(PERL) Introduction What is PERL? Practical Report and Extraction - PDF document

• Example: @color = qw (red, blue, green, black); $first = shift @color; # $first gets “red”, and @color becomes # (blue, green, black) unshift (@color, “white”); # @color becomes (white, blue, green, black) Internet & Web Based Technology 35

pop and push • They operate on the bottom of the array. – ‘pop’ removes the last element of the array. – ‘push’ replaces the last element of the array. Internet & Web Based Technology 36

• Example: @color = qw (red, blue, green, black); $first = pop @color; # $first gets “black”, and @color becomes # (red, blue, green) push (@color, “white”); # @color becomes (red, blue, green, white) Internet & Web Based Technology 37

Reversing an Array • By using the ‘reverse’ keyword. @names = (“Mina”, “Tina”, ‘Rina”) @rev = reverse @names; # Reversed list stored in ‘rev’. @names = reverse @names; # Original array is reversed. Internet & Web Based Technology 38

Printing an Array • Example: @colors = qw (red, green, blue); print @colors; # prints without spaces – redgreenblue print “@colors”; # prints with spaces – red green blue Internet & Web Based Technology 39

Sort the Elements of an Array • Using the ‘sort’ keyword, by default we can sort the elements of an array lexicographically. – Elements considered as strings. @colors = qw (red blue green black); @sort_col = sort @colors # Array @sort_col is (black blue green red) Internet & Web Based Technology 40

– Another example: @num = qw (10 2 5 22 7 15); @new = sort @num; # @new will contain (10 15 2 22 5 7) – How do sort numerically? @num = qw (10 2 5 22 7 15); @new = sort {$a <=> $b} @num; # @new will contain (2 5 7 10 15 22) Internet & Web Based Technology 41

The ‘splice’ function • Arguments to the ‘splice’ function: – The first argument is an array. – The second argument is an offset (index number of the list element to begin splicing at). – Third argument is the number of elements to remove. @colors = (“red”, “green”, “blue”, “black”); @middle = splice (@colors, 1, 2); # @middle contains the elements removed Internet & Web Based Technology 42

File Handling

Interacting with the user • Read from the keyboard (standard input). – Use the file handle <STDIN>. – Very simple to use. print “Enter your name: ”; $name = <STDIN>; # Read from keyboard print “Good morning, $name. \n”; – $name also contains the newline character. • Need to chop it off. Internet & Web Based Technology 44

The ‘chop’ Function • The ‘chop’ function removes the last character of whatever it is given to chop. • In the following example, it chops the newline. print “Enter your name: ”; chop ($name = <STDIN>); # Read from keyboard and chop newline print “Good morning, $name. \n”; • ‘chop’ removes the last character irrespective of whether it is a newline or not. – Sometimes dangerous. Internet & Web Based Technology 45

Safe chopping: ‘chomp’ • The ‘chomp’ function works similar to ‘chop’, with the difference that it chops off the last character only if it is a newline. print “Enter your name: ”; chomp ($name = <STDIN>); # Read from keyboard and chomp newline print “Good morning, $name. \n”; Internet & Web Based Technology 46

File Operations • Opening a file – The ‘open’ command opens a file and returns a file handle. – For standard input, we have a predefined handle <STDIN>. $fname = “/home/isg/report.txt”; open XYZ , $fname; while (<XYZ>) { print “Line number $. : $_”; } Internet & Web Based Technology 47

– Checking the error code: $fname = “/home/isg/report.txt”; open XYZ, $fname or die “Error in open: $!”; while (<XYZ>) { print “Line number $. : $_”; } – $. returns the line number (starting at 1) – $_ returns the contents of last match – $i returns the error code/message Internet & Web Based Technology 48

• Reading from a file: – The last example also illustrates file reading. – The angle brackets (< >) are the line input operators. • The data read goes into $_ Internet & Web Based Technology 49

• Writing into a file: $out = “/home/isg/out.txt”; open XYZ , “>$out” or die “Error in write: $!”; for $i (1..20) { print XYZ “$i :: Hello, the time is”, scalar(localtime), “\n”; } Internet & Web Based Technology 50

• Appending to a file: $out = “/home/isg/out.txt”; open XYZ , “>>$out” or die “Error in write: $!”; for $i (1..20) { print XYZ “$i :: Hello, the time is”, scalar(localtime), “\n”; } Internet & Web Based Technology 51

• Closing a file: close XYZ; where XYZ is the file handle of the file being closed. Internet & Web Based Technology 52

• Printing a file: – This is very easy to do in Perl. $input = “/home/isg/report.txt”; open IN, $input or die “Error in open: $!”; while (<IN>) { print; } close IN; Internet & Web Based Technology 53

Command Line Arguments • Perl uses a special array called @ARGV. – List of arguments passed along with the script name on the command line. – Example: if you invoke Perl as: perl test.pl red blue green then @ARGV will be (red blue green). – Printing the command line arguments: foreach (@ARGV) { print “$_ \n”; } Internet & Web Based Technology 54

Standard File Handles • <STDIN> – Read from standard input (keyboard). • <STDOUT> – Print to standard output (screen). • <STDERR> – For outputting error messages. • <ARGV> – Reads the names of the files from the command line and opens them all. Internet & Web Based Technology 55

– @ARGV array contains the text after the program’s name in command line. • <ARGV> takes each file in turn. • If there is nothing specified on the command line, it reads from the standard input. – Since this is very commonly used, Perl provides an abbreviation for <ARGV>, namely, < > – An example is shown. Internet & Web Based Technology 56

$lineno = 1; while (< >) { print $lineno ++; print “$lineno: $_”; } – In this program, the name of the file has to be given on the command line. perl list_lines.pl file1.txt perl list_lines.pl a.txt b.txt c.txt Internet & Web Based Technology 57

Control Structures

Introduction • There are many control constructs in Perl. – Similar to those in C. – Would be illustrated through examples. – The available constructs: • for • foreach • if/elseif/else • while • do, etc. Internet & Web Based Technology 59

Concept of Block • A statement block is a sequence of statements enclosed in matching pair of { and }. if (year == 2000) { print “You have entered new millenium.\n”; } • Blocks may be nested within other blocks. Internet & Web Based Technology 60

Definition of TRUE in Perl • In Perl, only three things are considered as FALSE: – The value 0 – The empty string (“ ”) – undef • Everything else in Perl is TRUE. Internet & Web Based Technology 61

if .. else • General syntax: if (test expression) { # if TRUE, do this } else { # if FALSE, do this } Internet & Web Based Technology 62

Examples: • if ($name eq ‘isg’) { print “Welcome Indranil. \n”; } else { print “You are somebody else. \n”; } if ($flag == 1) { print “There has been an error. \n”; } # The else block is optional Internet & Web Based Technology 63

elseif Example: • print “Enter your id: ”; chomp ($name = <STDIN>); if ($name eq ‘isg’) { print “Welcome Indranil. \n”; } elseif ($name eq ‘bkd’) { print “Welcome Bimal. \n”; } elseif ($name eq ‘akm’) { print “Welcome Arun. \n”; } else { print “Sorry, I do not know you. \n”; } Internet & Web Based Technology 64

while Example: (Guessing the correct word) • $your_choice = ‘ ‘; $secret_word = ‘India’; while ($your_choice ne $secret_word) { print “Enter your guess: \n”; chomp ($your_choice = <STDIN>); } print “Congratulations! Mera Bharat Mahan.” Internet & Web Based Technology 65

for • Syntax same as in C. • Example: for ($i=1; $i<10; $i++) { print “Iteration number $i \n”; } Internet & Web Based Technology 66

foreach • Very commonly used function that iterates over a list. • Example: @colors = qw (red blue green); foreach $name (@colors) { print “Color is $name. \n”; } • We can use ‘for’ in place of ‘foreach’. Internet & Web Based Technology 67

• Example: Counting odd numbers in a list @xyz = qw (10 15 17 28 12 77 56); $count = 0; foreach $number (@xyz) { if (($number % 2) == 1) { print “$number is odd. \n”; $count ++; } print “Number of odd numbers is $count. \n”; } Internet & Web Based Technology 68

Breaking out of a loop • The statement ‘last’, if it appears in the body of a loop, will cause Perl to immediately exit the loop. – Used with a conditional. last if (i > 10); Internet & Web Based Technology 69

Skipping to end of loop • For this we use the statement ‘next’. – When executed, the remaining statements in the loop will be skipped, and the next iteration will begin. – Also used with a conditional. Internet & Web Based Technology 70

Relational Operators

The Operators Listed Comparison Numeric String Equal == eq Not equal != ne Greater than > gt Less than < lt Greater or equal >= ge Less or equal <= le Internet & Web Based Technology 72

Logical Connectives • If $a and $b are logical expressions, then the following conjunctions are supported by Perl: – $a and $b $a && $b – $a or $b $a || $b – not $a ! $a • Both the above alternatives are equivalent; first one is more readable. Internet & Web Based Technology 73

String Functions

The Split Function ‘split’ is used to split a string into multiple pieces using a • delimiter, and create a list out of it. $_=‘Red:Blue:Green:White:255'; @details = split /:/, $_; foreach (@details) { print “$_\n”; } – The first parameter to ‘split’ is a regular expression that specifies what to split on. – The second specifies what to split. Internet & Web Based Technology 75

• Another example: $_= “Indranil isg@iitkgp.ac.in 283496”; ($name, $email, $phone) = split / /, $_; • By default, ‘split’ breaks a string using space as delimiter. Internet & Web Based Technology 76

The Join Function • ‘join’ is used to concatenate several elements into a single string, with a specified delimiter in between. $new = join ' ', $x1, $x2, $x3, $x4, $x5, $x6; $sep = ‘::’; $new = join $sep, $x1, $x2, $w3, @abc, $x4, $x5; Internet & Web Based Technology 77

Regular Expressions

Introduction • One of the most useful features of Perl. • What is a regular expression (RegEx)? – Refers to a pattern that follows the rules of syntax. – Basically specifies a chunk of text. – Very powerful way to specify string patterns. Internet & Web Based Technology 79

An Example: without RegEx $found = 0; $_ = “Hello good morning everybody”; $search = “every”; foreach $word (split) { if ($word eq $search) { $found = 1; last; } } if ($found) { print “Found the word ‘every’ \n”; } Internet & Web Based Technology 80

Using RegEx $_ = “Hello good morning everybody”; if ($_ =~ /every/) { print “Found the word ‘every’ \n”; } • Very easy to use. • The text between the forward slashes defines the regular expression. • If we use “!~” instead of “=~”, it means that the pattern is not present in the string. Internet & Web Based Technology 81

• The previous example illustrates literal texts as regular expressions. – Simplest form of regular expression. • Point to remember: – When performing the matching, all the characters in the string are considered to be significant, including punctuation and white spaces. • For example, /every / will not match in the previous example. Internet & Web Based Technology 82

Another Simple Example $_ = “Welcome to IIT Kharagpur, students”; if (/IIT K/) { print “’IIT K’ is present in the string\n”; { if (/Kharagpur students/) { print “This will not match\n”; } Internet & Web Based Technology 83

Types of RegEx • Basically two types: – Matching • Checking if a string contains a substring. • The symbol ‘m’ is used (optional if forward slash used as delimiter). – Substitution • Replacing a substring by another substring. • The symbol ‘s’ is used. Internet & Web Based Technology 84

Matching

The =~ Operator • Tells Perl to apply the regular expression on the right to the value on the left. • The regular expression is contained within delimiters (forward slash by default). – If some other delimiter is used, then a preceding ‘m’ is essential. Internet & Web Based Technology 86

Examples $string = “Good day”; if ($string =~ m/day/) { print “Match successful \n"; } if ($string =~ /day/) { print “Match successful \n"; } • Both forms are equivalent. • The ‘m’ in the first form is optional. Internet & Web Based Technology 87

$string = “Good day”; if ($string =~ m@day@) { print “Match successful \n"; } if ($string =~ m[day[ ) { print “Match successful \n"; } • Both forms are equivalent. • The character following ‘m’ is the delimiter. Internet & Web Based Technology 88

Character Class • Use square brackets to specify “any value in the list of possible values”. my $string = “Some test string 1234"; if ($string =~ /[0123456789]/) { print "found a number \n"; } if ($string =~ /[aeiou]/) { print "Found a vowel \n"; } if ($string =~ /[0123456789ABCDEF]/) { print "Found a hex digit \n"; } Internet & Web Based Technology 89

Character Class Negation • Use ‘^’ at the beginning of the character class to specify “any single element that is not one of these values”. my $string = “Some test string 1234"; if ($string =~ /[^aeiou]/) { print "Found a consonant\n"; } Internet & Web Based Technology 90

Pattern Abbreviations • Useful in common cases Anything except newline (\n) . \d A digit, same as [0-9] \w A word character, [0-9a-zA-Z_] \s A space character (tab, space, etc) \D Not a digit, same as [^0-9] \W Not a word character \S Not a space character Internet & Web Based Technology 91

$string = “Good and bad days"; if ($string =~ /d..s/) { print "Found something like days\n"; } if ($string =~ /\w\w\w\w\s/) { print "Found a four-letter word!\n"; } Internet & Web Based Technology 92

Anchors • Three ways to define an anchor: ^ :: anchors to the beginning of string $ :: anchors to the end of the string \b :: anchors to a word boundary Internet & Web Based Technology 93

if ($string =~ /^\w/) :: does string start with a word character? if ($string =~ /\d$/) :: does string end with a digit? if ($string =~ /\bGood\b/) :: Does string contain the word “Good”? Internet & Web Based Technology 94

Multipliers • There are three multiplier characters. * :: Find zero or more occurrences + :: Find one or more occurrences ? :: Find zero or one occurrence • Some example usages: $string =~ /^\w+/; $string =~ /\d?/; $string =~ /\b\w+\s+/; $string =~ /\w+\s?$/; Internet & Web Based Technology 95

Substitution

Basic Usage • Uses the ‘s’ character. • Basic syntax is: $new =~ s/pattern_to_match/new_pattern/; What this does? • Looks for pattern_to_match in $new and, if found, replaces it with new_pattern. • It looks for the pattern once. That is, only the first occurrence is replaced. • There is a way to replace all occurrences (to be discussed shortly). Internet & Web Based Technology 97

Examples $xyz = “Rama and Lakshman went to the forest”; $xyz =~ s/Lakshman/Bharat/; $xyz =~ s/R\w+a/Bharat/; $xyz =~ s/[aeiou]/i/; $abc = “A year has 11 months \n”; $abc =~ s/\d+/12/; $abc =~ s /\n$/ /; Internet & Web Based Technology 98

Common Modifiers Two such modifiers are defined: • /i :: ignore case /g :: match/substitute all occurrences $string = “Ram and Shyam are very honest"; if ($string =~ /RAM/i) { print “Ram is present in the string”; } $string =~ s/m/j/g; # Ram -> Raj, Shyam -> Shyaj Internet & Web Based Technology 99

Use of Memory in RegEx • We can use parentheses to capture a piece of matched text for later use. – Perl memorizes the matched texts. – Multiple sets of parentheses can be used. • How to recall the captured text? – Use \1, \2, \3, etc. if still in RegEx. – Use $1, $2, $3 if after the RegEx. Internet & Web Based Technology 100