1. Computer Science 2
Hashing

2. Learning Objectives Understand the benefits of hashing
Be able to generate a value based a string
Be able to write a function to generate a hashed value.
Add information into an array based on Hashed their hashed value.

3. Welcome Back Last week… Hashing Benefits Deficits Why What is hashing?
What do you recall about the algorithm we used last class to find a hashed value for a string?

4. Hashing Function Review: Using Base 2
Find the Ordinal value for each character and subtract 64 so that
‘A’ = 1, ‘B’ = 2, ‘C’=3, …
Ord(upcase(stringVariable[position])) – 64
Use Base 2 for the place values, like in Binary Numbers. AB = *2^1 + 2* 2^0 = = 4
BA = *2^1 + 1* 2^0 = = 5
CAB = 2*2^2 + 1*2^1 + 2* 2^0 = =12
Then MOD by this size of the array.
Hands on with Algorithm
For each of the following, find the hashed value. Use an array size of 5. 
Names Value Value MOD 5
AC _______ ________
JC ________ ________
EG ________ ________
BIL ________ ________

5. What about the array? How big should you make the array?
How do you initialize the array?
How do you put elements into the array?
What if there is someone already at the position?
How do you remove someone from the array?

6. Add the values from the Warm up into an array.
PSEUDOCODE: ADD
Get info
Hash the key field {The name}
Found = false {‘Cause you haven’t found where to put it.}
count = 0 {Counting to make sure time isn’t wasted when the array is full.}
repeat
if currentname (theArray[hashedValue].name) = ‘Empty’ or Deleted
put in the info at this spot
found = true {‘Cuz you’ve found where to put it}
else if name = currentname
Tell the user “It already exists”
found = true {You found the person}
else
rehash
add one to count
until (found) or (count is too big)
Add the values from the Warm up into an array.
Use +2 for rehashing.

7. PSEUDOCODE SHOW ONE Get info Hash the key field {The name}
Found = false {‘Cause you haven’t found where to put it.}
count = 0 {Counting to make sure time isn’t wasted when the array is full.}
repeat
if currentname (theArray[hashedValue].name) = ‘Empty’
Show “The person is not in the list”
found = true {‘Cuz you’ve found where to put it}
else if name = currentname
Show the information about the person
found = true {You found the person}
else
rehash
add one to count
until (found) or (count is too big)

8. PSEUDOCODE: DELETE ONE
Get info
Hash the key field {The name}
Found = false {‘Cause you haven’t found where to put it.}
count = 0 {Counting to make sure time isn’t wasted when the array is full.}
repeat
if currentname (theArray[hashedValue].name) = ‘Empty’
Show “The person is not in the list”
found = true {‘Cuz you’ve found where to put it}
else if name = currentname //What if someone had to rehash from this spot?
Delete the person (Copy ‘Deleted’ to the name field)
found = true {You found the person}
else
rehash
add one to count
until (found) or (count is too big)
Dry run with the warm up. Delete JC.

9. Assume all capital letters for this activity.
Dry Running the Pseudo Code: