1. CMPT 120 Lecture 29 – Unit 5 – Internet and Big Data
Algorithm – Searching and Sorting

2. Lists of Lists - Homework
How can we create myMatrix?
How can we access each of its elements?
myMatrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
How can we slice it?
How can we modify its elements?

3. First, the basic algorithm: Linear Search
Searching
First, the basic algorithm: Linear Search

4. Last Lecture – We gave it a try!
Is in this sequence?
Our reading calls it “The Sequential Search”
“target”

5. Last Lecture - Algorithm of Linear Search
For each element Is element == target? If so, found!

6. Another Example of Linear Search Algorithm
linearSearch(list, target)
set result to value TARGET_NOT_FOUND
set targetNotFound to value true
if list not empty
set currentElement to first element of list
while targetNotFound AND have not looked at every element of list
if currentElement == target
set result to current element
set targetNotFound to false
otherwise
set currentElement to next element of list
return result

7. Implementation of Linear Search
Using Repl.It

8. Let’s test it!
ourList = [ 0, 6, 9, 2, 5, 3, 7, 1, 2, 4 ] target: 2

9. What other test cases can we use?
ourList target

10. Observations Our linear search so far:
Finds the first occurrence of the target
What if the target occurs many times in the list
Returns True/False
What else could it return?

11. Linear search algorithm
Advantages
Simple to understand, implement and test
Disadvantages
Slow (time inefficient) because it looks at every element
Wait a minute! Not always!
We saw that for some of our test cases linear search did not look at every element
That is true! We’ll come back to this real soon!

12. “Time Efficient” Searching?

13. Hey! How about if we sort first!
Difficult to find things (CD’s) when they are disorganized
It could be quick (first CD we look at) or slow (last CD we look at)
Sorting
Although it will take some time to sort, it will make searching for a CD much quicker, every time!
Unpredictable!

14. First, the basic algorithms: Bubble, Selection and Insertion Sort
Sorting
First, the basic algorithms: Bubble, Selection and Insertion Sort

15. Sorting Visualization and Videos!
There are plenty of sorting visualization web sites and videos on the Internet to help you understand these algorithms

16. Selection Sort Algorithm
How it works:
Repeatedly selects the next smallest (or largest) element from the unsorted section of the list and swaps it into the correct position in the already sorted section of the list:
for index = 0 to len(data) – 2 do
select: let x = location of element with smallest value
in data from index to len(data) – 1
if index != x
swap: tempVar = data[index]
data[index] = data[x]
data [x] = tempVar

17. In-Class Demo

18. Next Lecture Let’s have a look at one more sorting algorithm:
insertion sort
Then let’s have a little activity which will allow us to discover a more efficient way of searching 