1. CMPT 120 Lecture 13 – Unit 2 – Cryptography and Encryption –
The realm of secret codes
Python – Functions and While loop

2. Functions

3. Last lecture we saw that …
Function
Takes
parameters
or not
Returns
a value
or not
Python code fragment that does one specific task
A function has a descriptive name

4. Last lecture, we were about to …

5. Execution Flow with Functions
Let’s introduce a new tool:
Python code visualizer

6. Different kinds of functions
Activity: Fill this table using functions we have used so far!
Function takes parameters
Function does not take parameters
Function returns a result (i.e., a returned value)
Function does not return a result
(i.e., a returned value)

7. Are there “functions” in the real world?
Mom says to Louise “Please, can you go make your bed?“
Specific task: Make bed
Parameter(s): None required
Returned value: None returned
Mom says to Louise “Here’s $5, please, can you go buy a bag of apples?“
Specific task:
Parameter(s):
Returned value:

8. Are there “functions” in the real world?
Mom says to Louise “Please, can you find my cell phone?“
Specific task:
Parameter(s):
Returned value:
Mom says to Louise “Please, can you put these bags on the kitchen counter?“
Specific task: Put bags on kitchen counter

9. Homework – Using Repl.It …
Let’s create a function that returns the largest number out of 3 numbers
Requirements:
cannot use the max( ) built-in function
cannot use the word max to name your function
Let’s create a function that returns the sum of the numbers in a list
Let’s create a function that prints “*” 5 times in a row

10. Review - Function Syntax of function definition
def <functionName>( [parameter(s)] ) :
< 1 or more statements >
return [expression]
def -> means “here is the definition of a function”
<functionName> (Good Programming Style - GPS)
Function name is descriptive -> it describes its purpose
Function name syntax: same as for variable name syntax
Function definition ->
Header of a function ->
Body of a function ->
One return statement

11. While loop

12. Activity
Add a while loop to our encryption/decryption program that allows the user to encrypt/decrypt messages until s/he enters ‘S’ or ‘s’

13. Back to our Guessing Game
Can we fix our Guessing Game?

14. Review - Syntax of a while loop
<stmt: initialize condition variable> while <Boolean condition> : <first statement to be repeated> <second statement to be repeated> ... <stmt: modify condition variable> <statement outside (after) the loop>

15. Review - Syntax of a while loop
<stmt: initialize condition variable>
while <Boolean condition> :
<first statement to be repeated>
<second statement to be repeated>
...
<stmt: modify condition variable>
<statement outside (after) the loop>
Important – About Indentation
Statements inside the loop (i.e., statements executed at each iteration of the loop) are the statements indented with respect to the while keyword
Statements outside the loop (before and after the loop) are the statements that are not indented with respect to the while keyword – these statements are considered to be at the same level of indentation as the while loop

16. Review - Difference between while and for loops

17. When best to use a while loop
If we know there is a condition that will occur in our program and its occurrence will dictate when we stop repeating a set of statements, then we use the iterative statement called while loop
That condition is often called a sentinel or flag
Examples:
User termination
User enters yes/no (flag) or some special value (sentinel)
User selects ‘X’ to eXit from a menu (menu-driven program)
Occurrence of an error
Reading data from a file -> EOF

18. Let’s challenge ourselves!
Let’s convert the previous while loop using the most appropriate iterative statement?
fruit = ["banana", "apple", "plum"]
index = 0
while index < len(fruit):
print(fruit[index])
index = index + 1

19. GPS: Not to be used in this course!
while True :
while 1 :
break
continue
pass
Why?

20. Next Lecture We are having our Midterm 1 in B9201 at 9:30am
See you there!