Date post: | 10-May-2015 |
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Vamshi Krishna S
Practical Extraction and Report Language
‘A general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development-CGI scripting, network programming, GUI development, and more.’
‘The language is intended to be PRACTICAL (easy to use, efficient, complete) rather than
BEAUTIFUL (tiny, elegant, minimal).’
And Some More:
‘Many earlier computer languages, such as Fortran and C, were designed to make efficient use of expensive
computer hardware. In contrast, Perl is designed to make efficient use of expensive computer programmers.
Perl has many features that ease the programmer's task at the expense of greater CPU and memory requirements.
These include automatic memory management; dynamic typing; strings, lists, and hashes; regular expressions;
Larry Wall invented PERL in the mid-1980'sLarry Wall was trained as a linguist, and the design of Perl is very
much informed by linguistic principles. Examples include Huffman coding (common constructions should
be short), good end-weighting (the important information should
come first), and a large collection of language primitives. Perl favors language constructs that are natural for humans to
read and write, even where they complicate the Perl interpreter.’Perl has rapidly become the language of choice for writing
programs quickly and robustly across a wide range of fields - ranging from systems administration, text processing, linguistic analysis, molecular biology and (most importantly of all) the creation of dynamic World Wide Web pages. It has been estimated that about 80% of dynamic webpages worldwide are being created by Perl programs.
PERL encompasses both the syntactical rules of the language and the general ways in which programs are organized
It is dynamically typed language. Relatively easy to learn (and easier to make a
mess too). incredibly flexible coding style (some argues it is
too flexible). Perl is interpreted not complied hence its
scripting language. It follows OOPs concepts.
http://perldoc.perl.org/perldoc.html
CPAN(comprehensive Pern Archive Network) : consists of Additional perl modules(more than 100,000 modules), documentation,various releases etc., HTTP://cpan.org/
Define the problem Search for existing code Plan your solution Write the code
Modify ->Debug ->Modify Run the code
Now-a-days On *nix OSes Perl comes installed automatically. And can be located at /usr/bin/perl and /usr/local/bin/perl
To install Perl on Windows :
http://strawberryperl.com/
http://www.activestate.com/activeperl
Open a terminal Make a perl dir(directory) in your home
dir Move into the perl directory Create a file named ‘hello_world.pl’ Open the file in your text editor Code the program Save the file Make the program executable Test the program.
# Unix perl -e 'print "Hello world\n"‘
# MS-DOS perl -e "print \"Hello world\n\""
Location of perl is normally in /usr/bin/perl and /usr/local/bin/perl
Perfix the script with #!/usr/bin/perlAnd also you can type in “use <version>” to use the latest version
#!/usr/bin/perl -wuse strict;use warnings;
my $message = ‘Welcome to perl
tutorial’;
print “\t hello world $message.!!\n”; print ‘hello world $message.!!\n’
#prints $message\n literally
Scalar: a single piece of information. Scalars can hold numbers or text $ is the identifier of scalar in perl There are special variables for the system: $ARGV,
$! @scores = (32, 45, 16, 5); @cars = (BMW,Renault,Jaguar,Ferrari); or @cars =
qw(BMW Renault Jaguar Ferrari);
my @sorted = sort @cars; my @backwards = reverse @scores;
$multilined_string = <<EOF; This is my multilined string note that I am terminating it with the word "EOF". EOF
Scalar values are represented as $var = <num/char> ; Dynamic typing
Array/List: an ordered collection of scalars my @array = ( 1, 2 ); my @words = ( "first", "second", "third" ); my @mixed = ("camel", 42, 1.23);
print $mixed[$#mixed]; # last element, prints out 1.23
SubscriptsAn array can be accessed one scalar at a time by specifying a dollar sign ($ ), then the name of the array (without the leading @ ), then the subscript inside square brackets.
For example:@myarray = (5, 50, 500, 5000);print "The Third Element is", $myarray[2], "\n";
Declaration of HASHes %scientists =
(
"Newton" => "Isaac",
"Einstein" => "Albert",
"Darwin" => "Charles",
"Feynman" => "Richard",
);
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
Hash subscripts are similar, only instead of square brackets curly brackets are used
my %fruit_color = ("apple", "red", "banana", "yellow");
To get at hash elements:
$fruit_color{"apple"}; # gives "red“ To get a lists of keys and values
with keys() and values().
my @fruits = keys %fruit_colors;
my @colors = values %fruit_colors;
Some scalar variables have special meaning in Perl. Of note are `$_`,`$!`, `$0`, and `$$`.
There are system defined functions for operations on Scalar variables, Arrays, Hashes, File Handlers, Regular Expressions, Sub routines, Modules etc., which appear like keywords some times and take arguments
Eg: Chomp, join, my, our, grep, mkdir, open, import,defined,undef,sort,reverse etc.,
For detailed description follow: http://perldoc.perl.org/perlfunc.html#Perl-Functions-by-Category
double quotes(“), single quotes(‘) and multi-line quoting(qq) :$var =10;$text = “There are $var apples” $text2 = ‘There are $var apples’$text3 = qq{ There
are$var
apples};
Functions are blocks of code which perform specific task
#takes no input and returns no output… common practice to use #‘main’ as the starting point in a script.sub main { …}
#Takes 2 *scalars* as input sums them and returns one scalar.sub sum_2_numbers {
my ($numA,$numB) = @_; #get passed in values
my $sum = $numA+$numB; #sum numbersreturn($sum); #return sum
}
if/else if ( condition ) {…} elsif ( other condition ) {…} else {…}
Unlessdie "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
While while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key};
}
Until$count = 10; until ($count == 0) { print "$count "; $count--;}
foreachforeach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
Logical operators: ==, !=, &&, ||, eq and ne
Undefined/” ”/0 values are treated as false
Scalars are evaluated as:numbers are evaluated as true if non-zerostrings are evaluated as true if non-empty
$var = “false”;
if($var)
{
say “$var is true!”;
}
Scripts can take inputs in two ways:Arguments
./print_args.pl ARG1 ARG2Prompted inputs from users
$user_text = <STDIN>
Things don’t always come out as expected. It is good to check the output of important functions for errors, it is highly recommended to validate any input from users or external sourcesDieWarn