1Perl●Practical Extraction and Report Language●Similar to Python in most senses●Intended to be practical rather than elegant●“There is more than one way”●But easier to write, than to read●Poorly written programs can be almost impossible to decipher●Widely used, so it helps to be able to understand it●Useful for writing short programs to quickly process text files2Running programs●The symbol # is used to comment lines of code●Every statement in perl ends with a semi-colon;●Hello world: print "Hello, world\n";●It is highly reccomended to use warnings and strict syntax:perl -w program.pluse strict●Very hard to discover bugs otherwise and even possible to miss them3Scalars●Can be strings, integers or floating point numbers:my $text = "aeou";my $number = 1;print $text;print "The number is $number\n";●Assignments and operations similar to C:$a = 1 + 2; # Add 1 and 2 and store in $a$a++; # Return $a and then increment it$b = $a; $a += $b; or $a = $a+$b; # Add $b to $a4●Concatenation$a = ‘Monday’; $b=‘Tuesday’;$c=$a.” “.$b;$c= ‘Monday Tuesday’; # Single quote: “don’t do anything to string”$d= “$a and $b”; # Double quote: “interpret any variables in string”print $d; # prints ‘Monday and Tuesday’;●There is a number of special scalars with strange names, that are used for all kinds of purposes.●The most important is the default variable: $_ ●It is the default argument to a number of functions and it's set implicitly by certain looping constructs.●print; # prints the contents of $_5Arrays●List of values:my @animals = ("camel", "llama", "owl");my @numbers = (23, 42, 69);my @mixed = ("aeou", 42, 1.23);●Arrays are zero-based. Single elements are scalars, so $ is used:print $numbers[0]; # prints 23●The special variable $#array gives the index of the last element of an array●Length of an array: (not $#array+1)●@array in scalar context (where Perl expects a scalar value) gives the number of elements in the array:if (@numbers < 5) { ... }6●Initialize an array to null:@colors=();●Functions push and pop#assign elements to array @colors@colors=(“red”,”blue”,”yellow”);#use push function to add an element to the end of arraypush(@colors,”green”);#colors now contains:“red”,”blue”,”yellow”,”green”#use pop function to remove an element from the end of arraypop(@colors);#colors now contains“red”, “blue”, “yellow”7More about arrays●To get multiple values from an array: @animals[0,1]; # gives ("camel", "llama"); @animals[0..2]; # gives ("camel", "llama", "owl"); @animals[1..$#animals]; # gives all except the first element●You can do various things: my @sorted = sort @animals; my @backwards = reverse @numbers;●There are also special arrays, such as @ARGV (the command line arguments) and @_ (the arguments passed to a subroutine).8Hashes●A different name for dictionaries. ●Initialize: %Mygrades=();●Assign elements:$Mygrades{‘CHM696’}=90;$Mygrades{‘CHM621’}=70;●Printing:while(($key,$value)=each(%Mygrades)){print “$key => $value\n”;}●keys() and values() return lists of keys and valuesmy @key_list = keys %hash;my @value_list = values %hash;9●More complex data types can be constructed using references●A reference is a scalar value and can refer to any other Perl data type. ●By storing a reference as the value of an array or hash element, we can create lists and hashes within lists and hashes. References10Conditionals●Testing strings ($a eq $b, $a ne $b):if($a eq ‘red’) { print “the color is $a\n”;}else { print “The color $a is not red\n”;}●Testing numbers ($a == $b, $a != $b):if ($a ==1){$a = $a+2;}elsif ($a ==2) {$a =$a+3;}else { $a++;}●Logicals (&&, ||, !):if(($a==1)||($b eq ‘red’)){print “Color $a is $b\n”;}11Loops●foreach: iterates through the array element by elementforeach $el (@colors) { print “The color is : $el\n”;}for($i=0;$i<=$#colors;$i++) { print “The color is : $colors[$i]\n”;}$i=0;while($i=<=$#colors) { print “$colors[$i]\n”; $i++;}12Files and I/O●Opening a file for reading:$filename =“MyFile.txt”;open(FILE,"/data/MyFile.txt") or die ("Cannot open file MyFile : $!\n");●If the file cannot be opened for reading the program will ‘die’ and the reason will be returned in the special variable $!●To open a file for writing (overwriting):open(FILE,”>OutFile.txt”);●To open for appending: open(FILE,”>>Append.txt”);●Close File: close(FILE);13●You can read from an open filehandle using the <> operator. ●In scalar context it reads a single line and in list context it reads the whole file in, assigning each line to an element of the list:my $line = <INFILE>;my @lines = <INFILE>;●Perl defaults allow for very short programs:while(<>){ print;}●will print whatever it gets from STDIN14#open input file for readingopen(IN,”InFile.txt”) or die “Can’t open file….$!\n”;#open output file for writingopen(OUT,>”OutFile.txt”) or die “Cant open file….$!\n”;while(<IN>){ #while there are still lines in InFile.txt $line=$_; #read in the lines one at a time chop($line); #remove end of line character if($line eq “Number 7”){ print OUT “$line\n”; }}close(IN); close(OUT); #close Files15Regular expressions●A regular expression is contained in slashes, and matching occurs with the =~ operator.●This is true if the string “the” appears in $sentence$sentence =~ /the/●REs are case sensitive, so if $sentence = "The quick brown fox"; then the above match will be false.$sentence !~ /the/ # is true because the (lower case) is not in $sentence●To eliminate case use i:$sentence =~ /the/i; # is true●Special characters are similar to Python. ^ $ * + ? \s \d \n \b16●Besides grouping, parentheses can be used to capture the results of parts of the RE match. The results go to $1, $2, etc.●Example: breaking an email address into parts:if ($email =~ /([^@]+)@(.+)/) { print "Username is $1\n"; print "Hostname is $2\n";}17●Substitution using REs:$sentence =~s/red/blue/; #replaces all instances of red in $sentence to blue●split:$example = “This sentence is split”;@name = split(/\s+/,$example);@name = “This”, “sentence”, “is”, “split”●split can also assign elements to variables:$name = “Bill:Kandylas”;($first_name,