Introduction to Perl Programming: Basics and Concepts

1. Perl Day 1

2. Programming • Computers know how to execute a sequence of instructions • Instructions must be very precise. • Think of a recipe • Preheat oven to 400F • Place 1 cup sugar and 1 cup butter into a bowl • Blend until creamy with a high speed mixer • Add in 1 medium sized egg, 2 cups flour, and 1 teaspoon baking soda, ½ spoon salt • Cut into circular shaped cookies 2” in diamater, place on a greased cookie sheet • Bake in preheated oven for 6.5 minutes.

3. Programming Language • Computers ultimately speak 1’s and 0’s, but we aren’t good at entering long strings of them. • Thus people came up with higher level languages to send instructions to the CPU • This is called Machine or Assembler language • That language is hard to understand, so later people developed higher level languages that are closer to English, which “compile” into assembler language • C, C++ are good examples of this • These languages are compiled for a specific processor (Intel/AMD, vs PowerPC vs. Spark). Thus a program compiled on an Intel processor won’t work on a PowerPC (Mac). • To get around this, people developed interpreted languages which have “interpreters” that are compiled for each platform, that run a even higher level language. • The cost to this is speed • The advantage is portability of code, code written once will execute on Mac, Windows, Unix etc.

4. Where to get perl • Most Unix machines have it built in. • This includes scooby and scooby2 if you have accounts there. • For windows and mac: • http://www.activestate.com/activeperl/ • You’ll use notepad, vi or some other TEXT editor to write your scripts. • There are also many IDEs you can use, but I’ll leave that up to you to research.

5. She bangs, she bangs… • All perl programs start with a line called a shabang. #!C:/perl/bin/perl.exe • If you are on Unix: #!/usr/bin/perl • Every line should end in a ; • If you wish to put a comment in your script, simply start the comment with a #: #This is a comment

6. Having the script talk to you • print print(“hello world\n”); • \n • All lines that are printed should end with a \n inside the quotes. This is to tell perl that you want a “new line” (same as hitting enter). • If you forget to put the \n, sometimes it won’t print at all.

7. What is a variable • Think of a variable as a bucket into which you can place something. • You can have an unlimited number of buckets. To create a bucket you simply use it. • Buckets have names which must be unique. • They are case sensitive so “a” is different from “A” • Buckets should be named so you can tell what’s in them. This will make your life easier and is mandatory as far as I’m concerned. • In Perl variable names are always named $[something], @[something], or %[something] • $ signifies a Scalar • @ signifies an Array • % signifies a Hash • e.g $a, $b, $enda, $this_is_long, $ThisIsLong

8. Double Quotes – Double the fun • Anytime in perl you use “ “ whatever is inside the quotes will be interpreted. • Anytime you use ‘ ‘ whatever is inside the quotes will be used literly. print(“$x \n”); • This will output the value of $x print(‘$x \n’); • This will output $x \n

9. Scalars • A scalar is a variable which holds exactly 1 value. • It usually contains: • a number, • a letter (character), • a set of characters (string) • e.g. $NumStudentsInClass=12; $ClassName=“Learning Perl”;

10. Buckets of fun • Now that you have a scaler, you can do things with it like print it out: print(“There are $NumStudentsInClass people in this class\n”);

11. Math • If a scalar contains a number, you can do math: $NumA=7; $NumB=10; $Num=$NumA+$NumB; Print(“$NumA + $NumB = $Num\n”); • Operators include: +, -, / (divide) and * (multiply)

12. Text • If your scalar has characters or a string in it, you add additional text with the . operator: $Word1=‘Hello’; $Word2=‘World’; $Words=$Word1.’ ‘.$Word2; *or* $Words=“$Word1 $Word2”; • You can add additional text to an existing scalar: $a = ‘Hello’; $a .= ‘ World’; print(“$a\n”); #This prints “hello world”

13. Conditionals • Sometimes you only want to do things if a scalar contains a certain value: $Num=105; if($Num==105) { print(“Big Number!\n”); }

14. Tests • If’s can have the following tests: == (checks if numbers are equal) < (checks if numbers are less than) > (checks if numbers are greater than) <= (checks if numbers are less than or equal) >= (checks if numbers are greater than or equal) != (checks if numbers are not equal) eq (checks if characters/strings are identical) ne (checks if characters/strings are not identical)

15. Maybe we need 2 tests • We can combine tests with and/or • AND: && • OR: || $Num=100; if(($Num>10) && ($Num<100)) { print(“Our numbers is between 10 and 100\n”); } $Name=“Enda”; $HeightInFeet=6; if(($Name eq “Enda”) || ($Height>5)) { print(“It’s either Enda, or a tall person\n”); }

16. What if we want to do one thing if something is true, and something else otherwise? • Each if can have unlimited number of elsif, and up to 1 else: $Month=3; if($Month==1) { print(“Jan\n”); } elsif($Month==2) { print(“Feb\n”); } elsif($Month==3) { print(“Mar\n”); } … elsif($Month==12) { print(“Dec\n”); } else { print(“Error: Month should be between 1 and 12\n”); }

17. Additional thing you can check • Does this variable exist? if(defined($a)) • What if you want something to happen if something is NOT true if(! (defined($a)))

18. More than one thing in a variable • Crazy talk? • No, one way to do this is called an Array. @a=(1,2,3); • There is no restriction on what the values in the array can be, one could be a number, the next a string, the next a number etc. • When you refer to the entire array you use @ instead of $ • Arrays are indexed from 0, so the first element in the array is said to be at “index” 0. The second element is said to be at “index” 1. • When you refer to an individual scalar in the array, you use $ again • e.g. @b=@a; print(“$a[0]\n”);

19. How many things are in the array? • You can find out by assigning the Array to a scalar • Yes, that doesn’t make sense, it’s a weird perl thing: @Months=(‘jan’,’feb’,’mar’,’apr’,’may’,’jun’,’jul’,’aug’,’sep’,’oct’,’nov’,’dec’); $NumMonths=@Months; print(“There are $NumMonths months\n”);

20. Command Line • Let’s assume you wrote a Perl Script in a file called doit.pl • You typically execute it like this: • C:\perl\bin\perl doit.pl • What if you want to pass “arguments” into your script: • C:\perl\bin\perl doit.pl 1 2 Enda • Those arguments are automatically stored in an Array called @ARGV • Assuming the above command line: • @ARGV=(1,2,’Enda’); • $ARGV[0] now contains 1 • $ARGV[1] now contains 2 • $ARGV[2] now contains ‘Enda’

21. Doh! • Often you want to ensure that you got at least 1 argument to your script. • If not there may be no reason to continue. • For example, imagine a script that takes one argument, which should be a number less than 5. It prints out the numbers before that number. • Sometime there is just no point in continuing, you should just die: • die “I asked you to send me a number, you gave me $ARGV[0]…I can do nothing with that…giving up”;

22. count.pl #!C:/Perl/bin/perl.exe $NumArguments=@ARGV; If($NumArguments<1) { die “Usage: count.pl NUMBER”; } If($ARGV[0]==5) { print(“1 2 3 4 5\n”); } elsif($ARGV[0]==4) { print(“1 2 3 4\n”); } elsif($ARGV[0]==3) { print(“1 2 3\n”); } elsif($ARGV[0]==2) { print(“1 2\n”); } elsif($ARGV[0]==1) { print(“1\n”); } else { die “Error, you must send me a number between 1 and 5 as the first argument”; }