This document provides an overview of arrays, references, multi-dimensional arrays, hashes, and sorting in Perl. It discusses array references, constructing and iterating over multi-dimensional arrays, default and customized sorting of arrays, and how to construct, manipulate, and reference hashes in Perl code. Examples are provided to demonstrate these key concepts.

1. Python & Perl
Lecture 17
Department of Computer Science
Utah State University

2. Outline
●
Array References
●
Multi-dimensional Arrays
●
Sorting Arrays
●
Hashes

3. Array References

4. Array References
●
It is possible to get references to arrays
●
An array reference is an address of the first element
●
If @ary is an array, then @ary is a named reference
to @ary
●
Example: $ary0_ref = @ary;
●
You can get multiple named references to the same
array

5. Example
array_refs_01.pl

6. Array References
●
If @ary is an array and $ary_ref is a reference to it,
then you can use $ary_ref to access individual
elements of @ary
●
Example:
my @ary = (1 .. 5);
my $ary_ref = @ary0;
$ary_ref->[0] ## refers to 1
$ary_ref->[4] ## referes to 5

7. Array References
●
You can assign values to references to destructively modify
arrays
●
For example, if $ary0_ref is a reference to and @ary0 and
a reference to it, then you can use $ary1_ref is a reference
to @ary1 the references can be used to assign elements of
one array
●
Here is how you can assign the first value of @ary1 to the
first value of @ary0:
$ary0_ref->[0] = $ary1_ref->[0];

8. Example
array_refs_02.pl

9. Multi-dimensional Arrays

10. Construction & Iteration
●
Multi-dimensional arrays can be constructed with named
and anonymous array references
●
Suppose @row0, @row1, and @row2 are arrays and
$row_ref0, $row_ref1, and $row_ref2 are named
references
●
Then you can construct a 2D array @rslt as
@rslt = ($row_ref0, $row_ref1, row_ref2);
●
$rslt[$r]->[$c] refers to element at r, c

11. Example
multidim_arrays_01.pl

12. Anonymous References
●
Using anonymous array references is more
straightforward if you do not need to refer to sub-
arrays by names
●
Example: @ary = ([1, 2, 3], ['a', 'b', 'c'])
constructs a 2x3 array with anonymous references
(i.e., references to [1, 2, 3] and ['a', 'b', 'c'] are
not named)
●
$ary[$r][$c] refers to element at r, c

13. Iterating over Anonymous References
●
You can use foreach to iterate over the sub-arrays
●
Example: foreach my $aref (@ary) { … }, $aref
iterates over anonymous references to sub-arrays
●
@{$aref} is the sub-array to which $aref currently refers
●
Example: foreach my $e (@{$aref}) { … } iterates over
the scalars in @{$aref}

14. Example
multidim_arrays_02.pl

15. Variable Length Rows
●
Rows in multi-dimensional arrays do not have to be of the
same length
●
You can use $#{} notation to obtain the number of
elements in each row
●
Example: if @ary2 is a 2D array, $#{ary2[$r]}+1 is the
length of row $r
●
Example: if @ary3 is a 3D array, $#{ary3[$r][$c]}+1 is
the length of row at $r, $c

16. Sorting Arrays

17. Default Sorting Settings
●
The sort function by default sorts its argument array/list
alphanumerically (i.e., it treats array/list elements as
strings)
●
In other words, even if all elements in in the argument
array/list are numbers, they are sorted as strings
●
The sort function returns a sorted copy of its array/list
argument

18. Example
sort_01.pl

19. Customized Sorting
●
You can explicitly customize sorting to work on strings and
numbers
●
If you want to have an array/list sorted numerically, use
{ $a <=> $b }
●
If you want to have an array/list sorted alphanumerically,
use { $a cmp $b }
●
Example: sort { $a <=> $b } (5 , 1, 3);
●
Example: sort { $a cmp $b } qw(Perl Python);

20. Example
sort_02.pl

21. Hashes

22. Hashes
●
A hash is a one-to-one mapping from keys to values
●
Keys are not ordered
●
A hash variable must be marked with the % type identifier
●
Three main type identifiers:
 $ - scalar
 @ - array
 % - hash

23. Hash Construction
●
A hash can be constructed from a list of comma-
separated key-value pairs
●
A hash can be constructed by inserting key-value pairs
into it
●
A hash can be constructed from a list with the =>
operator

24. construct_hash_01.pl
construct_hash_02.pl
Example

25. Hash Manipulation
●
Once a hash exists, one can:
 Get the keys
 Get the values
 Iterate through the key-value pairs
 Swap the keys and values
 Check if a key exists or is defined

26. manip_hash_01.pl
manip_hash_02.pl
Example

27. Reading & References
●
http://perldoc.perl.org/
●
James Lee. Beginning Perl, 2nd
Edition, APRESS
●
Dietel, Dietel, Nieto, McPhie. Perl How to Program,
Prentice Hall