PERL

1. PERL

2. Perl Power! • Who uses Perl? • Sites with heavy traffic that need excellent text processing capabilities   • Compiler optimization is also done in Perl  • System Administrators(the impatient ones, to be exact.) • The Swiss Army Knife Language •  It can perform a lot of things,from simple scripting to number-crunching • "There are many ways to do it" --> the Perl Philosophy • Similarities with other programming language • There is a striking similarity with C---after all, this is written in C! • Shell scripting in UNIX heavily influenced Perl 

3. The Fundamentals • Scalar Data Types Revisited • Lists and Arrays • Control Structures and Code Blocks • Subroutines • Regular Expressions (a.k.a. Pattern Matching)

4. Scalar Data Types • Scalar as String or Scalar as Integer? Both! • Could be: • $name = "Gizmo" ; • $age =3 ;  • $height = 3.141591 • $name = $age = 'This is a new string, overwriting both variables $name and $age!' • So what's the big deal with the '$'? • It is required for every scalar variable

5. Basic Operations • arithmetic • the usual +,-,*, /, % • e.g  • $ave = $grade/$units • concatenation • if in Python, we use +, here we use '.' (a period) • e.g: $string = $firstname.$lastname;

6. Basic Operations • the 'x' operator • if (again, in Python), we use '*' to duplicate strings, here we use 'x'. • e.g: $manylaughs = "he"x1000;

7. The substring method • Syntax; • $somesub = substring($original, startingpoint,lengthofsubstring); • e.g: $firstfour = substring($somestring, 0,4); • this is how you access the individual characters of a string • the basic data type is a scalar---string or number only.  • character is not basic data type, so it has to be extracted first from a string.

8. Putting Comments • Like in Python, the comments are      # enter comments here (See the legacy of Perl over Python? :P) 

9. Try this one!     (To print, use print method, just like in Python) #!/usr/bin/perl -w $name = "Slim Shady"; $num = 525600; $message = $name." knows that there are ".$num." minutes in a year.\nErgo, he knows that there are ".$num/60."hours in a year." ; print $message;

10. Formatted Output There is also a printf method in Perl syntax:     printf "%format", some variable;

11. Basic Input • Consider the snippet below first: • $input = <STDIN>; • chomp($input); • STDIN is a built-in file handle.  • if you type into a console, the data you've entered is directed into STDIN • $input gets the value of STDIN

12. Basic Input • chomp is just optional • it only trims the trailing newlines of the input • What if I use chop() instead? • yes it can also trim the newline ---but everytime we  call chop() removes ANY character at the end. Be careful.  

13. Quotes Matter! • What is the difference between • $name = "I shall return." ; • print "$name"; • print '$name';

14. Quotes Matter! • using single quotes print things LITERALLY. • Ergo,  doing a print '$name' will really print '$name'. • and yes, you've guessed it, using double quotes ("") enable variable interpolation (err, the opposite of literal) • print "$name" really prints "I shall return."

15. Control Structures and Blocks • The Control Structures in Perl tend to be like those in C. • Conditional Statements: •  if • if-else • if-elsif-else • unless  

16. Conditional Statement: if • if - same as in C. • e.g. • $gas_money=5;if ($gas_money < 10){  print "You will not get much gas for less than 10 bucks!";}print "Gas is expensive.";

17. Conditional Statement:if-else • if the "if" condition is not true. If you have just one other option, simply use an else statement after the if statement, like this: • if ($gas_money < 10){  print "You will not get much gas for less than 10 bucks!";}else{ print "You have enough money for now.";}print "Gas is expensive.";

18. Conditional Statement: if-elsif-else • If you want to check for a couple of conditions, you can use elsif to do another check and leave the else code as a last resort. • if (($gas_money < 10) && ($gas_money > 0)){  print "You will not get much gas for less than 10 bucks!";} elsif ($gas_money <= 0){ print "You need to get some money!";} else{ print "You have enough money for now.";}print "Gas is expensive.";

19. Conditional Statement: unless •  Sort of like the opposite of the if statement: • unless ($gas_money == 10){ print "You need exact change. 10 bucks please.";}

20. There are many ways to Loop it! • for - same as in C • syntax: • for (starting assignment; test condition; increment){ code to repeat}

21. There are many ways to Loop it! •  using for in Perl  • e.g. for ($i = 0; $i < 10; ++$i){print "$i\n";}

22. There are many ways to Loop it! • foreach - to go through each line of an array or any list-like structure(such as each line of a file) • e.g. • @food = ("sinigang","adobo","beef steak") • foreach $pagkain (@food) { •       print " ang sarap naman ng "."$pagkain!\n"; • }

23. There are many ways to Loop it! • while -The while loop repeats a portion of code, but it repeats it until the condition you provide comes back false • syntax: • while(some condition) • {code to repeat} • do-while: • do{code to repeat} • while(some condition)

24. There are many ways to Loop it! • using while while($ctr < ){ print “$ctr\n”; $ctr++; }# end while

25. Subroutines • functions in Perl are called subroutines. • Always pass by value • to call a subroutine we do this: • &sub1; • Note that there are no "()" after. We can deal with that later.

26. Subroutines • To define a subroutine:          sub sub1{              /*Then enter logic here*/         } • But where are the parameters?

27. Subroutines • Parameters are passed using the '@_' built-in variable, containing the value of the recently evaluated array. e.g.:  sub Sub1{      ($param1,$param2,...,$param) = @_;  } 

28. Subroutines • Bring into memory Dynamic and Static Scoping • Static: variables are "seen" from the place where it was declared. Any other function also defined by the same place can also see the variable

29. Subroutines • Are we really sure that the parameters won't go out of scope? • if the parameter's identifier is used outside the subroutine, would there be a difference e.g.:   sub Sub1{      ($param1,$param2,...,$param) = @_;  } 

30. Subroutines • Bring into memory Dynamic and Static Scoping • Dynamic: Values of the variables defined somewhere can still be seen even after it has "escaped" the place where it was defined.

31. Subroutines: Using my and local • my keyword: • variables defined with this keyword can only be seen by the defining method • sub Sub1{ • my ($var1) = @_; • sub Sub2{ •  print $var1; #--> error! • }  • } 

32. Subroutines: Using my and local • local keyword: • can be seen by the defining method, and other methods defined with the variable • sub Sub1{ • local ($var1) = @_; • sub Sub2{ • print $var1;  • }  • } 

33. Subroutines: Returning Values • the usual return in C and Java • without the return, the last evaluated expression will be returned • this exhibits "Dynamic Scoping", too. 

34. Exercise! Regular Expressions!!! • At least one a,then any number of b /a+b*/ Any 5 characters including new line /(.|\n){5}/ Match a string resembling an if statement in C if () /^if(\w+)/ The same word written in two or more times in a row (with possibly varying intervening whitespace), where a word is defind as a nonempty swquence of nonwhitespace character. / (^|\s)(\S+)(\s+\2)+(\s|$)/

35. Answer: • sub fac{ • my ($n) = @_; • if ($n <= 1){return 1;} • else { • return &fac($n-1)*$n; • } • } • %hasher; • for $i (1..9){ • $hasher{$i} = &fac($i); • print "$i\'s factorial: ",$hasher{$i}, "\n"; • }

36. Regular Expressions • This is the most powerful aspect of Perl • With great power comes great responsibility • “If Perl is a gun, then THIS is the bullet!” • One amazing search engine

37. One Basic Example $var = “Starry starry night, paint your\n palette blue and gray.”; if ($var =~ /rry s/){ print “Yey, something matched”; } else{ print “nah, nothing matched.”; } So what will it print? Why?

38. Examining the Pattern • our patterns that we wish to find are located inside the ‘//’ • /rry s/ • also note that there are no quotes “around” the pattern

39. Examining the Pattern • First, we assigned $var some string. • Then comes a conditional • if ($var =~ /rry s/){ • #perform operations here • } • else{ • #perform other operations here • } • What does that expression mean?

40. Examining the Pattern -‘=~’ • We introduce the matching operator • checks if a given variable contains the pattern described • So if $var is “Starry starry night, paint your palette blue and gray.”, • there really is some sequence of characters matching the pattern /rry s/ • it returns true!

41. Examining the Pattern • But what if /rRy s/? • patterns are CASE SENSITIVE • Ergo, it will return false with respect to $var

42. Quantifiers • They check the multiplicity of the character before it • added at the end • form: • /(…)QUANTIFIER /

43. More Patterns: QUANTIFIERS • introducing ‘.’ • e.g. /Sta./ as our pattern • the ‘dot’ means “any” non-null character, except a newline! • so, /Sta./ matches “Star” in $var!

44. Quantifiers • Introducing ‘*’ • It means any number of the character before it(even 0) • so, if our pattern is /Stare*/, IT WILL RETURN TRUE!. • BUT WHY? • * checks the letter ‘e’ before Star • but it appears nowhere in $var, still qualifying for ‘*’’s logic

45. Quantifiers • Yes, yes, ‘*’ can be pretty cumbersome for all of us at times • introducing ‘+’ • it assures that there is at least one instance of that character

46. Quantifiers • e.g.: • if we wish to find the pattern /Stare+/, it will now return false • + checks if there exists at least ONE or MORE ‘e’, given that it matches the pattern. • if our pattern, for variety, is /gray+/, it will return TRUE.

47. Quantifiers • Introducing ‘?’ • it means “to find for something that occurs ONCE or NONE AT ALL” • i.e., if the character is OPTIONAL • e.g. • $string = “colour”; • /colou?r/ will return true

48. Grouping Characters Together • We use ‘( )’ to group characters • Similar with the mathematical grouping notation • so if our pattern is /(nightt)+/ • our search will return false with respect with $var. • To search for ‘(‘ or ‘)’ itself, add \ in front.

49. Alternations (using ‘or’) • /x|y/, if you want to find EITHER x or y

50. Summary • /x*/ - check any number of x’s • /x+/ - check any number of “occurring” x’s • /x?/ - check if x is optional • /(xyz)+/ - groups characters as if it is a single entity; checks any number of occurring (xyz)’s • /…|…/ - it’s like ORing patterns • AND ONE NOTE: these “metacharacters” must be preceded by a \ to actually search them.