Introduction to Decision Structures and Boolean Variables

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

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

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 …?

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)

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

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)

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”

Boolean Expressions

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.”)

Boolean Expressions Writing a condition

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

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

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

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” )

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

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

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

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

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

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

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

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

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

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 __.”

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

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

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

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

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

The IF-ELSE Statement

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

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)

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)