Perl -basic Programming Concepts.

Perl By Tanuj Maheshwari

Introduction Perl stands for "Practical Extraction and

Report Language" Created by Larry Wall when awk ran out of steam Perl grew at almost the same rate as the Unix operating system

Introduction (cont.) Perl fills the gaps between program

languages of different levels A great tool for leverage High portability and readily available

Availability It's free and runs rather nicely on nearly

everything that calls itself UNIX or UNIXlike Perl has been ported to the Amiga, the Atari ST, the Macintosh family, VMS, OS/2, even MS/DOS and Windows The sources for Perl (and many precompiled binaries for non-UNIX architectures) are available from the Comprehensive Perl Archive Network (the CPAN). http://www.perl.com/CPAN  

Running Perl on Unix Setup path variable to point to the

directory where Perl is located Check /usr/local/bin or /usr/bin for “perl” Run a Perl script by typing “perl ” Alternatively, change the file attribute to executable and include “#!/usr/bin/perl” in the first line of your perl script The .pl extension is frequently associated to Perl scripts

Running Perl on Win32 ActivePerl allows Perl scripts to be executed in

MS-DOS/Windows Perl is being ported faithfully The #! directive is no longer used because it does not mean anything to MS-DOS/Windows Perl scripts are executed by typing “perl Alternatively, double clicking on the file if the extension .pl is being associated to the Perl interpreter

An Example #!/usr/bin/perl print “Hello World!”; The #! directive directs subsequent lines in the file to the perl executable All statements are terminated with ; as in C/C++/Java print by default outputs any strings to the terminal console. (such as printf in C or cout in C++) Perl completely parses and compiles the script before executing it

Variables Three main types of variables, Scalar Array Hash Variables are all global in scope unless

defined to be private or local Note: remember that hash and array are used to hold scalar values

Examples Assigning values to a scalar

$i = “hello world!”; $j = 1 + 1; ($i,$j) = (2, 3) Assigning values to an array

$array[0] = 1; $array[1] = “hello world!”; push(@array,1); #stores the value 1 in the end of @array $value = pop(@array); #retrieves and removes the last element #from @array @array = (8,@array); #inserts 8 in front of @array

Examples (cont.) Assigning values to a hash

$hash{‘greeting’} = “Hello world!”;

$hash{‘available’} = 1; #or using a hash slice @hash{“greeting”,”available”} = (“Hello world!”, 1); Deleting a key-value pair from a hash:

delete $hash{‘key’};

Conditional Statements Variables alone will not support switches or

conditions If-Then-Else like clauses are used to make decisions based on certain preconditions Keywords: if, else, elsif, unless Enclosed by ‘{‘ and ‘}’

A Conditional Statement Example print "What is your name? "; $name = <STDIN>; chomp ($name); if ($name eq "Randal") { print "Hello, Randal! How good of you to be here!\n"; } else { print "Hello, $name!\n"; # ordinary greeting } unless($name eq “Randal”) { print “You are not Randal!!\n”; #part of the ordinary greeting }

$name = <STDIN> reads from standard

input chomp is a built-in function that removes

Loops Conditional statements cannot handle

repetitive tasks Keywords: while, foreach, for , until, do-while, do-until Foreach loop iterates over all of the elements in an array or hash, executing the loop body on each element For is a shorthand of while loop until is the reverse of while

Loops (cont.) Do-while and do-until loops executes the

loop body once before checking for termination Statements in the loop body are enclosed by ‘{‘ and ‘}’

While Loop Syntax: while(some expression){ statements; … } Example:

#prints the numbers 1 – 10 in reverse order $a = 10; while ($a > 0) { print $a; $a = $a – 1; }

Until Loop Syntax: until(some expression){ statements; … } Example:

#prints the numbers 1 – 10 in reverse order $a = 10; until ($a <= 0) { print $a; $a = $a – 1; }

Foreach Loop Syntax: foreach [] (@some-list){ statements… } Example:

#prints each elements of @a @a = (1,2,3,4,5); foreach $b (@a) { print $b; }

Foreach Loop (cont.) Accessing a hash with keys function:

foreach $key (keys (%fred)) { # once for each key of %fred print "at $key we have $fred{$key}\n"; # show key and value }

For Loop Syntax: For(initial_exp; test_exp; re-init_exp ) { statements; … }

Example:

#prints numbers 1-10 for ($i = 1; $i <= 10; $i++) { print "$i "; }

Do-While and Do-Until Loops Syntax:

do {statments; do{ statements; } while some_expression; }until some_expression; Example the prints the numbers 1-10 in

reverse order:

$a = 10; do{ print $a; $a = $a – 1; }while ($a > 0);

$a = 10; do{ print $a; $a = $a - 1; }until ($a <= 0);

Built-in functions shift function Ex: $value = Shift(@fred) is similar to ($x,@fred) = @fred; unshift function Ex: unshift(@fred,$a); # like @fred = ($a,@fred);

reverse function @a = (7,8,9); @b = reverse(@a); # gives @b the value of (9,8,7) sort function @y = (1,2,4,8,16,32,64); @y = sort(@y); # @y gets 1,16,2,32,4,64,8

Built-In Functions (cont.) qw function Ex: @words = qw(camel llama alpaca); # is

equivalent to @words = (“camel”,”llama”,”alpaca”); defined function Returns a Boolean value saying whether the

scalar value resulting from an expression has a real value or not. Ex: defined $a; undefined function Inverse of the defined function

Built-In Functions (cont.) uc and ucfirst functions –vs- lc and lcfirst

functions  = uc(<string>)  = ucfirst(<string>)

$string = “abcde”; $string2 = uc($string); #ABCDE $string3 = ucfirst($string); #Abcde Lc and lcfirst has the reverse effect as uc and

ucfirst functions

Basic I/O STDIN and STDOUT STDIN Examples: $a = <STDIN>; @a = <STDIN>; while (defined($line = <STDIN>))

{ # process $line here } STDOUT Examples: print(list of arguments); print “text”; printf ([HANDLE], format, list of arguments);

Regular Expressions Template to be matched against a string Patterns are enclosed in ‘/’s Matching against a variable are done by the =~

operator Syntax: /<pattern>/ Examples: $string =~/abc/ #matches “abc” anywhere in

$string <STDIN> =~ /abc/ #matches “abc” from standard #input

Creating Patterns Single character patterns: “.” matches any single character except newline

(\n), for example: /a./ “?” matches zero or one of the preceding character Character class can be created by using “[“ and “]”. Range of characters can be abbreviated by using “-”, and a character class can be negated by using the “^” symbol. For examples:  [aeiouAEIOU] matches any one of the vowels  [a-zA-Z] matches any single letter in the English

alphabets  [^0-9] matches any single non-digit

Creating Patterns (cont.) Predefined character class abbreviations: \d == [0-9] \D == [^0-9] \w == [a-zA-Z0-9] \W == [^a-zA-Z0-9] \s == [ \r\t\n\f] \s == [^ \r\t\n\f]

Creating Patterns (cont.) Multipliers: *, + And {} * matches 0 or more of the preceding

character ab*c matches a followed by zero or more bs and

followed by a c + Matches 1 or more of the preceding

character ab+c matches a followed by one or more bs and

followed by a c {} is a general multiplier a{3,5} #matches three to five “a”s in a string a{3,} #matches three of more “a”s

Creating Patterns (cont.) a{3} #matches any string with more than

three “a”s in it Complex patterns can be constructed from

these operators For examples: /a.*ce.*d/ matches strings such as

“asdffdscedfssadfz”

Creating Patterns: Exercises Construct patterns for the following

strings: 1. "a xxx c xxxxxxxx c xxx d“ 2. a sequence of numbers 3. three or more digits followed by the string “abc” 4. Strings that have an “a”, one or more “b”s and at least five “c”s 5. Strings with three vowels next to each other. Hint: try

Creating Patterns: Exercises Answers: /a.*c.*d/ /\d+/ or /[0-9]+/ /\d\d\d.*abc/ or /\d{3,}abc/ /ab+c{5,}/ /[aeiouAEIOU]{3}/ Other possible answers?

Anchoring Patterns No boundaries are defined by the previous

patterns Word boundary: \w and \W \b and \B is used to indicate word boundaries and vice verse Examples: /fred\b/ #matches fred, but not frederick /\b\+\b/ #matches “x+y”, but not “x + y”, “++”

and ”+”. Why? /\bfred\B/ #matches “frederick” but not “fred

Anchoring Patterns (cont.) ^ and $ ^ matches beginning of a string $ matches end of a string Exampls: /^Fred$/ #matches only “Fred” /aaa^bbb/ #matches nothing

More on matching operators Additional flags for the matching operator: /<pattern>/i #ignores case differences  /fred/i #matches FRED,fred,Fred,FreD and etc… /<pattern>/s #treat string as single line /<pattern>/m #treat string as multiple line

More on Matching Operators (cont.) “(“ and “)” can be used in patterns to

remember matches Special variables $1, $2, $3 … can be used to access these matches For example: $string = “Hello World!”; if( $string =~/(\w*) (\w*)) { #prints Hello World print “$1 $2\n”; }

More on Matching Operators (cont.) Alternatively:

$string = “Hello World!”; ($first,$second) = ($string =~/(\w*) (\w*)); print “$first $second\n”; #prints Hello World Line 2: Remember that the =~ return values just like a function. Normally, it returns 0 or 1, which stands for true or false, but in this case, the existence of “(“ and “)” make it returns value of the matching patterns

Substitution Replacement of patterns in string s/<pattern to search>/<pattern to

replace>/ig i is to case insensitive g enables the matching to be performed more than once Examples: $which = “this this this”; $which =~ s/this/that/; #produces “that this this”

Substitution (cont.) $which =~ s/this/that/g; #produces “that that that” $which =~ s/THIS/that/i; #produces “that this this” $which =~ s/THIS/that/ig; #produces “that that that” Multipliers, anchors and memory operators can be

used as well: $string = “This is a string”; $string =~ s/^/So/; # “So This is a string” $string =~ s/(\w{1,})/I think $1/; # “I think This is a string”

Split and Join Functions Syntax:  =

split(/<pattern>/[,]);  = join(“<seperator>”,<array>); Examples:

$string = “This is a string”; @words = split(/ /,$string); #splits the string into #separate words @words = split(/\s/,$string); #same as above $string = join(“ “,@words); #”This is a string” Great functions in parsing formatted documents

Functions Automates certain tasks Syntax:

sub { … <statements> } Global to the current package. Since we are

not doing OOP and packages, functions are “global” to the whole program

Functions (cont.) Example:

sub say_hello { print “Hello world!\n”; } Invoking a function:

say_hello(); #takes in parameters &say_hello; #no parameters

Functions (cont.) Return values Two types of functions: void functions (also

known as routine or procedure), and functions void functions have no return values Functions in Perl can return more than one variable: sub threeVar { return ($a, $b, $c); #returns a list of 3 variables }

Functions (cont.) ($one,$two,$three) = threeVar(); Alternatively: @list = threeVar(); #stores the three values into a list Note:

($one, @two, $three) = threeVar(); #$three will not have #any value, why?

Functions (cont.) Functions can’t do much without parameters Parameters to a function are stored as a list

with the @_ variable Example:

sub say_hello_two { $string = @_; #gets the value of the parameter } Invocation:

say_hello_two(“hello world!\n”);

Functions (cont.) For example:

sub add { ($left,$right) = @_; return $left + $right; } $three = add(1,2);

Functions (cont.) Variables are all global even if they are

defined within a function my keyword defines a variable as being private to the scope it is defined For example: sub add { my($left,$right) = @_; return $left + $right; }

Functions (cont.) $three = add(1,2); #$three gets the value of 3

print “$one\n”; Print “$two\n”;

#prints 0 #prints 0

Exercises A trim() function that removes leading and

trailing spaces in a string Hint: use the s/// operator in conjunction with

anchors A date() function that converts date string,

“DD:MM:YY” to “13th of December, 2003” Hint: use a hash table to create a lookup

table for the month strings.

File I/O Filehandle Automatic filehandles: STDIN, STDOUT and

STDERR Syntax:

open(,”(<|>|>>)filename”); close(); Example:

open(INPUTFILE,”
File I/O (cont.) Handle access does not always yield true Check for return value of the open function Example: if(open(INPUT,”
… #do something else print “File open failed\n”;

File I/O (cont.) The previous method is the standard practice Unlike other languages, Perl is for lazy people Ifs can be simplified by the logical operator “||” For example:

open(INPUT,”
File I/O (cont.) Filehandles are similar to standard I/O handles <> operator to read lines For example:

open(INPUT,”){ chomp; print “$_\n”; } Use print <strings> to output

to a file

File I/O (cont.) File copy example:

open(IN,$a) || die "cannot open $a for reading: $!"; open(OUT,">$b") || die "cannot create $b: $!"; while () { # read a line from file $a into $_ print OUT $_; # print that line to file $b } close(IN) || die "can't close $a: $!"; close(OUT) || die "can't close $b: $!";

File I/O (cont.) File tests provides convenience for

programmers -e –r –w –x –d –f –l –T –B For example:

if(-f $name){ print “$name is a file\n”; } elsif(-d $name){ print “$name is a directory\n”; }

Special Variables $_, @_ $1, $2… - backreferencing variables

$_ = "this is a test"; /(\w+)\W+(\w+)/; # $1 is "this" and $2 is "is" $`, $& and $’ - match variables

$string = “this is a simple string”; /si.*le/; #$& is now “sample”, $` is “this is a” and $’ is #“string” And many more…refer to ActivePerl’s online

documentations for their functions

Packages and Modules Concentrate only on their usage in the

Greenstone environment Package: a mechanism to protect codes from tempering each other’s variables Module: reusable package that is stored in .dm The ppm (Perl Package Manager) for Linux and Win32 version of Perl manages installation and uninstallation of Perl packages

Packages and Modules (cont.) Install the module and put “use ModuleName”

or “require ModuleName” near the top of the program :: qualifying operator allow references to things in the package, such as $Module::Variable So “use Math::Complex module” refers to the module Math/Complex.pm new creates an instance of the object, then use the handle and operator -> to access its functions

Packages and Modules (cont.) use accepts a list of strings as well, such that the

we can access the elements directly without the qualifying operator For example: use Module qw(const1 const2 func1 func2 func3); const1, const2, func1, func2 and func3 can now be used directly in the program

Packages and Modules (cont.) Perl locates modules by searching the @INC

array The first instance found will be used for the module referenced within a program Where to locate modules are an automatic process as the Makefiles and PPM take care placing modules in the correct path

Packages and Modules (cont.) An example that uses the package CGI.pm:

use CGI; #uses the CGI.pm module $query = CGI::new(); #creates an instance of CGI $bday = $query->param("birthday"); #gets a named parameter print $query->header(); #outputs html header print $query->p("Your birthday is $bday."); #outputs text to html

Packages and Modules (cont.) Advantages: Encourages code reuse and

less work Disadvantages: 33% as fast as procedural Perl according to the book “object-oriented Perl”, generation of Perl modules involves some ugly codes

Packages and Modules (cont.) Huge library

Thanks Tanuj maheshwari