The Joy of Perl

I gave this talk to the youngsters at work today. They LOL'd.

1. The Joy of Perl
2. Perl is • Simple • Powerful • Elegant • Everywhere
3. The basics Perl has three main "types". • $scalar • @array • #hash (Plus a couple more, like references and file handles.)
4. When you expect a scalar value, use $. When you expect a list value, use @. When you expect a hash value, use #.
6. @array = (1, 2, 3); # Indexing into an array returns a single element, so use $ $array[0]; # An array slice is a list, so use @ @array[0..1];
7. Introducing the magic variable ($_) Perl has lots of magic variables (see perldoc perlvar), but $_ is the Daddy. Think of $_ as "the thing most likely to be the thing I want to do stuff with right now".
9. Simple example: enumerating an array
10. With an explicit loop variable: foreach $elem (@array) { print $elem; } With an implicit loop variable foreach (@array) { # foreach uses $_ as default loop variable print $_; }
11. print uses $_ by default too! foreach (@array) { print; } (Told you $_ was the Daddy.)
13. Functions
14. Declared with sub: sub hello { print "Hello\n" } hello();
15. Args passed in @_ sub greet { ($time_of_day, $name) = @_; print "Good $time_of_day, $name"; } greet("morning", "Simon");
16. Idiomatic Perl: process @_ with shift (shift operates on @_ by default) sub greet { $time_of_day = shift; $name = shift or die "Insufficient args"; print "Good $time_of_day, $name"; } greet("morning", "Simon");
17. Let's look at file handling
18. Simple example, explicit variables $words = "/usr/share/dict/words"; open (FH, $words) or die "On opening $words for reading: $!"; while (defined($line = <FH>)) { chomp $line; # strip trailing newline print "$line\n" if length($line) > 15; } close FH;
19. Same code, implicit use of $_ $words = "/usr/share/dict/words"; open (FH, $words) or die "On opening $words for reading: $!"; while (<FH>) { chomp; print "$_\n" if length > 15; } close FH;
20. Writing to a file handle open (IN, "/usr/share/dict/words") or die; open (OUT, "> long-words.txt") or die; while (<IN>) { chomp; print OUT "$_\n" if length > 15; } close IN; close OUT or die "On closing file handle after writing: $!";
21. Beautiful!
22. Testing file attributes -X FILEHANDLE -X $path Where X is one of a few dozen different characters. For example: -e File exists -d File is a directory -z File has zero size (is empty) -t Filehandle is opened to a tty -M Get time since modification of file, in days See perldoc perlfunc for more
23. Example 1 if (-t STDIN && -t STDOUT) { # running in interactive mode } Example 2 if (-M '/path/to/backup' > 3) { # No backup in 3 days! Send an email or something. }
24. Let's look at regular expressions
25. Regular expressions in Perl 1. Match with /PATTERN/ 2. Substitute with s/PATTERN/REPLACEMENT/ 3. Invoke with =~
26. Matching $words = "/usr/share/dict/words"; open (FH, $words) or die "On opening $words for reading: $!"; while (defined($line = <FH>)) { print $line if $line =~ /aa/; } close FH;
27. Same, using $_ $words = "/usr/share/dict/words"; open (FH, $words) or die "On opening $words for reading: $!"; while (<FH>) { print if /aa/; } close FH;
28. Substitution $greeting = "Hello NAME"; $greeting =~ s/NAME/Simon/; print $greeting; # Hello Simon
29. Let's talk about one-liners
30. $ perl -e 'PROGRAM HERE'
31. $ perl -e 'print scalar gmtime'
32. $ wc -l /usr/share/dict/words | \ perl -e '$n = <STDIN>; print int($n/1000), "K words"'
33. Some handy perl CLI flags
34. -n: loop over STDIN $ cat /usr/share/dict/words | \ perl -ne 'chomp; print "$_\n" if length > 15'
35. NB: this works too, of course: $ perl -ne 'chomp; print "$_\n" if length > 15' /usr/share/dict/words But I'm using cat as a simple placeholder for "a thing that writes to STDOUT" !
36. -l: enable automatic line ending processing Implicitly chomps input, adds newline to output. $ cat /usr/share/dict/words | perl -nle 'print if length > 15' (-l[octnum] to use character other than \n)
37. -p: loop over STDIN and print each line $ cat /usr/share/dict/words | perl -pe '$_ = uc $_'
38. BEGIN and END blocks Let's re-implement wc -l: $ cat /usr/share/dict/words | perl -ne '$n++; END { print $n }'
39. A word about safety
40. Perl creates variables on the fly when you reference them. Can be handy in one-liners, but easy to mess up. $value = 1 + 2; print $Value; # prints 0
41. strict pragma to the rescue use strict; # <-- use the strict pragma my $value = 1 + 2; # <-- now vars have to be declared with `my` print $Value; On running this: Global symbol "$Value" requires explicit package name at test.pl line 4. Execution of test.pl aborted due to compilation errors.
42. warnings pragma also useful use strict; use warnings; my $num = '3:' + 4; print $num; # prints 7 On interpreting: Argument "3:" isn't numeric in addition (+) at test.pl line 6.
43. Where next? • perldoc perl • perldoc perlrun - command-line flags • perldoc perlre - regular expressions • perldoc perlvar - for more magic variables
44. Questions?