1. 2ex.1 Lists and Arrays

2. 2ex.2 Comments on exercises Always run your script with “ perl -w ” and take care of all warnings  submitted scripts should not produce any warnings When you email us problems you run into, send the script, and copy the running of it (with “ perl -w ”) and the output from the command prompt window When submitting exercises by email write your name and the exercise number in the subject line (e.g. “ Israel Israeli perl ex. 2 ”) Write a separate file for each question, and name the scripts: “ ex2.1.pl ”, “ ex2.2.pl ”, “ ex2.3.pl ”… Use meaningful name for variables.

3. 2ex.3 Adding comments Comments: The # symbol, and anything from it to the end of the line is ignored. # get start and stop values from the user my $start = my $stop = # calculate string length my $length = $start - $stop + 1;

4. 2ex.4 Adding comments Comments: If you want to insert a comment of multiple lines, you can use =begin and =cut. =begin This program prints stuff. Here you can write any text you want and you don’t need any # =cut print "stuff\n";

5. 2ex.5 Undefined variables my $a; print($a+3); Use of uninitialized value in addition (+) 3 print("a is :$a:"); Use of uninitialized value in concatenation (.) or string a is ::

6. 2ex.6 Lists and arrays A list is an ordered set of scalar values: (1,2,3,"fred") An array is a variable that holds a list: my @a = (1,2,3,"fred"); print @a;123fred You can access an individual array element: print $a[1];2 $a[0] = "*"; print @a;*23fred 3210 scalar 4scalar 3scalar 2scalar 1 3210 "fred"321

7. 2ex.7 Lists and arrays You can easily get a sub-array: my @a = (1,2,3,"fred","bob"); print @a;123fredbob print $a[1];2 my @sub_a = @a[2..3] print @sub_a;3fred You can extend an array as much as you like: my @b = (1,2,3) $b[5] = 6; @b is now (1,2,3,undef,undef,6) 4 3210 "bob""fred"321

8. 2ex.8 Lists and arrays Assigning to arrays: my @a = (3..6); (3,4,5,6) my @b = qw(a b cat d); ("a","b","cat","d") my ($a,$b,@c) = (1..5); $a=1; $b=2; @c=(3,4,5) Counting array elements: print scalar(@a); 4

9. 2ex.9 Reading and printing arrays You can read lines from the standard input in list context: my @a = ; @a will store all the lines entered until the user hits ctrl-z. You can interpolate arrays and array elements into strings: print @b; abcatd print "@b"; a b cat d print "$b[2] is the third element of \@b"; cat is the third element of @b

10. 2ex.10 Manipulating arrays – push & pop my @a = (1,2,3,4,5); print @a;12345 push(@a,6); print @a;123456 ----------------------- my @a = (1,2,3,4,5); my $x = pop(@a); print $x;5 print @a;1234

11. 2ex.11 shift & unshift my @a = (1,2,3); print @a;123 unshift(@a,0); print @a;0123 --------------------- my @a = (1,2,3); my $x = shift(@a); print $x;1 print @a;23

12. 2ex.12 split & join my @a; my @b; @a = split(" ", "You talkin to me? You talkin to me?"); @a = ("You","talkin","to","me?","You","talkin","to","me?") @b = split("", "You talkin to me? You talkin to me?"); @b = ("Y","o","u"," ","t","a","l","k","i","n"," ",...) my $str = join("-", @a); print "$str\n"; "You-talkin-to-me?-You-talkin-to-me?"

13. 2ex.13 Reversing lists my @a = ("yossi","bracha","moshe"); print join(";", reverse(@a)); moshe;bracha;yossi (You can also reverse strings…)

14. 2ex.14 Sorting lists Default sorting is alphabetical: my @a = sort("yossi","bracha","moshe"); # @a is ("bracha","moshe","yossi") my @b = sort(1,3,9,81,243); # @b is (1,243,81,9) Other forms of sorting require subroutine definition: my @c = sort(compare_sub 1,3,9,81,243); We’ll get to that latter…

15. 2ex.15 Class exercise 2 Write the following scripts: 1.Read a number from the first line of input, and then read the rest of the lines and print the one selected by that number 2.Read a list of numbers separated by spaces, and print those numbers in reverse order, separated by slashes (/) 3.Read a list of words separated by spaces, sort and print them 4*.Like in 2, but double the first and the last numbers 5*.Like in 3, but reverse the order of the letters of the last word

16. 2ex.16 Perl Express http://www.perl-express.com http://www.perl-express.com

17. 2ex.17 The Perl-Express editor

18. 2ex.18 Output tab Output of run Perl Express – running a script Run the script Warnings and errors

19. 2ex.19 Perl Express – entering input Click “ Std. Input ”

20. 2ex.20 Click “ i/o ” Perl Express – entering input

21. 2ex.21 Go back to “ Std. Output ” Perl Express – entering input Enter input Note: Perl Express can’t be given input with a ctrl-Z, so you can’t run a script with: @a =

22. 2ex.22 Class exercise 2 (cont.) Run the script from question 2 in Perl Express

23. 2ex.23 The Debugger

24. 2ex.24 Debugging A complex program will never work correctly the first time you run it! So: Write the program one stage at a time and check that it works Use a debugger to execute the program step by step Next line that will be executed

25. 2ex.25 Next line that will be executed Start debuggerStep one line Run continuously Add breakpoint – to run until this point

26. 2ex.26 You can “ watch ” your variables as they change their values using the “ Watch List ” window Mark a variable name 1 Click “ Add Watch ” 2 The variable will be displayed 3

27. 2ex.27 Enter an expression that will be evaluated In order to “ watch ” arrays and more complex data use the “ Evaluate ” button

28. 2ex.28 Class exercise 2 (cont.) Follow the working of the script from question 2 in the debugger. Watch the value of each variable.