1. Introduction to Decision Structures and Boolean Variables

2. Warm-Up
Write a program that asks the user for their hourly rate of pay.
Then, ask the user how many hours they worked this week and calculate and print their wages for the week.
Format accordingly

3. Sequence Structures
Thus far, we’ve been programming “sequence structures”
That just means that the program will execute the statements in the order in which they appear in the source code
We would now like for programs to deviate from the linear structure to adapt according to conditions being met, or not met

4. The Problem
If you remember, when we were calculating the car payments, we left an option at the bottom of the code for if the price they were given was too high.
The problem was, it wasn’t really an option … it was more like a forced secondary deal
So, how do we get Python to make decisions …?

5. The Selection Statement
Allows program to “ask a question” and respond accordingly
Simplest form: perform an action only if a certain condition is met
If the condition is not met, then the action is not performed (we will learn how to accommodate for non-met conditions with a separate action later on)

6. The Selection Statement
In this program, we start by asking a question, “is it cold outside?”
If the answer is yes (aka “True”) then we execute an alternate set of commands
Otherwise, we continue with the program as-is

7. The Selection Statement
In Python, we use the keyword “if” to start a selection statement
We must also use colons to end the selection statement
The block of code reserved to execute if and only if the condition is met, must be indented within the selection statement (nested structure)

9. Writing a condition
Python doesn’t read the way you would, so you can’t just write your conditions as questions like, “Is it cold outside?”
The key is to ask a question and then compare the returned value (the answer to your question) to another value
This comparison is our condition
Each of these conditions must hold either “True” or “False”

10. Boolean Expressions

11. Example
answer = input(“Is it cold outside?”) if answer == “yes” : # this will only execute IF and only IF the condition holds TRUE print(“Go put on a jacket!”) # then the program will return to it’s original line of code print(“Go outside.”)

12. Boolean Expressions
Writing a condition

13. Boolean Expressions
Named after George Boole, a 19th century English philosopher and mathematician
Boole developed a system of mathematics that allows us to work with the concepts of “True” or “False”
Boole is considered one of the founders of modern computer science, as his work is reflected in the way computers process binary data

14. Boolean Expressions
Boolean expressions can be used as the condition of an “if” selection statement
They are generally formed using relational operators which allow you to test whether a specific relationship exists between two (or more) values

15. Relational Operators A > B # A is greater than B
A < B # A is less than B
A == B # A is equal to B
A >= B # A is greater than OR equal to B
A <= B # A is less than OR equal to B

16. Writing a Boolean Expression
charizard = 10 pikachu = 7 if charizard > pikachu: # charizard > pikachu print( “It seems that your # 10 > 7 Pikachu is no match for the # True, condition met almighty Charizard” )

17. Practice
# given these variables # evaluate A = 99 A > B B = 7 B < C C = -5 B >= C D = 92 C <= D A == B + D D >= A - C C != B

18. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C C = -5 B >= C D = 92 C <= D A == B + D D >= A - C C != B

19. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C D = 92 C <= D A == B + D D >= A - C C != B

20. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C True D = 92 C <= D A == B + D D >= A - C C != B

21. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C True D = 92 C <= D True A == B + D D >= A - C C != B

22. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C True D = 92 C <= D True A == B + D True D >= A - C C != B

23. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C True D = 92 C <= D True A == B + D True D >= A - C False C != B

24. Practice
# given these variables # evaluate A = 99 A > B True B = 7 B < C False C = -5 B >= C True D = 92 C <= D True A == B + D True D >= A - C False C != B True

25. More Boolean Operators
Don’t confuse “==“ with “=“:
“=“ is used to assign variables
“==“ is used to test if two values are equivalent
We use “!=“ to test if two values are different (“not equal to”)
“<=“ and “>=“ test for more than one relationship at a time

26. Challenge: Guess the Magic Number
Write a program that sets a magic number, anywhere from 1-10.
Then, ask the user to guess a number.
If they guess correctly, then print out “You guessed correctly, the number was __!”
If not, tell them, “Sorry, the magic number was __.”

27. Calculating Overtime
In America, if an employee works more than 40 hours in a week, he/she is entitled to overtime pay
Overtime pay is calculated at the rate of 1.5 times the normal hourly rate
The additional rate is only applied to the hours worked above 40 hours

28. Calculating Overtime Input: hourly rate of pay
Input: number of hours worked in a week
If hours worked is less than 40, simply multiply hours worked by hourly rate
If hours worked is greater than 40:
Multiply hourly rate by 40
Subtract 40 from hours worked
Multiply overtime hours by 1.5 times hourly rate
Add overtime pay to base pay

29. Challenge: Overtime Pay
Write a program that asks the user for an hourly rate of pay and the number of hours they’ve worked
If the number of hours worked is over 40, then calculate for overtime pay at 1.5 the original rate
Then show the user how much money they made for the week

30. Challenge: Overtime Pay
HINT: you may want to set a variable for the overtime pay and set it equal to zero, before the selection statement
This way, the overtime pay is only changed if the condition is met
You may want to do the same with the base

31. The IF-ELSE Statement
We wrote “IF” statements to check for a condition, and our programs followed the order of the program according to whether the conditions held “True” or “False”
Now, we will add in another branch of execution based on the value of the Boolean expression

32. The IF-ELSE Statement

33. The IF-ELSE Statement
We can do this by adding the reserved word “else”
This tells the program to check for the “if” condition and if it is not satisfied, run the code branched under the “else” statement
You need to add a colon at the end of each else statement

34. The IF-ELSE Statement
Previously, we had to use two “if” statements like this:
if hours > 40:
print(“Your total is”, (40*rate) + ( (hours - 40)*1.5*rate) )
if hours <= 40:
print(“Your total is”, hours*rate)

35. The IF-ELSE Statement
Now, with the “else”, we can write our codes like this:
if hours > 40:
print(“Your total is”, (40*rate) + ( (hours - 40)*1.5*rate) )
else:
print(“Your total is”, hours*rate)