Perl Arrays and Lists Learning Objectives: 1. To understand the format and the declaration of Arrays & Lists in Perl 2. To distinguish the difference between a List & an Array & their usage 3. To study the basic operations of Arrays & Lists in Perl

COMP111 Lecture 11 / Slide 2 Lists  A list is an ordered collection of scalar data.  A list begins and ends with parentheses, with the elements separated by commas (and optional spaces). (1,2, 3,4.1)  List elements can be constants or expressions: ("Bill", 4, "pie", "B. Gates") ($num, 17, $num+1+$i)  Memory for lists is dynamically allocated and removed as the program runs.

COMP111 Lecture 11 / Slide 3 Lists  The empty list (no elements) is represented by an empty pair of parenthesis: ( )# empty list  The list constructor “.. ” creates a list of values with increments of 1: (1.. 4) # same as (1, 2, 3, 4) (1, 5.. 7) # same as (1, 5, 6, 7) ($min.. $max) # depends on values of $min and $max ( ) # same as (1, 2, 3, 4, 5) ??!! (10.. 5) # same as ( ) -> it can’t count down

COMP111 Lecture 11 / Slide 4 Single-Word Lists  There is a shortcut for lists of single-word strings, the “ quote word ” function: ("bill", "gates", "pie", "toss") # usual version qw(bill gates pie toss) # same as above qw(bill gates pie toss) # also okay

COMP111 Lecture 11 / Slide 5 Arrays  An array contains a list (zero or more scalar values).  Array variable names are similar to scalar variable names, except the initial character is ” instead of “ $ = (1,2, copies = ("Bill", 4, "pie", "B. Gates"); $num = = ($num, 17, $num+1);  If a scalar value is assigned to an array variable, it becomes a single-element list = 4;# becomes (4) automatically

COMP111 Lecture 11 / Slide 6 Inserting Arrays  You can also insert array elements into = = (1, 10); # = # = 99); # (0,1,2,6,7,8,10,99)  Note that the inserted array elements are at the same level as the other elements, not in a “ sub-list ”.

COMP111 Lecture 11 / Slide 7 Left-Side Assignment  If a list only contains variables, you can use it on the left side of an assignment: ($a,$b,$c) = (1,2,3); # set $a=1, $b=2, $c=3 ($a,$b) = ($b,$a); # swap $a and $b = ($a,$b,$c); # set $d=$a # remove first element # and put it in $e # end up with: $a=2, $b=1, $c=3 # $d=2, $e) # no change  An array variable can only occur in the last position in the list, because the array variable is “ greedy ” and consumes all the remaining values.

COMP111 Lecture 11 / Slide 8 Array Length  If an array variable is assigned to a scalar variable, the number assigned is the length of the = (1,2,3); $n # $n gets 3, the length  The context determines whether the length of the array is used or the list: $n $n gets the length ($n) # $n gets the first element  The first assignment is a scalar assignment, is treated as a scalar, returning its length.  The second assignment is an array assignment, and gives the first element (silently discarding the rest).

COMP111 Lecture 11 / Slide 9 Array Subscripting  Each element of an array can be accessed by its integer position in the list.  The first element starts at position 0 (like C++ arrays).  The first element of array is accessed as = qw(bill gates pie toss); $name1 = $a[0]; # sets name1 to "bill" $name2 = $a[3]; # sets name2 to "toss" $a[1] = "clinton"; # a: qw(bill clinton pie toss)  Note that on the array name becomes a $ when accessing individual elements.

COMP111 Lecture 11 / Slide 10 Array Subscripting  You can use all the usual scalar operations on the array = (1,2,3); $a[0]++;# a: (2,2,3) $a[1] += 4;# a: (2,6,3) $a[2] += $a[0];# a: (2,6,5) # swap the first two elements ($a[0],$a[1]) = ($a[1],$a[0]); # a: (6,2,5)

COMP111 Lecture 11 / Slide 11 Array Slices  Accessing a list of elements from the same array is called a slice.  Perl provides a special shortcut for = # swap the first two # make all 3 elements like the = (7,4); # change the last two to 7 and 4 # a: (1,7,4)  Note that slices rather than $. This is because slices work with lists rather than scalar values.

COMP111 Lecture 11 / Slide 12 List Slices  Slices also work directly on lists: $c = (1,2,3,4,5)[2];# sets $c to = (1,2,3,4,5)[2,4];# to (3,5)  The second statement above is equivalent =

COMP111 Lecture 11 / Slide 13 “push” and “pop”  You can use push and pop to add and remove values from the end of an = (1,2,3); $new = 6; # same = # so far: (1,2,3,6) $oldvalue = # removes last element # so far: (1,2,3) # can push multiple values # so far: (1,2,3,4,5,6)  pop returns undef if given an empty array.

COMP111 Lecture 11 / Slide 14 “unshift” and “shift” an Array  You can use unshift and shift to add and remove values from the beginning of an = (1,2,3); $new = 6; # same = # so far: (6,1,2,3) $old = # removes first element # so far: (1,2,3) # can unshift multiple values # same = # so far: (4,5,6,1,2,3)  shift returns undef if given an empty array.

COMP111 Lecture 11 / Slide 15 “reverse” and “sort” an Array  The reverse function reverses the array, returning the resulting = = = reverse(1,2,3);# same  The sort function returns a sorted array in ascending ASCII = qw(small medium = large, medium, = sort(qw(small medium large)); # =

COMP111 Lecture 11 / Slide 16 Scalar and List Context  If an operator or function expects a scalar argument, the argument is evaluated in a scalar context. $n $n gets the length  If an operator or function expects a list argument, the argument is evaluated in a list context. ($n) # $n gets the first element  A scalar value used within a list context is promoted to a single-element = 1; = (1)

COMP111 Lecture 11 / Slide and $ (at sign) Refers to the entire array or slice of an array (when used with [ ]). $ (dollar sign) Refers to one element of the array, used with [ ]

COMP111 Lecture 11 / Slide 18 Array chomp  The chomp function also works for array variables, removing any ending newlines in each element = qw(unix\n shell\n perl); is now qw(unix shell perl)  Array chomp is especially useful when reading a file or input from a user.  in a list context will return all remaining lines up to the end of the file, or until the user hits = ;# read input in a list context remove all trailing newlines

COMP111 Lecture 11 / Slide 19 Example: Max Value  How to find the biggest value in an array of = (23,4,56,99,36,24); $max = $a[0]; for($i=1; $i++){ if($a[$i] > $max){ $max = $a[$i]; } print "max is: $max\n";