Download - Perl -Basic Programming concepts.

By Tanuj Maheshwari

Perl

IntroductionPerl 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 levelsA great tool for leverageHigh portability and readily available

AvailabilityIt's free and runs rather nicely on nearly

everything that calls itself UNIX or UNIX-like

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 UnixSetup path variable to point to the

directory where Perl is locatedCheck /usr/local/bin or /usr/bin for “perl”Run a Perl script by typing “perl

<filename>” 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 Win32ActivePerl allows Perl scripts to be executed in

MS-DOS/WindowsPerl is being ported faithfullyThe #! directive is no longer used because it

does not mean anything to MS-DOS/WindowsPerl scripts are executed by typing “perl

<filename> Alternatively, double clicking on the file if the

extension .pl is being associated to the Perl interpreter

An Example#!/usr/bin/perlprint “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

VariablesThree main types of variables, Scalar ArrayHash Variables are all global in scope unless

defined to be private or localNote: remember that hash and array are

used to hold scalar values

ExamplesAssigning 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 StatementsVariables alone will not support switches or

conditionsIf-Then-Else like clauses are used to make

decisions based on certain preconditionsKeywords: if, else, elsif, unlessEnclosed 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 newline character

LoopsConditional statements cannot handle

repetitive tasksKeywords: while, foreach, for , until, do-while,

do-untilForeach 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 LoopSyntax:

while(some expression){statements;…

}Example:

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

print $a; $a = $a – 1;}

Until LoopSyntax:

until(some expression){statements;…

}Example:

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

print $a; $a = $a – 1;}

Foreach LoopSyntax:

foreach [<variable>] (@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 LoopSyntax:

For(initial_exp; test_exp; re-init_exp ) { statements;

…}

Example:#prints numbers 1-10for ($i = 1; $i <= 10; $i++) {

print "$i ";

}

Do-While and Do-Until LoopsSyntax:

do {statments; do{ statements;} while some_expression; }until

some_expression;

Example the prints the numbers 1-10 in reverse order:$a = 10; $a = 10;do{ do{ print $a; print $a; $a = $a – 1; $a = $a - 1;}while ($a > 0); }until ($a <= 0);

Built-in functionsshift 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<result> = uc(<string>)<result> = 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/OSTDIN and STDOUTSTDIN 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 ExpressionsTemplate to be matched against a stringPatterns are enclosed in ‘/’sMatching against a variable are done by the =~

operatorSyntax: /<pattern>/Examples:

$string =~/abc/ #matches “abc” anywhere in $string

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

Creating PatternsSingle 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

characterab+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 stringa{3,} #matches three of more “a”s

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

three “a”s in itComplex patterns can be constructed from

these operatorsFor examples:

/a.*ce.*d/ matches strings such as

“asdffdscedfssadfz”

Creating Patterns: ExercisesConstruct patterns for the following strings:

1. "a xxx c xxxxxxxx c xxx d“2. a sequence of numbers3. 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 character class and general multiplier

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

Anchoring PatternsNo boundaries are defined by the previous

patterns Word boundary: \w and \W\b and \B is used to indicate word boundaries

and vice verseExamples:

/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 stringExampls:

/^Fred$/ #matches only “Fred”/aaa^bbb/ #matches nothing

More on matching operatorsAdditional 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

SubstitutionReplacement of patterns in strings/<pattern to search>/<pattern to

replace>/igi is to case insensitiveg enables the matching to be performed

more than onceExamples:

$which = “this this this”;$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 FunctionsSyntax:

<return value(s)> = split(/<pattern>/[,<variable>]);

<return value> = 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

FunctionsAutomates certain tasks Syntax:

sub <name>{

…<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 valuesTwo types of functions: void functions (also

known as routine or procedure), and functionsvoid functions have no return valuesFunctions 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 parametersParameters to a function are stored as a list

with the @_ variableExample:

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 functionmy keyword defines a variable as being

private to the scope it is definedFor example:

sub add{

my($left,$right) = @_; return $left + $right;}

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

3

print “$one\n”; #prints 0 Print “$two\n”; #prints 0

ExercisesA trim() function that removes leading and

trailing spaces in a stringHint: 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/OFilehandleAutomatic filehandles: STDIN, STDOUT and

STDERR Syntax:

open(<handle name>,”(<|>|>>)filename”);close(<handle name>);

Example:open(INPUTFILE,”<inputs.txt”); #opens file

handle…Close(INPUTFILE); #closes file handle

File I/O (cont.)Handle access does not always yield trueCheck for return value of the open functionExample:

if(open(INPUT,”<inputs.txt”))… #do something

elseprint “File open failed\n”;

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

open(INPUT,”<inputs.txt”) ||die “File open failed\n”;

Use $! variable to display additional operating system errorsdie “cannot append $!\n”;

File I/O (cont.)Filehandles are similar to standard I/O handles<> operator to read linesFor example:

open(INPUT,”<inputs.txt”);while(<INPUT>){

chomp;print “$_\n”;

}Use print <handle_name> <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 (<IN>) { # 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 –BFor 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 ModulesConcentrate only on their usage in the

Greenstone environmentPackage: a mechanism to protect codes from

tempering each other’s variablesModule: reusable package that is stored in

<Name of Module>.dmThe 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


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


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

ThanksTanuj maheshwari