Decision Structures and Boolean Variables

Sequence Structures Thus far, we’ve been programming “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 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 We would now like for programs to deviate from the linear structure to adapt according to conditions being met, or not met

Calculating Overtime For instance, if an employee works more than 40 hours in a week, he/she is entitled to overtime pay For instance, 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 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 The additional rate is only applied to the hours worked above 40 hours

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

Calculating Overtime Currently, we don’t know enough on Python to deviate from a linear sequence structure Currently, we don’t know enough on Python to deviate from a linear sequence structure We cannot account for when the user inputs a total amount of hours worked that is above 40 to pay for overtime We cannot account for when the user inputs a total amount of hours worked that is above 40 to pay for overtime

The Selection Statement Allows program to “ask a question” and respond accordingly Allows program to “ask a question” and respond accordingly Simplest form: perform an action only if a certain condition is met 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) 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?” 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 If the answer is yes (aka “True”) then we execute an alternate set of commands Otherwise, we continue with the program as-is Otherwise, we continue with the program as-is

The Selection Statement Python will read your inputs of “yes” and “no” as holding the values of “True” or “False” Python will read your inputs of “yes” and “no” as holding the values of “True” or “False”

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

Boolean Expressions WRITING A CONDITION

Writing a condition Python doesn’t read the way you would, so you can’t write your conditions as questions like, “Is it cold outside?” Python doesn’t read the way you would, so you can’t write your conditions as questions like, “Is it cold outside?” The key is to write a selection statement in a way Python might understand and that matches the question you are trying to ask The key is to write a selection statement in a way Python might understand and that matches the question you are trying to ask All selection statements must have a condition to be “tested” as either True or False All selection statements must have a condition to be “tested” as either True or False Think about asking questions that can be answered using “yes” or “no” Think about asking questions that can be answered using “yes” or “no”

Boolean Expressions

Named after George Boole, a 19 th century English philosopher and mathematician Named after George Boole, a 19 th century English philosopher and mathematician Boole developed a system of mathematics that allows us to work with the concepts of “True” or “False” 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 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 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 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 greater than B A < B # A is less than B A < B # A is less than B A == B # A is equal to B A == B # A is equal to B A >= B # A is greater than OR equal to B A >= B # A is greater than OR equal to B A <= B # A is less than OR equal to B A <= B # A is less than OR equal to B

Boolean Expressions All Boolean expressions in Python, we say “evaluate” to either “True” or “False” All Boolean expressions in Python, we say “evaluate” to either “True” or “False”

Writing a Boolean Expression sticks_stones = 10 words = 7 if sticks_stones > words: # sticks_stones > words print( “Sticks and stones# 10 > 7 may break my bones, but# True, condition met words will never hurt me” )

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 < CFalse C = -5 B >= C True D = 92 C <= D True A == B + D True D <= A + C True C != BTrue

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

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

Challenge: Overtime Pay Write a program that asks the user for an hourly rate of pay and the number of hours they’ve worked 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 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 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 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 This way, the overtime pay is only changed if the condition is met You may want to do the same with the base You may want to do the same with the base