Lecture#3 Outline

Chapter 3 - Lists and Arrays

List - An ordered collection of scalars
Array - A variable that contains a list

A list can have different 'types' of values

Use the @ for an array....

@myarray = (hello, goodbye, hey);
print $myarray[0]; #prints hello
print $myarray[2]; # hey
print $myarray[10]; # undef
$myarray[10] = "hi there";

$myarray[3-9] are undef
$myarray[2.5] = $myarray[2] => hey
$#myarray = 10 - index of last element in array
$myarray[-1] = $myarray[10] => "hi there"
$myarray[-11] = $myarray[0] => "hello"
$myarray[-20] => error

List Information

() - Empty list
(1,2,) - comma is ignored
("hello", 2) - Different types ok!
(1..10) - List of 10 items
($a..$b) - List from $a to $b, $b > $a

Quoted Words

("a", "b", "c", "d", "e")

can also be represented as:

qw/ a b c d e /
qw! a b c d e !
qw( a b c d e )
qw# a b c d e #
qw[ a b c d e ]
qw< a b c d e >

Other uses for lists:

Multiple assignments:
($a, $b, $c) = ("hi", "bye", "hey");

Easy to write the classic 'swap' routine
($b, $a) = ($a, $b);

Pop and Push

pop - takes last element off array
push - puts an element on the end

@array = 1..5;
$a = pop @array; #$a gets 5, @array = (1,2,3,4)
$a = pop (@array); #$a gets 4, @array=(1,2,3)
pop @array; #@array=(1,2)

push (@array, 3); #@array = (1,2,3)
@array2 = (4,5);
push @array, @array2; #@array=(1,2,3,4,5);

Shift and Unshift

Works on the other end of an array
@array = ("Apple", "Baker");
$a=shift @array; #$a="Apple", @array=("Baker")
shift (@array); #@array=()
$a=shift @array; #$a=undef
unshift @array, 1; #@array = (1)
@array2 = (3,2);
unshift @array, @array2; #@array=(3,2,1)

Arrays are interpolated into strings separated by spaces
@array=("how","are","you");
print "Hey, @array?\n"; #prints Hey, how are you?
print "Hey $array[2]?\n"; #prints Hey, you?
print @array; #howareyou

Foreach

@myarray = (1..10);
foreach $num (@myarray) {
$num=$num-1;
}

@myarray = (0,1,2,3,4,5,6,7,8,9) - foreach uses the actual element

Can use the 'default' variable...
@myarray = (1..10);
foreach (@myarray) {
$_=$_-1;
}

$_="something";
print;
prints whatever's in $_

Sort/Reverse

Sorts or reverses arrays. Must be assigned to a variable
@array = (4,3,2,1);
sort (@array); #Does nothing
@array = sort @array; #Sorts the array
print reverse @array; #Prints array in reverse order

Versions of Perl before 5.6 use Quicksort internally...later versions, including the one on the eos system use mergesort.

Scalar and list context

When perl is expecting a scalar or a list...EX:
5+ something...Perl is expecting a scalar
sort something...Perl expects a list
If an array appears in scalar context, it returns the number of elements:

@array=(1,2,3);
$x = @array; #$x now has 3 in it

assigning reverse to a scalar does something strange...
@array=("Alan","Bob");
@backwards=reverse @array; #("Bob", "Alan")
$backwards=reverse @array; #nalAboB

You can force scalar context...
@array = ("Alan", "Bob");
print "This list has @array elements\n"; #This list has Alan Bob elements
print "This list has" , scalar @array, " elements\n"; #This list has 2 elements

One other anomaly....

Assigning <STDIN> to an array reads the whole file and puts each line in one element of the array:

@array = <STDIN>;

File
Hi there
this is my file

$array[0] = Hi there\n
$array[1] = this is my file\n

CSC255 - Alan Watkins - North Carolina State University