introduction to perl part i
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Introduction to Perl
Part I
By: Cédric Notredame
(Adapted from BT McInnes)
2
What is Perl?
Perl is a Portable Scripting Language No compiling is needed. Runs on Windows, UNIX, LINUX and cygwin
Fast and easy text processing capability Fast and easy file handling capability Written by Larry Wall “Perl is the language for getting your job done.”
Too Slow For Number Crunching Ideal for Prototyping
3
How to Access Perl
To install at home Perl Comes by Default on Linux, Cygwin, MacOSX www.perl.com Has rpm's for Linux www.activestate.com Has binaries for Windows
Latest Version is 5.8 To check if Perl is working and the version number
% perl -v
4
Resources For Perl
Books: Learning Perl
By Larry Wall Published by O'Reilly
Programming Perl By Larry Wall,Tom Christiansen and Jon Orwant Published by O'Reilly
Web Site http://safari.oreilly.com
Contains both Learning Perl and Programming Perl in ebook form
5
Web Sources for Perl
Web www.perl.com www.perldoc.com www.perl.org www.perlmonks.org
6
The Basic Hello World Program
which perl pico hello.pl Program:
#! /…path…/perl -wprint “Hello World!\n”;
Save this as “hello.pl” Give it executable permissions
chmod a+x hello.pl Run it as follows:
./hello.pl
7
“Hello World” Observations
“.pl” extension is optional but is commonly used The first line “#!/usr/local/bin/perl” tells UNIX where
to find Perl “-w” switches on warning : not required but a really
good idea
Variables and Their Content
9
Numerical Literals
Numerical Literals 6 Integer 12.6 Floating Point 1e10 Scientific Notation 6.4E-33 Scientific Notation 4_348_348 Underscores instead of
commas for long numbers
10
String Literals
String Literals “There is more than one way to do it!” 'Just don't create a file called -rf.' “Beauty?\nWhat's that?\n” “” “Real programmers can write assembly in any
language.”
Quotes from Larry Wall
11
Types of Variables
Types of variables: Scalar variables : $a, $b, $c Array variables : @array Hash variables : %hash File handles : STDIN, STDOUT, STDERR
Variables do not need to be declared Variable type (int, char, ...) is decided at run time
$a = 5; # now an integer
$a = “perl”; # now a string
12
Operators on Scalar Variables
Numeric and Logic Operators Typical : +, -, *, /, %, ++, --, +=, -=, *=, /=, ||, &&, ! ect
… Not typical: ** for exponentiation
String Operators Concatenation: “.” - similar to strcat
$first_name = “Larry”;
$last_name = “Wall”;
$full_name = $first_name . “ “ . $last_name;
13
Equality Operators for Strings
Equality/ Inequality : eq and ne
$language = “Perl”;
if ($language == “Perl”) ... # Wrong!
if ($language eq “Perl”) ... #Correct
Use eq / ne rather than == / != for strings
14
Relational Operators for Strings
Greater than Numeric : > String : gt
Greater than or equal to Numeric : >= String : ge
Less than Numeric : < String : lt
Less than or equal to Numeric : <= String : le
15
String Functions
Convert to upper case $name = uc($name);
Convert only the first char to upper case $name = ucfirst($name);
Convert to lower case $name = lc($name);
Convert only the first char to lower case $name = lcfirst($name);
16
A String Example Program Convert to upper case
$name = uc($name); Convert only the first char to upper case
$name = ucfirst($name);
Convert to lower case $name = lc($name);
Convert only the first char to lower case $name = lcfirst($name);#!/usr/bin/perl$var1 = “larry”;$var2 = “moe”;$var3 = “shemp”;……Output: Larry, MOE, sHEMP
17
A String Example Program
#!/usr/local/bin/perl
$var1 = “larry”;
$var2 = “moe”;
$var3 = “shemp”;
print ucfirst($var1); # Prints 'Larry'
print uc($var2); # Prints 'MOE'
print lcfirst(uc($var3)); # Prints 'sHEMP'
18
Variable Interpolation
Perl looks for variables inside strings and replaces them with their value
$stooge = “Larry”
print “$stooge is one of the three stooges.\n”;
Produces the output:Larry is one of the three stooges.
This does not happen when you use single quotesprint '$stooge is one of the three stooges.\n’;
Produces the output:$stooge is one of the three stooges.\n
19
Character Interpolation
List of character escapes that are recognized when using double quoted strings \n newline \t tab \r carriage return
Common Example :
print “Hello\n”; # prints Hello and then a return
20
Numbers and Strings are Interchangeable
If a scalar variable looks like a number and Perl needs a number, it will use it as a number
$a = 4; # a number
print $a + 18; # prints 22
$b = “50”; # looks like a string, but ...
print $b – 10; # will print 40!
Control Structures: Loops and Conditions
22
If ... else ... statements
if ( $weather eq “Rain” )
{
print “Umbrella!\n”;
}
elsif ( $weather eq “Sun” ) {
print “Sunglasses!\n”;
}
else {
print “Anti Radiation Armor!\n”;
}
23
Unless ... else Statements
Unless Statements are the opposite of if ... else statements.
unless ($weather eq “Rain”) {
print “Dress as you wish!\n”;
}
else {
print “Umbrella!\n”;
}
And again remember the braces are required!
24
While Loop
Example :$i = 0;
while ( $i <= 1000 ) {
print “$i\n”;
$i++;
}
25
Until Loop
The until function evaluates an expression repeatedly until a specific condition is met.
Example:
$i = 0;
until ($i == 1000) {
print “$i\n”;
$i++;
}
26
For Loops
Syntax 1: for ( $i = 0; $i <= 1000; $i=$i+2 )
{ print “$i\n”; }
Syntax 2: for $i(0..1000)
{ print “$i\n”; }
27
Moving around in a Loop
next: ignore the current iteration last: terminates the loop.
What is the output for the following code snippet:for ( $i = 0; $i < 10; $i++)
{
if ($i == 1 || $i == 3) { next; }
elsif($i == 5) { last; }
else
{print “$i\n”;}
}
Answer
0
2
4
29
Exercise
Use a loop structure and code a program that produces the following output:
AAAAAAAAABAAABAAAABAAAAABAAAAAABAAAB
…..
TIP: $chain = $chain . “A”;
30
Exercise
#! /usr/bin/perl
for ($i=0, $j=0; $i<100; $i++)
{
if ( $j==3){$chain.=“B”;$j=0;}
else {$chain.=“A”; $j++;}
print “$chain\n”;
}
31
Exercise: Generating a Random Sample
A study yields an outcome between 0 and 100 for every patient. You want to generate an artificial random study for 100 patients:
Patient 1 99Patient 2 65Patient 3 89….
Tip:- use the srand to seed the random number generator
-use rand 100 to generate values between 0 and 100 :
rand 100
32
Exercise
for ($i=0; $i<100; $i++)
{
$v=rand 100;
#print “Patient $i $v\n”;
printf “Patient %d %.2f\n\n”, $i, $v;#%s : chaines, strings#%d : integer
#%f : floating points
}
Collections Of Variables: Arrays
34
Arrays
Array variable is denoted by the @ symbol @array = ( “Larry”, “Curly”, “Moe” );
To access the whole array, use the whole array print @array; # prints : Larry Curly Moe
Notice that you do not need to loop through the whole array to print it – Perl does this for you
35
Arrays cont…
Array Indexes start at 0 !!!!!
To access one element of the array : use $ Why? Because every element in the array is scalar
print “$array[0]\n”; # prints : Larry
Question:
What happens if we access $array[3] ?
Answer1 : Value is set to 0 in Perl Answer2: Anything in C!!!!!
36
Arrays cont ...
To find the index of the last element in the arrayprint $#array; # prints 2 in the previous
# example
Note another way to find the number of elements in the array:$array_size = @array; $array_size now has 3 in the above example
because there are 3 elements in the array
37
Sorting Arrays
Perl has a built in sort function Two ways to sort:
Default : sorts in a standard string comparisons order sort LIST
Usersub: create your own subroutine that returns an integer less than, equal to or greater than 0
Sort USERSUB LIST The <=> and cmp operators make creating sorting
subroutines very easy
38
Numerical Sorting Example
#!/usr/local/bin/perl -w
@unsortedArray = (3, 10, 76, 23, 1, 54);
@sortedArray = sort numeric @unsortedArray;
print “@unsortedArray\n”; # prints 3 10 76 23 1 54
print “@sortedArray\n”; # prints 1 3 10 23 54 76
sub numeric
{
return $a <=> $b;
}
# Numbers: $a <=> $b : -1 if $a<$b , 0 if $a== $b, 1 if $a>$b
# Strings: $a cpm $b : -1 if $a<$b , 0 if $a== $b, 1 if $a>$b
39
#!/usr/local/bin/perl -w
@unsortedArray = (“Larry”, “Curly”, “moe”);
@sortedArray = sort { lc($a) cmp lc($b)} @unsortedArray;
print “@unsortedArray\n”; # prints Larry Curly moe
print “@sortedArray\n”; # prints Curly Larry moe
String Sorting Example
40
Foreach
Foreach allows you to iterate over an array Example:
foreach $element (@array) { print “$element\n”;}
This is similar to : for ($i = 0; $i <= $#array; $i++) { print “$array[$i]\n”;}
41
Sorting with Foreach
The sort function sorts the array and returns the list in sorted order.
Example :
@array( “Larry”, “Curly”, “Moe”);
foreach $element (sort @array)
{
print “$element ”;
}
Prints the elements in sorted order:
Curly Larry Moe
42
Exercise: Sorting According to Multiple Criterion
Use the following initialization to sort individuals by age and then by income:
Syntax
@sortedArray = sort numeric @unsortedArray;sub numeric
{ return $a <=> $b;
}Data
@index=(0,1,2,3,4);@name=(“V”,“W”,”X”,”Y”,”Z”);@age=(10,20, 15, 20, 10);@income=(100,670, 280,800,400);
Output: Name X Age A Income I
…
Tip:-Sort the index, using information contained in the other arrays.
43
Exercise: Sorting According to Multiple Criterion
@index=(0,1,2,3,4,5);@name=(“V”,“W”,”X”,”Y”,”Z”);@age=(10,20, 15, 20, 10);@income=(100,670, 280,800,400);
foreach $i ( sort my_numeric @index){
print “$name[$i] $age[$i] $income[$i];}
sub my_numeric {
if ($age[$a] == $age[$b]){return $income[$a]<=>$income[$b]; }
else {return $age[$a]<=>$age[$b]; }
}
Manipulating Arrays
45
Strings to Arrays : split
Split a string into words and put into an array@array = split( /;/, “Larry;Curly;Moe” );
@array= (“Larry”, “Curly”, “Moe”);
# creates the same array as we saw previously
Split into characters@stooge = split( //, “curly” );
# array @stooge has 5 elements: c, u, r, l, y
46
Split cont..
Split on any character@array = split( /:/, “10:20:30:40”);
# array has 4 elements : 10, 20, 30, 40
Split on Multiple White Space@array = split(/\s+/, “this is a test”;
# array has 4 elements : this, is, a, test
More on ‘\s+’ later
47
Arrays to Strings
Array to space separated string@array = (“Larry”, “Curly”, “Moe”);
$string = join( “;“, @array);
# string = “Larry;Curly;Moe”
Array of characters to string@stooge = (“c”, “u”, “r”, “l”, “y”);
$string = join( “”, @stooge );
# string = “curly”
48
Joining Arrays cont…
Join with any character you want@array = ( “10”, “20”, “30”, “40” );
$string = join( “:”, @array);
# string = “10:20:30:40”
Join with multiple characters@array = “10”, “20”, “30”, “40”);
$string = join(“->”, @array);
# string = “10->20->30->40”
49
Arrays as Stacks and Lists
To append to the end of an array :@array = ( “Larry”, “Curly”, “Moe” );
push (@array, “Shemp” );
print $array[3]; # prints “Shemp”
To remove the last element of the array (LIFO)$elment = pop @array;
print $element; # prints “Shemp” @array now has the original elements
(“Larry”, “Curly”, “Moe”)
50
Arrays as Stacks and Lists
To prepend to the beginning of an array@array = ( “Larry”, “Curly”, “Moe” );unshift @array, “Shemp”;print $array[3]; # prints “Moe”print “$array[0]; # prints “Shemp”
To remove the first element of the array $element = shift @array;print $element; # prints “Shemp” The array now contains only :
“Larry”, “Curly”, “Moe”
51
Exercise: Spliting Instructions
Remove shift: beginning, pop: end
Add Unshift: beginning, push: end
Use split, shift and push to turn the following string:
“The enquiry 1 was administered to five couples”“The enquiry 2 was administered to six couples”“The enquiry 3 was administered to eigh couples”
Into“five couples were administered the enquiry 1”
….
52
Exercise: Spliting Use split, shift and push to turn the following string:
$s[0]= “The enquiry 1 was administered to five couples”;$s[1]= “The enquiry 2 was administered to six couples”;$s[2]= “The enquiry 3 was administered to eigh couples”;foreach $s(@s)
{@s2=split (/was administered to/, $s);$new_s=“$s2[1] were admimistered $s2[0]”;print “$new_s\n”;
}
Multidimentional Arrays
54
Multi Dimensional Arrays
Better use Hash tables (cf later) If you need to:
@tab=([‘Monday’,’Tuesday’],
[‘Morning’,’Afternoon’,’Evening’]);
$a=$tab[0][0] # $a == ‘Monday’
$tab2=(‘midnight’, ‘Twelve’);
$tab[2]=\@tab2 # integrate tab2 as the last row of tab
Thank you
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