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An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #6:
Nested Branches,
Multiple Alternatives
Sample “IF” Problem
In a scheduling program, we want to check whether two
appointments overlap. For simplicity, appointments start at a
full hour, and we use military time (with hours 0–24).
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“Hand Tracing”
• Hand Tracking is a technique that you can use in order to determine if your program is working correctly.
• Mentally execute your program’s statements and keep track of the value of each variable.
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Example: Let’s Talk About TaxesFederal Tax Rate Schedule
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Hand Tracing The Tax Program
Tax 1 Tax 2 Income MaritalStatus
80000 m
Hand Tracing The Tax Program
Tax 1 Tax 2 Income MaritalStatus
0 0 8000 m
Hand Tracing The Tax Program
Tax 1 Tax 2 Income MaritalStatus
0 0 8000 m
6400 4000
XXX XXX
Hand Tracing The Tax Program
Tax 1 Tax 2 Income MaritalStatus
0 0 8000 m
6400 4000 10400
XXX XXX
What Is A “Nested Branch”?
• It is often necessary to include an if statement inside another. Such an arrangement is called a nested set of statements.
• Example:
Is the club full?
Arrive at
the club
Are you on the
VIP list?
Wait in car
Go right in
Wait in lineY
Y
N
N
Example of a “Nested Branch”
if (peopleInClub < maxPeopleInClub) :
if (youName == VIPListName) :
goRightIn
else :
waitInLine
else :
waitInCar
Multiple Alternatives
• What should you do when you have to make a decision with more than one alternative?
• Example: Telling a student what their grade in a class was…
Example: Telling Students What Their Grade In A Course Is
If (classScore >=90) :
print(“You got an A!”)
else:
if (classScore >= 80) :
print(“You did ok, you got a B!”)
else:
if (classScore >=70) :
print(“So-so, you got a C”)
else:
if (classScore >= 60) :
print(“Oh –oh, you got a D”)
else :
print(“Dang it, you got an F”)
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Problems With “Super Nesting”
• Difficult to read
• Shifted too far to the right due to indentation
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A Better Way: elif
If (classScore >=90) :
print(“You got an A!”)
elif (classScore >= 80) :
print(“You did ok, you got a B!”)
elif (classScore >=70) :
print(“So-so, you got a C”)
elif (classScore >= 60) :
print(“Oh –oh, you got a D”)
else :
print(“Dang it, you got an F”)
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Note that you have to test the more specific conditions first.
Example: Supermarket Coupons
A supermarket awards coupons depending on how much a customer spends on
groceries. For example, if you spend $50, you will get a coupon worth eight percent
of that amount. The following table shows the percent used to calculate the coupon
awarded for different amounts spent. Write a program that calculates and prints the
value of the coupon a person can receive based on groceries purchased.
What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif
What We Covered Today
1. Hand Tracing
2. Nested Branch
3. elif
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What We’ll Be Covering Next Time
1. While Loop
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