1. Lecture 20 – Practice Exercises 4
CMPT 120
Lecture 20 – Practice Exercises 4

2. Weekly Exercise 7 - Feedback
Feedback on our algorithms of last Friday!
Question 1 - Problem Statement:
Given 3 numbers, find the largest one.
Solution – Algorithm:
Get three numbers -> number1, number2, number3
Set maxNumber to number1
if number2 > maxNumber
Set maxNumber to number2
if number3 > maxNumber
Set maxNumber number3
To help you create a
solution, use examples.
Using 15, 30, 45, here are all possible permutations:
number1
number2
number3 15 30 45

3. Weekly Exercise 7 - Feedback
These are NOT acceptable answers: 
Python program
A solution that says:
Get 3 numbers
Compare these 3 numbers and select the largest
Display/print the largest
Assign 3 numbers to variables
Include functions that convert the user’s response into a number
Create conditions that seeks out the largest value out of the 3
Give the user 3 numbers: 3, 8, 2
Ask the user which number is the largest
Let the user know whether s/he has selected correctly

4. Goals for Today!
Get better acquainted with recursion by box tracing the execution of a recursive program
Practise designing a solution (algorithm) to a problem
Recursive as well as non-recursive solution
Practise implementing a program (solution) described by an algorithm

5. Question 1 Go to https://repl.it/repls/EthicalSaddlebrownPentagon
Fork this link (press on the Fork button on the left of the Run button) -> what does this do?
and answer the questions on the given sheet (next 2 slides)
In order to answer these questions, you may wish to box trace the code by either
Copying the program into the Python Code Visualizer and visualizing its execution OR
Hand trace the execution of the program by drawing your own boxes (box tracing)
Either way, the goal is to represent using a box the execution of the function every time it is called (called “frames” in the Python Code Visualizer) and use this box to keep track of the values of local variables, parameters and returned value

6. Name: ________________________________
Student number: ______________________
Question 1
1. What does the program produce (print on the screen) when it has completely executed?
2. What is the purpose of func(…)? Be as specific as possible.
3. What is the value of the argument of func(…) when it is called for the 3rd time? Remember: an argument is the value you put within the ( ).
4. What is the data type of the value the function func(…) returns?
5. What is the base case of the function func(…)?
The base case is (occurs) when
if len(a) > 0 : is false. Then, as
part of the base case, the else
and the return statements are executed
Result: 3 in [5, 3, 8, 9]
It returns the number of odd integers in its parameter a.
a -> [8, 9]
int (i.e., integer)
Base case:
if len(a) > 0 :
...
else : b = 0
return b

7. Question 1 6. What value does the 6th execution of func(…) return?
7. What happens when the recursive case of func(…) is reached?
8. What happens when the base case of func(…) is reached?
9. What is the value of a[0] during the 4th execution of func(…)?
10. Which execution of func(…) adds the value 2 to the value 1?
There is no 6th execution of func(…) .
func( ) calls itself with a list that contains all elements of its parameter a except the first element. This way, the argument to func( ) is getting smaller at every call and eventually reaches the length 0. that is when the base case is reached.
func( ) assigns the value 0 to the variable b and returns the value of b.
The value of a[0] is 9.
The value 2 is added to 1 during the first execution of func( ), i.e., the first time we call func( )).

8. Question 2 Problem Statement:
Possible solution:
Problem Statement:
Write a Python function that reverse a string iteratively -> using a loop (iteration) as opposed to recursion
How to proceed – use Repl.it:
Write a header
Design your solution (algorithm) and write it as comment in your program
Write down, as comments, the test cases you will use to test your program
Translate your comments into Python statements
Execute and test your program

9. Question 3 Problem Statement:
Possible solution:
Problem Statement:
Write a Python function that reverse a string recursively
How to proceed – use Repl.it:
As part of the header
Write down the recursive description of a string
(The reason you are asked to do this is to make clear in your mind the recursive nature of a string)
As a comment in your program
Write down the base case
Write down how the string is getting smaller at every recursive call
Write down the test cases you will use to test your program - use the test cases from Question 2
Then write your program and test it