IT441 Network Services Administration

1. IT441Network Services Administration Prof. Alfred J Bird, Ph.D., NBCT http://www.cs.umb.edu/~abird abird@cs.umb.edu http://it441-s14 -bird.wikispaces.umb.edu/ Office – McCormick 3rd floor 607 Office Hours – Tuesday and Thursday 4:00PM to 5:15PM

2. Homework • Study and try to understand the program filetest.pl on pages 201-202. See if you can enter it and make it work.

3. The Transliteration Operator • This string operator resembles the substitute operator from a regex: s/// but it functions very differently • tr/old/new/ will not replace occurrences of the string old with the string new. It will replace any occurrence of the letter o with the letter n, any occurrence of the letter l with the letter e and any occurrence of the letter d with the letter w.

4. The Transliteration Operator • How can we use the transliteration operator to change every occurrence of a comma into a period? • How can we use the transliteration operator to change every occurrence of the letter a to the number 1? • How can we use the transliteration operator to change the case of all letters in a string?

5. The Transliteration OperatorA Coding Exercise • How can we use the transliteration operator to change the case of all letters in a string? • Write a small program to: • Input the file string.txt into your program • Change all occurrences of small letters into capital letters • Print the resulting string to the screen

6. The Transliteration OperatorA Coding Exercise • How can we use the transliteration operator to change the case of all letters in a string? • Write a small program to: • Input the file string.txt into your program • Change all occurrences of small letters into capital letters • Print the resulting string to the screen See transtest.pl in my home directory

7. Hashes • We have already studied two (2) ways to store data in a Perl program: • Scalars • Arrays • Each has its own strengths and weaknesses • In Perl (and many other programming environments) there is a third way to store data. It is referred to as a HASH • Hashes are very useful and very powerful!

8. Data Structures • Remember that a scalar variable always starts with a $, for example $scalar • Remember that an array structure always starts with a @, for example @array • Well a hash always starts with a %, for example %hash

9. What is a HASH? • A hash is a data structure that consists of pairs of datum, one called the key and one called the value. • Some people call a hash an associative array. • A hash is stored in no particular order. • In a hash the keys must be unique but the values have no such restrictions.

10. What Can We Use a Hash For? • A classic example of a use for a hash in Systems Administration is the MACIP pairing • To create this type of hash we would enter: my %ipMac = ( ‘192.168.124.1’, ‘2a:09:4e:3c:31:42’ ‘192.168.124.2’, ‘0e:88:4e:2a:56:07’ ‘192.168.124.3’, ‘1a:32:6f:5c:6b:1a’ ); • This hash uses the IP as the key and the MAC as the value.

11. What Can We Use a Hash For? • We could also enter the previous hash as my %ipMac = ( 192.168.124.1 => ‘2a:09:4e:3c:31:42’ 192.168.124.2 => ‘0e:88:4e:2a:56:07’ 192.168.124.3 => ‘1a:32:6f:5c:6b:1a’ ); • We call the symbol => the quoting comma because it acts as a comma and quotes the string to the left of it.

12. What Can We Use a Hash For? • Another possible use for a hash is using student numbers as the key and student names as the value. • Why could we not reverse the key, value pairs in this case? • Can you think of some other examples where a hash would be a good choice for a data structure?

13. Another Way to Define the Hash • Since hashes and arrays have a lot in common we can change back and forth between them. • Given the following array: @array = qw( Thao Saigon Hoang Boston Kevin Dorchester Gary Houston Susan Manchester Allison Chicago) • We can create a hash directly by an assignment statement: %where = @array • What are the pairs in this hash?

14. Another Way to Define the Hash • This way of defining an hash is equivalent to the following: %where = ( Thao => ‘Saigon’ Hoang => ‘Boston’ Kevin => ‘Dorchester’ Gary => ‘Houston’ Susan => ‘Manchester’ Allison => ‘Chicago’ );

15. Working with Hash Values • To look up a value in a hash we use something similar to the index notation for an array. However there are two (2) differences: • Instead of locating the value by number, we locate it by the key • Instead of using square brackets [ ] we use curly braces{ } • Here is an example using the hash %where: • print “Hoang lives in $where{Hoang}”;

16. Modifying the Hash • Adding a new element to the hash is very simple. All that is needed is to type an assignment statement: $where{Al} = “Quincy”; • Changing an element in the hash is just as simple: $where{Kevin} = “Quincy”; • Remember, Kevin used to live in Dorchester but he now lives in Quincy • Or we can remove an element from the hash using the delete function: delete $where{Gary} • Will remove Gary and Houston from the hash

17. Hash Functions • There are a few functions that operate on hashes. • keys(%hash) returns a list of all keys in %hash • values(%hash) returns a list of all values in %hash • each(%hash) returns each key/value pair in %hash • delete $hash{key} deletes the key/value pair in %hash • exists $hash{key} returns true if a entry with key exits in %hash

18. A Coding Exercise • Do you remember where in the Ubuntu directory tree structure a file is located and what it is named that stores the IP address and the MAC information for computers on the networks we set up in IT341? (I hope so) • Write a small program that will: • Read in the file • Extract the IP addresses and the corresponding MAC addresses • Create a hash named %ipMac containing that information • Print out a table containing the information from the hash

19. For next time • Read pages 115 to 129 in the textbook. • Try exercises 1-3 on pages 129-130 • Skim pages 131-152