Perl Arrays and Lists

Software Tools Perl Arrays and Lists

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.

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..4) # same as (1, 2, 3, 4) (1.2 .. 5.7) # same as (1, 2, 3, 4, 5) ??!! (1, 5 .. 7) # same as (1, 5, 6, 7) ($min .. $max) # depends on values of $min and $max (10 .. 5) # same as ( ) -> it can’t count down

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

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 “$”. @numbers = (1,2, 3,4.1); @all = @numbers; # copies array to @all @list1 = ("Bill", 4, "pie", "B. Gates"); $num = 2; @group = ($num, 17, $num+1); • If a scalar value is assigned to an array variable, it becomes a single-element list automatically: @a = 4; # becomes (4) automatically

Inserting Arrays • You can also insert array elements into lists: @numbers = (6,7,8); @numbers = (1, 2, @numbers, 10); # (1,2,6,7,8,10) @numbers = (0, @numbers); # (0,1,2,6,7,8,10) @numbers = (@numbers, 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”.

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 $b ($d,@bill) = ($a,$b,$c); # set $d=$a and @bill=($b,$c) ($e,@bill) = @bill; # remove first element of @bill # and put it in $e # end up with: $a=2, $b=1, $c=3 # $d=2, $e=1, @bill=(3) • 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.

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

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 the @a array is accessed as $a[0]: @a = 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 the @ on the array name becomes a $ when accessing individual elements.

Array Subscripting • You can use all the usual scalar operations on the array elements: @a = (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)

Array Slices • Accessing a list of elements from the same array is called a slice. • Perl provides a special shortcut for slices: @a = (1,2,3); @a[0,1] = @a[1,0]; # swap the first two elements @a[0,1,2] = @a[1,1,1]; # make all 3 elements like the 2nd @a[1,2] = (7,4); # change the last two to 7 and 4 # a: (1,7,4) • Note that slices use @ rather than $. This is because slices work with lists rather than scalar values.

List Slices • Slices also work directly on lists: $c = (1,2,3,4,5)[2]; # sets $c to 3 @b = (1,2,3,4,5)[2,4]; # sets @b to (3,5) • The second statement above is equivalent to: @x = (1,2,3,4,5); @b = @x[2,4];

Index Expressions • You can use expressions for the subscripts just like in C++: @list1 = (5,6,7); $n = 2; $m = $list1[$n]; # $m = 7 $p = $list1[$n-1]; # $p = 6 ($p) = (5,6,7)[$n-1]; # same thing using slice

Slice Expressions • You can use also array expressions to index slices if you want to be tricky: @nums = (5,6,7); @i = (2,1,0); @rev = @nums[@i]; # same as @nums[2,1,0] # or ($nums[2], $nums[1], $nums[0]) # or (7,6,5)

Bad Subscripting • If you access an array beyond the end of the array, the undef value is returned without warning. • undef is 0 when used as a number, the empty string when used as a string. @nums = (5,6,7); $nums[3] = "bill"; # @nums: (5,6,7,"bill") $nums[5] = "g."; # @nums: (5,6,7,"bill",undef,"g.") • Assignment to an array element with a subscript less than zero is a fatal error.

Backward Subscripting • You can use $#bill to get the index value of the last element of @bill. • Accessing an array with a negative subscript counts back from the end. So, another way to get the last element of @bill is $bill[-1]. @bill = qw(cheap rich lucky likespie); print $#bill; # prints 3 print $bill[$#bill]; # prints likespie print $bill[$#bill-1]; # prints lucky print $bill[-1]; # prints likespie print $bill[-2]; # prints lucky print $bill[-3]; # prints rich

push and pop • You can use push and popto add and remove values from the end of an array. @a = (1,2,3); $new = 6; push(@a,$new); # same as @a = (@a,$new) # so far: (1,2,3,6) $oldvalue = pop(@a); # removes last element of @a # so far: (1,2,3) push(@a,4,5,6); # can push multiple values # so far: (1,2,3,4,5,6) • popreturns undef if given an empty array.

unshift and shift • You can use unshift and shiftto add and remove values from the beginning of an array. @a = (1,2,3); $new = 6; unshift(@a,$new); # same as @a = ($new, @a) # so far: (6,1,2,3) $old = shift(@a); # removes first element of @a # so far: (1,2,3) unshift(@a,4,5,6); # can unshift multiple values # same as @a = (4,5,6,@a) # so far: (4,5,6,1,2,3) • shiftreturns undef if given an empty array.

reverse and sort • The reverse function reverses the array, returning the resulting list: @a = (1,2,3); @b = reverse(@a); # @b=(3,2,1), @a unchanged @b = reverse(1,2,3); # same thing @b = reverse(@b); # @b=(1,2,3) • The sort function returns a sorted array in ascending ASCII order: @size = qw(small medium large); @sortsize = sort(@size); # large, medium, small @sortsize = sort(qw(small medium large)); # same @a = (1,2,4,8,16,32,64); @b = sort(@a); # @b=(1,16,2,32,4,64,8)

Array Variable Interpolation • Array elements can be interpolated in double-quoted strings: @comp111 = qw(unix shell perl); print "$comp111[0] programming is mainly done "; print "in $comp111[2]\n"; $n=3; print "$comp111[0] programming is mainly done "; print "in $comp111[$n-1]\n"; # same thing • Arrays and array slices will be interpolated (printed) with spaces between the elements: @a = (1,2,"bill",3); print "a: @a\n"; # prints a: 1 2 bill 3 print "a1: @a[1,2]\n"; # prints a1: 2 bill

Scalar and List Context • If an operator or function expects a scalar argument, the argument is evaluated in a scalar context. $n = @nums; # $n gets the length of @nums • If an operator or function expects a list argument, the argument is evaluated in a list context. ($n) = @nums; # $n gets the first element of @nums • A scalar value used within a list context is promoted to a single-element array. @nums = 1; # @nums = (1)

@ and $ Review @ (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 [ ]

Array chomp • The chomp function also works for array variables, removing any ending newlines in each element : @comp111 = qw(unix\n shell\n perl); chomp(@comp111); # @comp111 is now qw(unix shell perl) • Array chomp is especially useful when reading a file or input from a user. • <STDIN> in a list context will return all remaining lines up to the end of the file, or until the user hits CTRL-D. @lines = <STDIN>; # read input in a list context chomp(@lines); # remove all trailing newlines

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

Perl Arrays and Lists