$y = 1;

if($x == $y){
print "$x is equal to $y";
}

The first and second lines define two variables $x and $y with their values initialized to 1.

In the fourth line we use the "if "statement to print a message if $x is equal to $y.

The message is only printed if the expression ($x == $y) is evaluated as true.

III. PERL "IF-ELSE" CONTROL STRUCTURE

If you need an alternative choice, Perl provides an "if-else" control structure as follows:

if(condition){
if-statements;
}

else{
else-statements;
}

If the condition is false the else-statements will be executed; for example:

$x = 2;
$y = 3;

if($x == $y){
print "x is equal to y";
}

else{
print "x is not equal to y";
}

IV. PERL "IF-ELSE-IF" CONTROL STRUCTURE

A. MULTIPLE CHOICES

Perl also provides an "if-else-if" control structure to make multiple choices based on conditions.

if(condition1){
if-statements;
}

else if(condition2){
else if-statements;
}

else if(condition3){
else if-statements;
}

...

else{
else-statements;
}

Here is the source code example:

$x = 2;
$y = 3;

if($x > $y){
print "x is greater than y";
}

else if ($x < $y){
print "x is less than y";
}

else{
print "x is equal to y";
}

B. READABILITY

Perl helps you create a self documenting style of easy to read code.

A year after you write a program you should be able to read and understand what you wrote.

The following is another way to format your Perl code:

if (condition1) {

    task;

} elsif (condition2) {

    a different task;

} else {

    a task if all else fails;

}

V.   PERL SCRIPT SOURCE CODE
Copy paste the following into your text editor.

#Unicode
use utf8;

#Provides undefined value warnings.
use warnings;

#enables -w flag.
use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
use strict;

#stack trace
use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

#Specify the file.
my($file) = "C:/Strawberry/ifElse.pl";
#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";
my($x) = 2;
my($y) = 3;

if($x == $y){
print "x is equal to y";
}

else{
print "x is not equal to y";
}
#Close the file.
close FILE;

From your text editor, "File Save As" ifElse.pl using the path to the folder of your Perl download.

From the Perl prompt type the following:

C:\WINDOWS\system32>C:\Strawberry\ifElse.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\ifElse.pl
x is not equal to y
C:\WINDOWS\system32>

The my( ) function makes the variables local to the main package. Packages are classes.

A running Perl program has a built-in namespace called "main", which is the default name. For example, a subroutine called Sub1 can be called as Sub1( ) or main::Sub1( ).

With a variable the appropriate sigil is placed in front of the namespace; so a scalar variable called $var1 can also be referred to as $main::var1, or even $::var1. Other namespaces can be created at any time.

Now you have a "learning feed-back loop." Change the values of $x and $y. Think about what you expect Perl will do. Then run the program and see if Perl does what you expect.

If pearl does not do what you expect, look at your code and make the appropriate changes, then rerun the program.

You have learned how to use the Perl "if, else if, and else" statements to write simple logic in your Perl scripts.

Elcric Otto Circle

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PERL REGEX REGULAR EXPRESSIONS

noreply@blogger.com (Ron Tinnel) — Fri, 11 Nov 2011 20:44:00 +0000

PERL REGEX REGULAR EXPRESSIONS

REVISED: Sunday, March 3, 2013

You will learn how to use Perl REGEX, regular expressions.

I. REGEX

In its simplest form, a regular expression is just a word or phrase to search for.

REGEX are Perl REGular EXpressions. Perl REGEX syntax make it easy to do the following:

Complex string comparisons.

Complex string selections.

Complex string replacements.

Parsing based on the above abilities.

A. COMPLEX STRING COMPARISONS

1. The following is a very basic string logical comparison:

$string =~ m/sought_text/;

The above returns true if the string $string contains substring "sought_text", and false otherwise.

2.  If you only want the strings where the sought_text appears at the ^ "very beginning" of $string, write:

$string =~ m/^sought_text/;

3.  The $ operator indicates "end of string". If you want to find out if the sought_text is the very last text in the $string, write:

$string =~ m/sought_text$/;

4.  If you want the comparison to be true only if $string contains the sought_text and nothing but the sought_text, write:

$string =~ m/^sought_text$/;

5. If you want the comparison to be case insensitive add the letter i after the ending delimiter:

$string =~ m/^sought_text$/i;

6.  Wild Cards

.  Match any character
\w  Match "word" character alphanumeric, plus "_"
\W  Match non-word character
\s  Match whitespace character
\S  Match non-whitespace character
\d  Match digit character
\D  Match non-digit character
\t  Match tab
\n  Match newline
\r  Match return
\f  Match formfeed
\a  Match alarm bell, beep, etc.
\e  Match escape
\021  Match octal char 21
\xf0  Match hex char f0

7.  Repetition

* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m}      Match at least n but not more than m times

8.  Using Groups ( ) in Matching

Groups are regular expression characters surrounded by parentheses ( ).

a. Groups are used to allow alternative phrases; e.g.:

/(Jack|Jill|Hill)/i

For single character alternatives, use character classes [ ]. Everything inside the brackets represents one character, listing all its alternative possibilities. Character classes are alternative single characters within square brackets [ ]. There are two commonly used special characters inside the square brackets:

A hyphen (-) is used to indicate all characters in the colating sequence between the character on the hyphen's (-) left and the character on the hyphen's (-) right.

An uparrow (^) at immediately following the opening square bracket [ means "anything but these characters", and effectively negates the character class.

Character classes have three main advantages:

i. Shorthand notation, as [AEIOUY] instead of (A|E|I|O|U|Y).
ii. Character Ranges, such as [A-Z].
iii. One to one mapping from one class to another; e.g.: tr/[a-z]/[A-Z]/.

b. Groups are also used as a means of retrieving selected text in selection, translation and substitution, used with scalers; $1, $2; etc.

B. COMPLEX STRING SELECTIONS

Replace every "Jack" with "Jill"

$string =~ s/Jack/Jill/;

C. COMPLEX STRING REPLACEMENTS

Translations are like substitutions, except they happen on a letter by letter basis instead of substituting a single phrase for another single phrase.

What if you wanted to make all vowels upper case:

$string =~ tr/[a,e,i,o,u,y]/[A,E,I,O,U,Y]/;

Change everything to upper case:

$string =~ tr/[a-z]/[A-Z]/;

Change everything to lower case

$string =~ tr/[A-Z]/[a-z]/;

II. SYMBOLS

=~

This operator appears between the string var you are comparing, and the regular expression you are looking for. In selection or substitution a regular expression operates on the string var rather than comparing; for example:

$string =~ m/Jack/;
#return true if var $string contains the name Jack

$string =~ s/Jack/Jill/;
#replace Jack with Jill

!~

Just like =~, except negated. Returns true if it does not match.

/

This is the usual "delimiter" for the text part of a regular expression; for example:

$string =~ m/Jack/;
#return true if var $string contains the name Jack

$string =~ s/Jack/Jill/;
#replace Jack with Jill

If the sought-after text contains slashes, it is easier to use pipe symbols (|) for delimiters.

This is the "match" operator. The match operator comes before the opening delimiter. The match operator means read the string expression on the left of the =~, and see if any part of it matches the expression within the delimiters following the m. If the delimiters are slashes (/), the m is optional and often not included. Whether the match operator is there or not, it is still a match operation; for example:

$string =~ m/Jack/;
#return true if var $string contains the name Jack

$string =~ /Jack/;
#same result as previous statement

^

This is the "beginning of line" symbol. When used immediately after the starting delimiter, it signifies "at the beginning of the line"; for example:

$string =~ m/^Jack/;
#true only when "Jack" is the first text in the string

$

This is the "end of line" symbol. When used immediately before the ending delimiter, the "end of line" symbol signifies "at the end of the line"; for example:

$string =~ m/Jack$/;
#true only when "Jack" is the last text in the string

i

This is the "case insensitivity" operator when used immediately after the closing delimiter; for example:

$string =~ m/Jack/i;
#true when $string contains "Jack" or "jAcK"

III. TABLES

Some characters have a special meaning to the searcher. These characters are called metacharacters.

METACHARACTERS

CHAR	MEANING
`^`	beginning of string
`$`	end of string
`.`	any character except newline
`*`	match 0 or more times
`+`	match 1 or more times
`?`	match 0 or 1 times; or: shortest match
`\|`	alternative
`( )`	grouping; “storing”
`[ ]`	set of characters
`{ }`	repetition modifier
`\`	quote or special

REPETITION

a*
zero or more a’s
a+
one or more a’s
a?
zero or one a’s (i.e., optional a)
a{m}
exactly m a’s
a{m,}
at least m a’s
a{m,n}
at least m but at most n a’s
repetition?
same as repetition but the shortestmatch is taken

SPECIAL NOTATIONS WITH \

Single characters
`\t`	tab
`\n`	newline
`\r`	return (CR)
`\xhh`	character with hex. code `hh`

“Zero-width assertions”
`\b`	“word” boundary
`\B`	not a “word” boundary

Matching
`\w`	matches any single character classified as a “word” character (alphanumeric or “`_`”)
`\W`	matches any non-“word” character
`\s`	matches any whitespace character (space, tab, newline)
`\S`	matches any non-whitespace character
`\d`	matches any digit character, equiv. to `[0-9]`
`\D`	matches any non-digit character

CHARACTER CLASS [...]

Inside a "character class" denoted by [...] the following rules apply:

`[characters]`	matches any of the characters in the sequence
`[x-y]`	matches any of the characters from `x` to `y` (inclusively) in the ASCII code
`[\-]`	matches the hyphen character “`-`”
[`\n`]	matches the newline; other single character denotations with \ apply normally, too
`[^something]`	matches any character except those that `[something]` denotes; that is, immediately after the leading “`[`”, the circumflex “`^`” means “not” applied to all of the rest

EXPRESSION	MATCHES
`abc`	`abc` (that exact character sequence, but anywhere in the string)
`^abc`	`abc` at the beginning of the string
`abc$`	`abc` at the end of the string
`a\|b`	either of `a` and `b`
`^abc\|abc$`	the string `abc` at the beginning or at the end of the string
`ab{2,4}c`	an `a` followed by two, three or four `b`’s followed by a `c`
`ab{2,}c`	an `a` followed by at least two `b`’s followed by a `c`
`ab*c`	an `a` followed by any number (zero or more) of `b`’s followed by a `c`
`ab+c`	an `a` followed by one or more `b`’s followed by a `c`
`ab?c`	an `a` followed by an optional `b` followed by a `c`; that is, either `abc` or `ac`
`a.c`	an `a` followed by any single character (not newline) followed by a `c`
`a\.c`	`a.c` exactly
`[abc]`	any one of `a`, `b` and `c`
`[Aa]bc`	either of `Abc` and `abc`
`[abc]+`	any (nonempty) string of `a`’s, `b`’s and `c’s` (such as `a`, `abba`, `acbabcacaa`)
`[^abc]+`	any (nonempty) string which does not contain any of `a`, `b` and `c` (such as `defg`)
`\d\d`	any two decimal digits, such as `25`; same as \d{2}
`\w+`	a “word”: a nonempty sequence of alphanumeric characters and low lines (underscores), such as `cp3o` and `r2d2` and c`ool_1`
`100\s*mk`	the strings `100` and `mk` optionally separated by any amount of white space (spaces, tabs, newlines)
`abc\b`	`abc` when followed by a word boundary (e.g. in `abc!` but not in `abcd`)
`perl\B`	`perl` when not followed by a word boundary (e.g. in `perlert` but not in `perl stuff`)

You have learned how to use Perl REGEX, regular expressions.

Elcric Otto Circle

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WRITING AND READING PERL 5 SCRIPT

noreply@blogger.com (Ron Tinnel) — Tue, 08 Nov 2011 09:28:00 +0000

WRITING AND READING PERL 5 SCRIPT

REVISED: Sunday, March 3, 2013

You will learn how to write and read Perl 5 script.

I. PERL 5 FILE OUTPUT AND FILE INPUT SOURCE CODE EXAMPLES

Perl's strengths are in text manipulation and parsing; therefore, Perl 5 has complete file input and output capabilities, especially syntax for line-at-a-time sequential input.

A. PERL 5 FILE OUTPUT SOURCE CODE EXAMPLE

"Copy Paste" the following Perl 5 program into your text editor:

#Unicode

use utf8;

#Provides undefined value warnings.

use warnings;

#enables -w flag.

use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
#my variables (lexical variables) are faster than globals.

use strict;

#Stack trace.

use Carp ();

local $SIG{__WARN__} = \&Carp::confess;

open STDOUT, ">c:/strawberry/mary.txt";

print "Mary had a little lamb.\n";

print "Its fleas were white as snow.\n";

print "And every where that Mary went,\n";

print "the fleas were sure to go.\n";

close STDOUT;

"File Save As" writeMary.pl using the path of the Perl 5 download; e.g.:

c:\strawberry\writeMary.pl

Open Perl 5, depending on your platform you will have a Perl 5 prompt similar to the following:

C:\WINDOWS\system32>

From the above Perl 5 prompt type:

C:\WINDOWS\system32>c:\strawberry\writeMary.pl

Press Enter.

Perl 5 will display the following on your Perl 5 prompt:

C:\WINDOWS\system32>

If you were expecting to see the poem display on the Perl 5 prompt screen, surprise!

Lets examine the code line by line and see what happened to our poem.

Line 19: open STDOUT, ">c:/strawberry/mary.txt";

Our open has two arguments: The first argument is a filehandle, in this example the filehandle is STDOUT. The two arguments are separated by a comma. The second argument is a single string comprising how to open it, the path, and what to open. If the "expression" begins with ">", as ours does, the file is opened for output. When you name a file in an "open for writing" statement, as we have done, the file may or may not exist before opening. If it does exist, first it is wiped clean as soon as it's opened. If it does not exist, first it is created with no contents. mary.txt does not exist; therefore, mary.txt is created with no contents. open returns true when it works, and false when it fails.

A print statement sends output to STDOUT by default. You can specify the file you want to use by opening a filehandle to it. Then you use the new filehandle in your print statement, instead of the default STDOUT filehandle. There is no comma after the filehandle in the print statement. You can name a filehandle anything you like. By convention, a filehandle is written in upper case, but it does not have to be.

Since a print statement sends output to STDOUT by default we did not have to write lines 2-5 as shown below. However, if you change the program it will work because both are the same.

Line 20: print STDOUT "Mary had a little lamb.\n";

Line 21: print STDOUT "Its fleas were white as snow.\n";

Line 22: print STDOUT "And every where that Mary went,\n";

Line 23: print STDOUT "the fleas were sure to go.\n";

When you're finished, you close the filehandle as shown below.

Line 24: close STDOUT;

In summary, we created a file, which we opened for output, named mary.txt located at path c:/strawberry/ then we wrote our poem to that file and closed that file. That is why it did not appear displayed on the Perl 5 prompt screen.

B. PERL 5 FILE INPUT SOURCE CODE EXAMPLE

"Copy Paste" the following Perl 5 program into your text editor:

#Unicode

use utf8;

#Provides undefined value warnings.

use warnings;

#enables -w flag.

use diagnostics;

#Helps you catch typos.

#Forces you to use my() function to declare all variables.

#my variables (lexical variables) are faster than globals.

use strict;

#Stack trace.

use Carp ();
local $SIG{__WARN__} = \&Carp::confess;

#Specify the file

my $file = "c:/strawberry/mary.txt";

#Open the file and read data

#Die with grace if it fails

open (FILE, "<$file") or die "Can't open $file: $!\n";
my @lines = <FILE>;
print @lines;
close FILE;

"File Save As" readMary.pl using the path of the Perl 5 download; e.g.:

c:\strawberry\readMary.pl

From the Perl 5 prompt type the following:

C:\WINDOWS\system32>c:\strawberry\readMary.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>c:\strawberry\readMary.pl
Mary had a little lamb.
Its fleas were white as snow.
And every where that Mary went,
the fleas were sure to go.

C:\WINDOWS\system32>

Lets examine the code line by line and see what happened to our poem.

Line 19: #Specify the file.
Line 20: my $file = "c:/strawberry/mary.txt";

As the comment in Line 1: states, Line 2: specifies the file by assigning "c:/strawberry/mary.txt" to $file.

As shown below, the file $FILE is opened or if the file can not be opened an error message is displayed on the screen.

Line 21: #Open the file and read data
Line 22: #Die with grace if it fails
Line 23: open (FILE, "<$file") or die "Can't open $file: $!\n";

Our open has two arguments: The first argument is a filehandle, in this example the filehandle is FILE. The two arguments are separated by a comma. The second argument is a single string comprising how to open it, the path, and what to open. If the "expression" begins with "<", as ours does, the file is opened for input, reading. When you name a file in an "open for reading" statement, as we have done, the file may or may not exist before opening. If it does exist it's opened. open returns true when it works, and false when it fails. If our open had failed the condition would have been false and the or would have printed both the error message "Can't open $file: and the appropriate system error message provided by $!.

As shown below the file is read into the array @lines.

Line 24: #Read the file into the array @lines.
Line 25: my @lines = <FILE>;

print displays the poem on the screen.

Line 26: #Print the file to the screen.
Line 27: print @lines;

We are done! Therefore, as shown below, the file is closed.

Line 28: #Close the file.
Line 29: close FILE;

C. PERL 5 FILE INPUT OUTPUT SUMMARY

MODE	DESCRIPTION
+<	READ,WRITE
+>	READ,WRITE,TRUNCATE,CREATE
+>>	READ,WRITE,CREATE,APPEND

You have learned how to write and read Perl 5 script.

Elcric Otto Circle

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PERL 5 SUBROUTINE

noreply@blogger.com (Ron Tinnel) — Tue, 08 Nov 2011 09:25:00 +0000

PERL 5 SUBROUTINE

REVISED: Sunday, March 3, 2013

You will learn the basic fundamentals of the Perl 5 subroutine.

I. PERL 5 SUBROUTINE INTRODUCTION

A. PERL 5 SUBROUTINES

Perl 5 subroutines are user-defined functions.

Subroutines allow you to reuse blocks of code in a program over and over again.

Subroutine definitions can be anywhere in a Perl 5 program. However, you should pick a location and consistently place them in that location; e.g., either at the beginning or end of your program.

Subroutine definitions are global; they are accessible from every part of your program. A Perl 5 subroutine can return any kind of value. When creating a subroutine definition you can use a return statement but you do not have to. If you do not use a return statement, the return value of the subroutine is the value of the last statement executed.

A subroutine name begins with an ampersand (&) and consists of letters, digits, and underscores, but can not start with a digit. You use the subroutine name, with the prefix ampersand (&) from within an expression, to invoke or call the subroutine. Any time you call a subroutine, you implicitly call a subroutine which is inside the current package.

To define a subroutine, use the keyword sub, the name of the subroutine without the ampersand (&), then an indented block of code in curly braces ({}) which makes up the body of the subroutine. The concept of a block of code is important; therefore, remember the area between the two braces is called a block.

B. PERL 5 MODULE

A module is similar to a subroutine. The difference is that modules do not live in your program, they are their own separate script outside your code.

A module is incorporated into a program with use. You can use as many modules as you like in one program and you can write your own modules.

Make a new folder within the Perl 5 lib folder; e.g., myModules. Instead of saving to your new Perl 5 lib folder myModules with a .pl extension, save with a .pm extension for Perl Module. To use a module named firstModule.pm start your script with use myModules::firstModule;.

C. PERL 5 PACKAGE

A Perl 5 script (.pl file) must always contain exactly zero package declarations.

A Perl 5 module (.pm file) must always contain exactly one package declaration, corresponding exactly to its name and location. For example, module Mathematics/Powers.pm must begin with package Mathematics::Powers.

D. PERL 5 ARGUMENTS

All arguments in Perl 5 are passed by reference. Therefore, it is important to immediately assign the input-only arguments to local variables, and only work on the local variables.

In Perl 5, you can pass only one kind of argument to a subroutine: a scalar. To pass any other kind of argument, you need to convert it to a scalar. You do that by passing a reference to it. A reference to anything is a scalar.

E. REFERENCING AND DE-REFERENCING VARIABLES

-> This operator works the same as it does in C. It means "element so and so of the de-referenced reference". In-place de-referencing is provided by the -> operator. Most Perl 5 objects are references to a hash. One typical usage is an object containing a list of hashes. The list of hashes could easily represent a data table, with array elements being rows (records) and hash elements being columns (fields).

Whenever you call a subroutine, the perl 5 interpreter creates a list of any parameters passed to the subroutine and stores this list in a special version of the variable @_ that is usable only by the called subroutine. Arguments to a subroutine are accessible inside the subroutine as list @_. Any change the subroutine performs to @_ or any of its members like $_[0], $_[1], etc, are changes to the original argument. However, assigning @_ or its elements to other variables makes a separate copy. Changes to the separate copy are unknown outside of the subroutine.

The following table discusses the referencing and de-referencing of variables.

VARIABLE	INSTANTIATING THE SCALAR	INSTANTIATING REFERENCE TO SCALAR	REFERENCING SCALAR	DE-REFERENCING SCALAR	ACCESSING AN ELEMENT
$scalar	$scalar = "dog";	$ref = \"dog";	$ref = \$scalar	$$ref or ${$ref}	N/A
@list	@list = ("dog", "cat");	$ref = ["dog", "cat"];	$ref = \@list	@{$ref}	${$ref}[3] $ref->[3]
%hash	%hash = ("animal" => "dog", "camel" => "Bactrian");	$hash = {"animal" => "dog", "camel" => "Bactrian"};	$ref = \%hash	%{$ref}	${$ref}{"rat"} $ref->{"rat"}
FILE			*$ref = \FILE**	{$ref} or scalar <$ref>

F. PERL 5 TWO STRING ARGUMENTS SUBROUTINE EXAMPLE

The following table shows the subroutine syntax for a subroutine with two string arguments:

TWO STRING ARGUMENTS
CALL	SUBROUTINE
mysub($filename, $title);	sub mysub($$) { }

G. PERL 5 SUBROUTINE EXAMPLE

sub max {

    my($max) = shift(@_);

    foreach my $temp (@_) {

        $max = $temp if $temp > $max;

    }

    return($max);

}

You have learned the basic fundamentals of the Perl 5 subroutine.

Elcric Otto Circle

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