Chapter 6

1
Chapter 6 Hashes

• Wayne Machuca
• MHCC

2
Hash Reputation

• Hashes are sometimes regarded as a difficult
  subject in Perl
• Due largely to misleading or inconsistant
  syntax

3
Objectives

• Objectives
• Use hash prefixes
• Initialize a hash
• Create a hash
• Load and search a hash
• Remove keys from hashes

4
What are Hashes?

• A hash is an unordered collection of values, each
  of which is identified by a unique key
• A value can be retrieved by its key,
• one can add to or delete from the collection
• A data structure with these properties is called
  a dictionary

Abhijit Menon-Sen (October 01, 2002 )How Hashes
Really Workhttp//www.perl.com/lpt/a/2002/10/01/h
ashes.html
5
A Dictionary

• One simple way to implement a dictionary is to
  use a linked list of keys and values (that is, a
  list where each element contains a key and the
  corresponding value).
• To find a particular value, one would need to
  scan the list sequentially, comparing the desired
  key with each key in turn until a match is found,
  or we reach the end of the list.

Abhijit Menon-Sen (2002)
6
Problem

• This approach becomes progressively slower as
  more values are added to the dictionary,
• because the average number of elements we need to
  scan to find a match keeps increasing.

Abhijit Menon-Sen (2002)
7
Solution?

• Performing binary searches on a sorted array of
  keys instead of using a linked list
• but performance would still degrade as the
  dictionary grew larger
• Transform every possible key into a unique array
  index, then we could store each value in a
  corresponding array entry
• although this strategy is simple and fast, it has
  many disadvantages and is not always useful

Abhijit Menon-Sen (2002)
8
Solution

• What we need is a practical compromise between
  speed and memory usage
• Hashes represent just such a compromise.

Abhijit Menon-Sen (2002)
9
What are Hashes?

• Associative Arrays
• Same as a normal array
• It is a list of scalar values
• But, uses keys rather than a simple numeric
  subscript or index
• Array entries are called slots or buckets
• Most important thing
• Remember the order that you specify the hash
• Key ? then Data

10
Hash Clarification

• The Hash provides a useful look up tool similar
  to the key in a database table.
• A unique key is used to locate a given value
• Hashes are uniformly fast
• Can get large with little degration in speed
• Hashes are useful for handling input from HTML
  forms, CGI environment variables and more.

11
Hash Prefix

• When you want to refer to the whole hash, use the
  prefix
• However, you can also use the and _at_ prefix
  depending on what you are talking about.

12
Examples

• If you are looking to return a single item from
  the hash, you writeitem hashnamekeyvalue
  You are using the hash to return a single
  scalar value.

13
Examples

• If you are looking to return a number of values,
  you would use the array _at__at_arrayVal
  _at_hashNamekey1, key2You are returning
  multiple (array of) values.

14
Example

• Supposing that you wanted to extract a subset of
  one array._at_arraySub _at_hashName_at_arrayExtrYo
  u have an array called _at_arrayExtr which contains
  the keys you want to pull. This gets filtered
  through the hash and the result is placed in
  _at_arraySub

15
Initializing a Hash

• The first thing to do is initialize the space.

16
Creating the Hash

• Now lets add data. You can do it one at a time or
  several at a time

17
Multiple entries can be a pain

• Try this instead

• Called the comma-arrow operator.

18
Loading and Searching Hashes

• Weve already loaded the hash,
• Now lets print what we have

19
The Result
Shark returned Jaws
keys in a loop printed each
20
Or
Returns
21
Add a single item to an existing hash
prodPrice (Crab gt 5, Tuna
gt 7, Shrimp gt 4) prodPrice'Be
ef' 10 foreach line(keys prodPrice)
print "line \n"
22
Referencing a single item in a hash
prodPrice (Crab gt 5, Tuna gt
7, Shrimp gt 4) print
"prodPrice'Crab' \n\n" name 'Crab' value
prodPricename print "value \n\n"
Literal
Variable
23
Two Great Sites

• http//www.devshed.com/c/a/Perl/Hash-Mania-With-Pe
  rl/1/
• http//www.cs.mcgill.ca/abatko/computers/programm
  ing/perl/howto/hash/

24
Removing a key from the Hash
Note the syntax of the delete statement
Delete on the item with thekey value of Shark
25
Results
26
Now

• You can locate a single known item from the hash
  with the simple commandvalue
  animalsSharkwhich returned the value of
  Jaws.

27
Now

• This time, lets check to see if the hash value
  exists

28
Results

• Notice that Perl did not fail, it just printed
  blanks

• So you will want to check the values existence
  rather than assume

29
Check for Existence

• Replace this into your program

30
Check for Existence

• Then add this code next

Run it
31
Results

• When you check the solution

32
Lab 5

• In Lab 5 you will create an application that will
  check the value of quantity on hand for a
  particular item on demand.
• Create a hash with the products and quantities.
  You select the products and quantity values. You
  must have at least 20 items in the hash.

33
Lab 5

• Create a user interface.
• Include the company name
• Identification of the program
• List of available items
• A prompt for the user to enter the item name
• Validate the item name
• If the item does not match, display a message
  similar to we do not carry that item
• Prompt the user to enter another or quit

34
Lab 5

• When you locate the item requested
• Allow the user to change the on hand quantity or
  not
• Validate the change amount
• Verify that the amount can be deducted from the
  OHQ
• If OHQ 10 do not allow deduction of 20
• NO NEGATIVE OHQ!
• If valid, deduct the amount and print the hash
• If not valid, return an error message and prompt
  to try again or quit.

35
Additional Fun

• You can also make 2 dimensional arrays in Perl
• A 2D array is essentially a hash where the value
  is an array also.
• You have to think about it like a table of values
  where all of the data on each row is dependant on
  the key of that row
• Unlike a table, you cannot identify the elements
  of the 2nd dimension by name

36
Think like this
Item Name Cost Per Qty on Hand Qty on Hand Reorder Point
Crab 5 5 100 25
Tuna 6 6 50 10
Shrimp 4 4 75 10
Salmon 10 10 60 5
50 depends on Tunaor, 50 lbs refers to the
quantity on handof Tuna only
The data on the row is dependanton the key value
of the row
37
Think like this
If I wanted to know how many lbs ofTuna I had, I
would start with the Key and work across
Item Name Cost Per Qty on Hand Qty on Hand Reorder Point
Crab 5 5 100 25
Tuna 6 6 50 10
Shrimp 4 4 75 10
Salmon 10 10 60 5
38
Using this technique

• If we increased the number of offerings to
  ltunlimitedgt
• We would not have to ask for order quantities for
  every single item
• Rather, the user would enter the item to purchase
  (say Tuna),
• And after appropriate validation, the program
  returns the price and on hand qty

39
Using this technique

• Additionally,
• You store all of your data in one area
• You do not have to match single value arrays to
  get all of the information that you need.
• Array processing is less cumbersome,
• But,
• You need to remember the element order of the
  values.
• Or, you could accidently charge the qty on hand
  as the price per pound