2.1

Lists and Arrays

2.1

2.2Summary of 1st lesson

• Single quoted and double quoted strings

• Backslash (\) – the escape character: \t \n

• Operators: numbers: + - * / ** strings: . x

• Variables: my $name;

• Reading input: $name = <STDIN>;

• Functions: length($name); substr($name,2,2);

2.3

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 ::

2.4Lists 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

2.4

012 3

scalar 1scalar 2scalar 3scalar 4

2.5Lists 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)

2.5

012 3

scalar 1scalar 2scalar 3scalar 4

2.6Lists 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

2.6

2.7Reading and printing arrays

You can read lines from the standard input in list context:

my @a = <STDIN>;

@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

2.7

2.8Manipulating 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

2.8

2.9shift & 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

2.9

2.10split & join

my @a = split(",", "hello,how are you?,goodbye");

print "$a[1]\n";

how are you?

Now @a holds the list: ("hello","how are you?","goodbye")

my $str = join(":", @a);

print "$str\n";

hello:how are you?:goodbye

2.10

2.11Reversing lists

my @a = ("yossi","bracha","moshe");

print join(";", reverse(@a));

moshe;bracha;yossi

(You can also reverse strings…)

2.11

2.12Sorting 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…

2.12

2.13

The Debugger

2.13

2.14Debugging

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

2.15

Next line that will be executed

Start debugger Step one line

Run continuously Add breakpoint – to run until this point

2.16Choose “i/o” for interactive input

1

View printed output2

Enter input3

2.17You can “watch” your variables as they change their values using the “Watch List” window

Mark a variable name1

Click “Add Watch”

2

2.18

Enter expression

In order to “watch” arrays and more complex data use the “Evaluate Watch” button

2.19

Controls:Ifs and Loops

2.20Controls: if ?

Controls allow non-sequential execution of commands, and responding to different conditions.

else { print "Are you doing anything tomorrow night?\n";}

print "How old are you?\n";my $age = <STDIN>;if ($age < 18) { print "Sorry, I’m not allowed to chat with minors\n";}

2.21if, elsif, else

It’s convenient to test several conditions in one if structure:

if ($age < 18) { print "Sorry, I’m not allowed to chat with minors\n";} elsif ($age < 25) { print "Are you doing anything tomorrow night?\n";} elsif ($age < 35) { print "Are you married?\n";} else { print "Do you need help crossing the street?\n";}

2.22Comparison operators

ComparisonNumericString

Equal==eq

Not equal!=ne

Less than<lt

Greater than>gt

Less than or equal to

<=le

Greater than or equal to

>=ge

if ($age == 18)...

if ($name eq "Yossi")...

if ($name ne "Yossi")...

if ($name lt "n")...

2.23Boolean operators

if (($age==18) && ($name eq "Yossi"))...

if (!($name ne "Yossi"))...

if (!($name eq "Yossi:"

&& $age==18))...

And &&

Or ||

Not !

2.24Controls: Loops

Commands inside a loop are executed repeatedly (iteratively):

while ($name ne "Yossi") { chomp($name = <STDIN>); print "Hello $name!\n";}

foreach $name (@names) { print "Hello $name!\n";}

* There are also until, do-while and do-until loops

2.25Loops

$i=0;while ($i<scalar(@names)) { $name = $names[$i]; print "Hello $name!\n"; $i++;}

for ($i=0; $i<scalar(@names); $i++) { $name = $names[$i]; print "Hello $name!\n";}

A for loop is controlled by three statements:

• 1st is executed before the first iteration

• 2nd is the stop condition

• 3rd is executed before every re-iteration

These are equivalent

2.26Breaking out of loops

next – skip to the next iteration last – skip out of the loop

my @lines = <STDIN>;foreach $line (@lines) { if (substr($line,0,1) ne ">") { next; } print(substr($line,1)); if (substr($line,0,4) eq ">ehd") { last; }}

2.27Breaking out of loops

die – end the program and print an error message to the standard error <STDERR>

if ($score<0) { die "score must be positive"; } score must be positive at test.pl line 8.

Note: if you end the string with a "\n" then only your message will be printed

* warn does the same thing as die without ending the program