Lecture19-20

Programming in Perl

Randy Julian
Lilly Research Laboratories

The Crash Course…
Getting Perl (CPAN, PPM)
Getting/using the ptkdb debugger
print “hello worldn”
About variables: strings, numbers, arrays
Operators and expressions
About flow control
Introduction to regular expressions
Using Perl file I/O
Basic subroutines and modules

An assembly of material harvested from various sources

1

Getting Perl
Windows:
www.activestate.com
download “ActivePerl” (Either 5.6.x or 5.8.x)
Run windows installer
Linux:
Probably already installed (standard on most)
Latest version (5.6.x/5.8) via either RPM (from
ActiveState or RedHat) or from source:
cpan.perl.org

Getting the ptkdb debugger
Windows:
C:WINDOWS>perl -e “use Devel::ptkdb;”
If this gives an error, you need to install the debugger
PPM: Perl Package Manager
C:WINDOWS>PPM
PPM>install Devel::ptkdb
Linux:
$ perl -MCPAN -e ‘install “Devel::ptkdb” ’

From: “Perl for C Programmers”

2

A Perl Tutoral
Modified from: Nano Gough
http://www.computing.dcu.ie/~ngough/perl/tutorial.ppt

3

Perl Tutorial
Running Perl
Printing
Scalar Variables
Operations and Assignment
Arrays
Loops
Conditionals
File handling
Regular Expressions
Substitution
Split
Associative Arrays

Running Perl

#!/usr/local/bin/perl ( tells the file to run through perl)

Use .pl extension

perl programName (to run the program)

perl -d programName (to run using debugger)

perl – w programName (to run with warnings)

4

Printing

#The hash symbol (#) is use to comment lines of code
; Every statement in perl ends with a semi-colon (;)

print “Hello World. I love perl.”;
#prints: Hello World.I love perl.

print “Hello WorldnI love perln”;
#prints:
Hello World.
I love perl.

Scalar Variables
Strings/Numbers:
$name = “mary”;
$age = 27;
$income = 1_000_000; # underscores are ignored in numbers

Operations and Assignment
(* multiplication) ( division) (- subtraction)

$a = 1 + 2; # Add 1 and 2 and store in $a
$a = 5 % 2; # Remainder of 5 divided by 2
++$a; # Increment $a and then return it
$a++; # Return $a and then increment it
--$a; # Decrement $a and then return it
$a--; # Return $a and then decrement it

5

Operations and Assignment contd..

$a = 5; $b=7;
$a = $b; # Assign $b to $a ($a=7)
$a += $b; or $a=$a+b; # Add $b to $a ($a=12)
$a -= $b; or $a=$a-$b; # Subtract $b from $a ($a=-2)

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’;

Arrays
Initialize an array/set 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 array
push(@colors,”green”);
#colors now contains:
“red”,”blue”,”yellow”,”green”
#use pop function to remove an element from the end of array
pop(@colors);
#colors now contains
“red”, “blue”, “yellow”

6

#Functions shift and unshift
@colors=(“red”,”blue”,”yellow”);
$new_el=“green”;
#use unshift to append $new_el to start of array
unshift(@colors, $new_el);
@colors is now:
“green”,“red”,”blue”,”yellow”

#use shift to remove an element from the front of array
shift(@colors);
@colors is now:
“red”,”blue”,”yellow”

Accessing an element of the array

@colors = (“red”,”blue”,”yellow”);
print “$colors[0]” #prints: red

$#colors #index of last element of array
print “$colors[$#colors]; #prints: yellow

print @colors #prints: redblueyellow
print “@colors” #print: red blue yellow

$colors = "@colors"; #assigns colors to string
print $colors; #prints: red blue yellow

7

Loops
#Loops can be used to iterate through elements of an array
foreach Loop
foreach $el (@colors)
{
print “The color is : $eln”;
}

#The foreach loop iterates through the array element by #element. In
#the first iteration $el is assigned the value of the first element of
#colors (ie; red) etc..
#The result of executing this foreach statement is:
The color is : red
The color is : blue
The color is : yellow

Testing
Numbers
$a == $b # Is $a numerically equal to $b?
# don’t use $a=$b as this will not compare but just assign $b to $a
$a != $b # Is $a numerically unequal to $b?
$a<$b / $a>$b # Is $a less than/greater than $b
$a <=$b / $a >=$b # Is a less than or equal to/ g.t or eq to $b

Strings
$a eq $b # Is $a string-equal to $b?
$a ne $b # Is $a string-unequal to $b?

#You can also use logical and, or and not:
($a && $b) # Is $a and $b true?
($a || $b) # Is either $a or $b true? !($a)

8

Loops contd…
For Loop
for($i=0;$i<=$#colors;$i++)
{
print “The color is : $colors[$i]n”;
}

While Loop
$i=0;
while($i=<=$#colors)
{
print “$colors[$i]n”;
$i++;
}

Conditionals
#if $a is equal red print the color is red
If($a eq ‘red’) { print “the color is $an”;}
#in any other case (if $a not equal to red) print $a is not red
else { print “The color $a is not redn”;}

#if $a is equal to 1 , add 2 to $a
If($a ==1){ $a = $a+2;}
#elsif $a is equal to 2, add 3 to $a
elsif ($a ==2) {$a =$a+3;}
#in any other case add 1 to $a
else { $a++;}

#if $a is equal to 1 AND $b is equal to red: print Color 1 is red
If(($a==1)||($b eq ‘red’)){print “Color $a is $bn”;}

9

Some other string comparison operators:
eq - equality
ne - not equal
lt - less than
le - less than equal to
gt - greater than
ge - greater than equal to

File handling
Opening a file
$filename =“MyFile.txt”;
open(FILE,"/data/MyFile.txt") || die ("Cannot open file MyFile : $!n");

File: Filehandle for MyFile.txt
Die: If the file cannot be opened for reading the program will ‘die’ (ie quit
execution) and the reason for this will be returned in $!

The above file has been opened for reading : open(FILE,”filename”);
To open a file for writing: open(FILE,”>OutFile.txt”);
Outfile.txt will be overwritten each time the program is executed
To open a file for appending: open(FILE,”>>Append.txt”);
Close File: close(FILE);

10

File processing
#open input file for reading
open(IN,”InFile.txt”)|| die “Can’t open file….$!n”;
#open output file for writing
open(OUT,>”OutFile.txt”)|| die “Cant open file….$!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 $line meets conditional print to OutFile.txt
if($line eq “Number 7”)
{ print OUT “$linen”; } #endif
}#endWhile
close(IN); close(OUT); #close Files

Regular expressions

#A regular expression is contained in slashes, and matching occurs with
the =~ operator.
#The following expression is true if the string the appears in variable
$sentence.

$sentence =~ /the/
#The RE is case sensitive, so if $sentence = "The quick brown fox";
then the above match will be false.

$sentence !~/the/ (True) because the (lower case) is not in $sentence

#To eliminate case use i
$sentence =!/the/i; (True) because case has been eliminated with i

11

These Special characters can be used to match the following:
. # Any single character except a newline
^ # The beginning of the line or string
$ # The end of the line or string
* # Zero or more of the last character
+ # One or more of the last character
? # Zero or one of the last character

s+ (matches one or more spaces)
d+ (matches one or more digits)
t (matches a tab)
n (matches a new line)
b (matches a word boundary)

test.txt:
open(FILE,"test.txt");
#
while(<FILE>)
{ 100 This matches.
$line=$_;
chop($line); 200 This does not
if($line !~ /this/i) That does not.
{
print "$linen";
}
}

close(FILE)

#
Output:
That does not.

12

An Example using RE’s
TASK : We have a file containing lines in different formats. We want to
pick out the lines which start with a digit and end in a period.

while(<FILE>)
{
$line=$_;
chop($line);
if($line =~ /^d+(.*).$/)
{
print "$linen";
}
}
close(FILE)
^d+ (specifies that $line must begin with a digit)
(.*) This digit can be followed by any character any no. of times
. This is followed by a period (The slash is included to make the ‘.’ literal )
$. This specifies that the previous character (‘.’) must be the last on the line

test.txt:

while(<FILE>) #
{
100 This matches.
$line=$_;
chop($line); 200 This does not
if($line =~ /^d+(.*).$/)
That does not.
{
print "$linen";
}
}

close(FILE)

Output: 100 This matches.

13

RE’s contd
[a-z] (matches any lower case letter)
[a-zA-z] (matches any letter)

In the previous example a line was matched under the following
condition:
if($line =~/^(d+)(.*).$/)

The RE would match the line: 10 people went to the concert.
(d+) = 10; This is assigned to default variable $1
(.*) = “people went to the concert”; This is assigned to $2
Perl groups the elements specified by () together and assigns it a
default variable: $1,$2…etc.
print “$1n”; # prints : people went to the concert

Substitution

#substitution is a useful facility in perl which can be used to
replace one element with another

#replaces all instances of lafayette (lc) in $sentence to Lafayette (uc);
$sentence =~ s/lafayette/Lafayette/;

#replaces all instances of red in $sentence to blue
$sentence =~s/red/blue/;

Example
$sentence= “the red and white dress”;
$sentence =~s/red/blue/;
$sentence is now = “the blue and white dress”

14

Split

#split is a useful function : splits up a string and puts it on an
#array
$example = “Luke I am your father”;
@name=split(/s+/,$example);
@name = “Luke”, “I”, “am”, “your”, “father”

#using split you can also assign elements to variables
$name = “Luke:Skywalker”;
($first_name, $surname)=split(/:/,$name);
$first_name = “Luke”;
$surname=“Skywalker”;

Associative arrays / hashes
The elements of associative arrays have keys with associated values
Initialize
%Mygrades=();
Assign elements
$Mygrades{‘CHM696’}=90;
Printing
while(($key,$value)=each(%Mygrades))
{print “$key => $valuen”;}
ptkdb has trouble with each()
Prints:
CHM696 => 90
CHM621 => 70
CHM624 => 50

15

Plain Old Documentation (POD)
# The lines from here to the =cut are part of Sample run:
# Perl’s internal documentation system. If you
# want view the documentation use the perl test_ave.pl
# command: Read 64 items
Moving average
# pod2text <script>
4.59666666666667
# 4.60666666666667
=pod 4.64
4.67666666666667
=head1 NAME 4.71
...
test_ave.pl - Test the moving average program
=head1 AUTHOR
=head1 SYNOPSIS
Steve Oualline,
E<lt>oualline@www.oualline.comE<gt>.
perl test_ave.pl
=head1 COPYRIGHT
=head1 DESCRIPTION
Copyright 2002 Steve Oualline.
The I<test_ave.pl> reads a series of numbers This program is distributed under the GPL.
from I<num.txt>
and print a moving average spanning three data =cut
points.

=head1 EXAMPLES

Sub-routines
my @raw_data = ();
my $flag = read_data("num.txt");
if (not $flag) {
die("Could not read file");
}
sub read_data($)
{
my $file = shift; # The file to read
open DATA_FILE, "<$file" or return (undef);
@raw_data = <DATA_FILE>;
chomp(@raw_data);
close(DATA_FILE);
return (1); # Success
}

16

sub moving_average($)
{
my $increment = shift; # Increment for average
my $index; # Current item for average
my @result; # Averaged data

for ($index = 0;
$index <= $#raw_data - $increment;
$index++) {
my $total = sum(@raw_data[$index..$index + $increment -1]);
push (@result, $total / $increment);
}
return(@result);
} sub sum(@)
{
my @array = @_; # The array to sum
my $the_sum = 0; # Result so far
here a selection of
the @raw_data foreach my $element (@array) {
$the_sum += $element;
array is passed to }
sum() using the return ($the_sum);
}
.. operator

17

Lecture19-20

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