Transcript Getting Started with Perl - Information Technology Services

Getting Started with Perl
Jeffrey ROACH
28 November
What is Perl? What is it good for?
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Perl is a scripting language
Perl is a prototyping language
Perl is designed for relatively short scripts
Perl programs are best written by a single
programmer
• Perl is ideal for:
– Test processing: System Administration, Back-end
web administration, Bioinformatics
Popularity of Perl
The decline of Perl
• Replaced by PHP and Python
• Syntax is very different from other languages
– Programming constructs programmers expect are
non-supported
– Subroutines are available, but weak
– Object-oriented techniques are available, but weak
and slow
• Perl is not suitable for large scale, multideveloper projects
• Perl 6 has been coming next year for the last five
years
Nevertheless
• Perl remains useful as a scripting language
• Perl is installed in all Linux/Unix/Mac OS X
machines
• Perl is easily installed on Windows (Active State)
• Perl is free and a good place to start learning
good practice
• Perl allows non-programmers to write small
programs that can do worthwhile things quickly
Learning Perl
• Perl is a big, messy language
• Two hours is not sufficient to even crack the
surface
• What we will do:
– Learn seven (7) basic concepts
– Use this foundation to build some small, useful (at
least a little) tools
– Decide whether you want to learn more on your
own: Learning Perl The Hard Way
Getting Started on Kure or Killdevil
• Step 1: Get the course materials
cd /netscr/<your_onyen>
cp –r /netscr/roachjm/Perl .
ls
• Step 2: Choose an editor
nano 01_helloworld.pl
vi 01_helloworld.pl
emacs 01_helloworld.pl
(OR)
(OR)
01_helloworld.pl
01_helloworld.pl
#!/usr/bin/perl
use strict;
use warnings;
print "Hello, World!\n";
print 'Hello, World';
print "<--- No New Line\n";
print 'Hello, World!\n';
print "\n";
Notes
• Indicates which perl to use
– Allows ./01_helloworld.pl
– Use chmod u+x <file.pl>
• Strict and warnings pretty
much standard
• Note distinction between “”
and ‘’
• Note the new line characer
“\n”
02_variables.pl
02_variables.pl
#!/usr/bin/perl
use strict;
use warnings;
my $a = 1;
print "a: $a\n";
my $b = 2;
print "b: $b\n";
$b = $a + $b;
print "Added a to b.\n";
print "b: $b\n";
…
my $first_name = "Jeff";
my $last_name = "ROACH";
print "Full Name: $first_name $last_name\n";
…
Notes
• Usual start
• Standard practice
• Variables hold values
• Values may be numbers or
strings
• Perl is pretty promiscuous
about this
03_arrays.pl
03_arrays.pl
…
my @a = (1,2,3,4,5);
print "@a\n";
print '$a[0] $a[1] $a[2] $a[3] $a[4]:';
print "\t$a[0] $a[1] $a[2] $a[3] $a[4]\n";
print '$a[-1] $a[-2] $a[-3] $a[-4] $a[-5]:';
print "\t$a[-1] $a[-2] $a[-3] $a[-4] $a[-5]\n";
my $len_a = scalar(@a);
for (my $i=0; $i<$len_a; $i++) {
print "$a[$i] ";
}
print "\n";
…
Notes
• Arrays are variables that
hold more than one value
• Individual values are
indexed by an integer
• Array is @a
• First element is $a[0]
• Arrays can store numbers
and strings
• Index must always be
integer
04_hashes.pl
04_hashes.pl
…
my %a = ('one'=>1, 'two'=>2, 'three'=>3,
'four'=>4, 'five'=>5);
print "%a\n";
print '$a{\'one\'} $a{\'two\'} $a{\'three\'}
$a{\'four\'} $a{\'five\'}:';
print "\t$a{'one'} $a{'two'} $a{'three'} $a{'four'}
$a{'five'}\n\n";
foreach (keys %a) {
print "$_ => $a{$_} ";
}
print "\n\n";
…
Notes
• Same as arrays, but with
arbitrary indices
• Hash is %a
• Element at ‘f’ is $a{‘f’}
• Also called dictionaries,
associative arrays, maps
05_subs.pl
05_subs.pl
sub greeting {
my @param = @_;
print "Hello, $param[0], how are
you?\n\n";
return 0;
}
print "Round 1:\n";
foreach (@names) {
greeting($_);
}
Notes
• Subroutines were the basis
for Structured Programming
circa 1970 – 1975
• Natural way to break larger
programs into smaller
blocks
• Improves readability, code
re-use, and code quality
• Perl support is somewhat
underwhelming
06_scope.pl
06_scope.pl
Notes
my @names = ('Jeff', 'Jon',
'David', 'Sam');
• Scope is the value that
subroutines add
• Scope restricts the value that
variables take to a particular
subroutine
• Essentially a context
• Prevents name conflicts and
allows larger programs to be
composed of smaller parts
• Hierarchical
• Concept expanded in objectoriented programming
sub scope1 {
print "In Scope1:\n";
print "\tNames: @names\n";
my @new_names = ('Ffej',
'Noj', 'Divad', 'Mas');
print "\tNew Names:
@new_names\n";
}
07_files.pl
07_files.pl
sub read_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
…
}
…
print "\nRead File\n";
read_file("File1.txt");
print "\nWrite File\n";
copy_file("File1.txt","File2.txt");
Notes
./07_files.pl File1.txt File2.txt
diff File1.txt File2.txt
cp File1.txt File2CP.txt
diff file2.txt File2CP.txt
• Read and write from files
• Fundamental importance
08_echo.pl
08_echo.pl
#!/usr/bin/perl
use strict;
use warnings;
Notes
./08_echo.pl This is a test
echo This is a test
my $argc = scalar(@ARGV);
print "@ARGV\n\n";
for (my $i=0; $i<$argc; $i++) {
print "$ARGV[$i] ";
}
print "\n\n";
foreach (@ARGV) {
print "$_ ";
}
print "\n";
• Uses @ARGV builtin array
09_stats.pl
09_stats.pl
my $argc = scalar(@ARGV); # You can …
Notes
./09_stats.pl 1 2 3 4 5
print "@ARGV\n";
my $sum = 0.0;
my $sumsq = 0.0;
for (my $i=0; $i<$argc; $i++) {
$sum = $sum + $ARGV[$i];
$sumsq = $sumsq + $ARGV[$i]*$ARGV[$i];
}
my $mean = $sum / $argc;
my $stddev = sqrt(($sumsq - $sum*$sum/$argc) / ($argc
- 1));
print "n: $argc\n";
print "mean: $mean\n";
print "stddev: $stddev\n";
• # denotes comments
• Uses @ARGV builtin array
• Single pass standard
deviation
10_cat.pl
10_cat.pl
sub read_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
while (my $line = <FILE>) {
print $line;
}
close(FILE);
return 0;
}
Notes
./10_cat.pl File1.txt File2.txt
cat File1.txt File2.txt
• Basic command line args
• Basic file I/O
11_wc.pl
11_wc.pl
sub count_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
my $characters = 0;
my $words = 0;
my $lines = 0;
while (my $line = <FILE>) {
$characters = $characters + length($line);
my @word_array = split(' ',$line);
$words = $words + scalar(@word_array);
Notes
./11_wc.pl File1.pl
wc File1.pl
• Basic file I/O
• Accumulator
• String Split
$lines = $lines + 1;
}
close(FILE);
return "$characters $words $lines";
}
• Character, word, line order
reversed
12_cut.pl
12_cut.pl
sub cut_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
Notes
./12_cut.pl File1.csv
cut –d , -f 1,4 File1.csv
while (my $line = <FILE>) {
chomp($line);
my @line_array = split(',',$line);
my $column1 = $line_array[0];
my $column4 = $line_array[3];
print "$column1,$column4\n";
}
close(FILE);
return 0;
}
• Split on comma
• Print selected columns
13_grep.pl
13_grep.pl
while (my $line = <FILE>) {
Notes
./13_grep.pl File1.log
chomp($line);
if ($line =~ m/System Sleep/) {
$times = $times + 1;
my @line_array = split(' ',$line);
my $date1 = $line_array[0];
my $date2 = $line_array[1];
my $time = $line_array[2];
print "System Sleep at: $date1 $date2 $time\n";
}
}
• Chomp procedure
• If/then control structure
• Regular expression
• Log file analysis
14_head.pl
14_head.pl
sub read_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
my $lines_written = 0;
while ((my $line = <FILE>) and ($lines_written < 10)) {
print $line;
$lines_written = $lines_written + 1;
#Not allowed with strict
#if ($lines_written == 10) {
# break;
#}
}
close(FILE);
return 0;
}
Notes
./14_head.pl File1.log
head File1.log
• Use of boolean and in while
• Failed use of break
• Common use of comments
15_awk.pl
15_awk.pl
sub hist_file {
my @params = @_;
my $filename = $params[0];
open(FILE,'<',$filename) or die $!;
my %cities = ();
while (my $line = <FILE>) {
chomp($line);
my @line_array = split(',',$line);
if ($line_array[4] eq '"Active"') {
my $city = $line_array[0];
my $hours = $line_array[5];
Notes
• ./15_awk.pl File2.csv
• AWK is actually is own
programming
• String eq not =
15_awk.pl
15_awk.pl
my $city_total = $cities{$city};
if ($city_total) {
$cities{$city} = $city_total +
$hours;
} else {
$cities{$city} = $hours;
}
}
}
close(FILE);
return %cities;
}
Notes
• If/Then/Else checking for
key
• Returns cities hash
Conclusions
• Perl has tons of possibilities
• Expressive: You can write things a million
different ways
• Useful: You can make useful little tools relatively
easily
• Organic development and prototyping
• For further self-study:
– Learning Perl The Hard Way
• There is also Python, Ruby, PHP, and Lua