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Getting Started with Perl

Getting Started with Perl. Jeffrey ROACH 28 November. What is Perl? What is it good for?. 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:

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Getting Started with Perl

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  1. Getting Started with Perl Jeffrey ROACH 28 November

  2. What is Perl? What is it good for? • 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

  3. Popularity of Perl

  4. 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, multi-developer projects • Perl 6 has been coming next year for the last five years

  5. 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

  6. 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

  7. 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 (OR) vi 01_helloworld.pl (OR) emacs 01_helloworld.pl

  8. 01_helloworld.pl 01_helloworld.pl Notes Indicates which perl to use Allows ./01_helloworld.pl Use chmodu+x <file.pl> Strict and warnings pretty much standard Note distinction between “” and ‘’ Note the new line characer “\n” #!/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";

  9. 02_variables.pl 02_variables.pl Notes Usual start Standard practice Variables hold values Values may be numbers or strings Perl is pretty promiscuous about this #!/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"; …

  10. 03_arrays.pl 03_arrays.pl Notes Arrays are variables that hold more than one value Individual values are indexed by an integer Array is @a First element is $a[0] Arrays can store numbers and strings Index must always be integer … my @a = (1,2,3,4,5); print "@a\n"; print '$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"; …

  11. 04_hashes.pl 04_hashes.pl Notes Same as arrays, but with arbitrary indices Hash is %a Element at ‘f’ is $a{‘f’} Also called dictionaries, associative arrays, maps … my %a = ('one'=>1, 'two'=>2, 'three'=>3, 'four'=>4, 'five'=>5); print "%a\n"; print '$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"; …

  12. 05_subs.pl 05_subs.pl 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 sub greeting { my @param = @_; print "Hello, $param[0], how are you?\n\n"; return 0; } print "Round 1:\n"; foreach (@names) { greeting($_); }

  13. 06_scope.pl 06_scope.pl Notes 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 object-oriented programming my @names = ('Jeff', 'Jon', 'David', 'Sam'); sub scope1 { print "In Scope1:\n"; print "\tNames: @names\n"; my @new_names = ('Ffej', 'Noj', 'Divad', 'Mas'); print "\tNew Names: @new_names\n"; }

  14. 07_files.pl 07_files.pl 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 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");

  15. 08_echo.pl 08_echo.pl Notes ./08_echo.pl This is a test echo This is a test Uses @ARGV builtin array #!/usr/bin/perl use strict; use warnings; 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";

  16. 09_stats.pl 09_stats.pl Notes ./09_stats.pl 1 2 3 4 5 # denotes comments Uses @ARGV builtin array Single pass standard deviation my $argc = scalar(@ARGV); # You can … 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";

  17. 10_cat.pl 10_cat.pl Notes ./10_cat.pl File1.txt File2.txt cat File1.txt File2.txt Basic command line args Basic file I/O sub read_file { my @params = @_; my $filename = $params[0]; open(FILE,'<',$filename) or die $!; while (my $line = <FILE>) { print $line; } close(FILE); return 0; }

  18. 11_wc.pl 11_wc.pl Notes ./11_wc.pl File1.pl wc File1.pl Basic file I/O Accumulator String Split Character, word, line order reversed 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); $lines = $lines + 1; } close(FILE); return "$characters $words $lines"; }

  19. 12_cut.pl 12_cut.pl Notes ./12_cut.pl File1.csv cut –d , -f 1,4 File1.csv Split on comma Print selected columns sub cut_file { my @params = @_; my $filename = $params[0]; open(FILE,'<',$filename) or die $!; 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; }

  20. 13_grep.pl 13_grep.pl Notes ./13_grep.pl File1.log Chomp procedure If/then control structure Regular expression Log file analysis while (my $line = <FILE>) { chomp($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"; } }

  21. 14_head.pl 14_head.pl Notes ./14_head.pl File1.log head File1.log Use of boolean and in while Failed use of break Common use of comments 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; }

  22. 15_awk.pl 15_awk.pl Notes ./15_awk.pl File2.csv AWK is actually is own programming String eq not = 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];

  23. 15_awk.pl 15_awk.pl Notes If/Then/Else checking for key Returns cities hash my $city_total = $cities{$city}; if ($city_total) { $cities{$city} = $city_total + $hours; } else { $cities{$city} = $hours; } } } close(FILE); return %cities; }

  24. 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

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