Understanding Perl: A Flexible and Powerful Scripting Language

1. CSC 4630 Meeting 15 March 14, 2007 Happy Pi Day

2. Perl • Practical Extraction and Support Language • A glue language under UNIX • Written by Larry Wall • Claimed to be the most portable of scripting languages • See www.perl.com

3. Perl by Example • Perl is more powerful than awk, but looks somewhat the same. • Perl is more flexible than shell scripts, but uses some of the same constructs. • Perl is based on C, and in fact the interpreter is written in C, but is easier to use than C.

4. Perl by Example (2) Beginning assumptions and notations • Statements are terminated by ; (semicolon) • Comments start anywhere on a line with # and end at the end of the line. Thus, you can write explanations on the line for the line. But they can’t be two line explanations. • Built-in functions neither require nor forbid ( ) around their arguments

5. Perl by Example (3) • First line of Perl program should be a directive to the interpreter: #!/usr/local/bin/perl –w • Scalar variables have names that start with $ and have values that are either strings or double precision real numbers.

6. Exercise 1 #!/usr/local/bin/perl –w print (“Hello, world!\n”);

7. Exercise 2 #!/usr/local/bin/perl –w print “What is your name? “; $name = <STDIN>; chomp ($name); print “Hello, $name!\n”;

8. Exercise 3 #!/usr/local/bin/perl –w print “What is your name? “; $name = <STDIN>; chomp ($name); if ($name eq “Randal”) { print “Hello, $name! Glad you’re here\n” } else { print “Hello, $name!\n”; # ordinary greeting }

9. Exercise 4 #!/usr/local/bin/perl –w $secretword = “llama”; # the secret word print “What is your name?”; chomp($name = <STDIN>); if ($name eq “Randal”) { print “Hello, $name! Glad you’re here\n” } else { print “Hello, $name!\n”; print “What is the secret word?”; chomp($guess = <STDIN>); while ($guess ne $secretword) { print “Wrong, try again. Secret word?”; chomp($guess = <STDIN>); } }

10. Non-scalar Variables For example, arrays: • Identifier starts with an @ • Can be initialized with a list of strings: @words = (“camel”, “llama”, “alpaca”); • Or with the qw operator (which saves keystrokes) @words = qw (camel llama alpaca); • Elements accessed as scalars, so $words[0] is camel, $words[1] is llama NB: Index origin is 0 • $i = 2 ; $words[$i] has value alpaca

11. Exercise 5 #!/usr/local/bin/perl –w @swords = qw (camel llama alpaca); #set multiple secret words print “What is your name?”; chomp($name = <STDIN>); if ($name eq “Randal”) { print “Hello, $name! Glad you’re here\n” } else { print “Hello, $name!\n”; print “What is the secret word?”; chomp($guess = <STDIN>); $i = 0; #index in swords array $correct = “maybe”; #status of search while ($correct eq “maybe”) { if($guess eq $swords[$i]) { $correct = “yes”} # set flag on success elsif ($i < 2) {++$i} # not success, try next secret word else { print “Wrong, try again. Secret word?”; # no word matches, ask again chomp($guess = <STDIN>); $i=0; # reset index in secret word array } # end if sequence } # end while not correct } # end if not Randal

12. Scalar operators • Numeric: + - * / ** % • Numeric comparison: < <= == >= > != • String: . (for concat) x (for multiple concat) • String comparison: lt le eq ge gt ne • Identifier: $ <letter> <letter | digit | under > … • Assignment: = (carries value of expression) binary assignment: += *= .= etc. increment: prefix, postfix ++$i $i++ increment then assign vs. assign then increment