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

Functions

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

Last lecture, we were about to … https://repl.it/repls/TimelyJovialAssociate

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

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)

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:

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

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

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

While loop

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’

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

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>

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

Review - Difference between while and for loops

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

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

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

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