Selection and Iteration

Week 5 – Selection and Iteration

Whatisin thislesson?

1. The Decision Structure
2. The Relational Operators
3. The if Statement
4. The if-else Statement
5. Logical Operators
6. The Match Case in Python version 3.0
7. Introduction to Repetition Structures
8. The while Loop
9. The for Loop:
10. Processing List with Menu 11th January 2026 Tram Tran

Decision Structures and Boolean Logic

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1. The Decision Structure A control structure is a logical design that controls the order in which a set of statements execute.
2. A sequence structure is a set of statements that execute in the order that they appear.
3. A decision (or selection) structure is a control structure that can execute a set of statements and perform a specific action only if a certain condition exists. sequence decision

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The Decision Structure Decision (selection) structure: Diamond symbol represents a true/false condition Single alternative decision structure provides only one alternative path of execution.

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The Relational Operators A relational operator determines whether a specific relationship exists between two values. Table 4-1 Relational operators

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The Relational Operators Boolean Expressions and Relational Operators Table 4-2 Boolean expressions using relational operators

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The if Statement Concept: The if statement is used to create a decision structure, which allows a program to have more than one path of execution. The if statement causes one or more statements to execute only when a Boolean expression is true.

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The if Statement Decision (selection) structure: In Python … if condition: statement statement statement etc.

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The if-else Statement Concept: An if-else statement will execute one block of statements if its condition is true, or another block if its condition is false.

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The if-else Statement A dual alternative decision structure provides two possible paths of execution – one path is taken if a condition is true, and the other path is taken if the condition is false.

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The if-else Statement Dual alternative In Python … if condition: statement statement etc. else: statement statement etc.

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Nested Decision Structures and the if-elif-else Statement Concept: To test more than one condition, a decision structure can be nested inside another decision structure.

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Nested Decision Structures and the if-elif-else Statement

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Nested Decision Structures and the if-elif-else Statement The if-elif-else Statement if condition_1: statement statement etc. elif condition_2: statement statement etc. else: statement statement etc. if score < 60: print ‘Your grade is F.’ elif score < 70: print ‘Your grade is D.’ elif score < 80: print ‘Your grade is C.’ elif score < 90: print ‘Your grade is B.’ else: print ‘Your grade is A.’

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Logical Operators Concept: The logical and operator and the logical or operator allow you to connect multiple Boolean expressions to create a compound expression. The logical not operator reverses the truth of a Boolean expression.

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Logical Operators Table 4-3 Logical operators

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Logical Operators Table 4-4 Compound Boolean expressions using logical operators

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Logical Operators The and Operator if temperature < 20 and minutes > 12: print ‘The temperature is in the danger zone.’ Table 4-5 Truth table for the and operator

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Logical Operators The or Operator if temperature < 20 or temperature > 100: print ‘The temperature is too extreme.’ Table 4-6 Truth table for the or operator

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Logical Operators The not Operator if not(temperature > 100): print ‘This is below the maximum temperature.’ Table 4-7 Truth table for the not operator

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Logical Operators Checking Numeric Ranges with Logical Operators

if x >= 20 and x <= 40:

print ‘The value is in the acceptable range.’ if x < 20 or x > 40: print ‘The value is outside the acceptable range.’ # This is an error! if x < 20 and x > 40: print ‘The value is outside the acceptable range.’

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Boolean Variables Concept: A Boolean variable can reference one of two values: True or False. Boolean variables are commonly used as flags, which indicate whether a specific condition exists.

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Boolean Variables

if sales_quota_met == True:

print ‘You have met your sales quota!.’ if sales_quota_met: print ‘You have met your sales quota!.’ OR

if sales >= 50000:
sales_quota_met = True

else:

sales_quota_met = False

1. Match Case in Python version 3.0 1-25

AnyQuestion?

Repetition Structures

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1. Introduction to Repetition Structures

Concept: The repetition structure causes a statement or set of statements to execute repeatedly.

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Introduction to Repetition Structures Condition-Controlled and Count-Controlled Loops A condition-controlled loop uses a true/false condition to control the number of times that it repeats. A count-controlled loop repeats a specific number of times.

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The while Loop: a Condition-Controlled Loop Concept: A condition-controlled loop causes a statement or set of statements to repeat as long as a condition is true. In Python you use the while statement to write a condition-controlled loop.

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The while Loop: a Condition-Controlled Loop In Python … while condition: statement statement etc. The logic of a while loop

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The while Loop: a Condition-Controlled Loop The while Loop is a Pretest Loop, which means it tests its condition before performing an iteration.

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The for Loop: a Count-Controlled Loop Concept: A count-controlled loop iterates a specific number of times. In Python you use the for statement to write a count-controlled loop.

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The for Loop: a Count-Controlled Loop In Python … for variable in [value1, value2, etc.]: statement statement etc.

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The for Loop: a Count-Controlled Loop Using the range Function with the for Loop for num in [0, 1, 2, 3, 4]: print num for num in range(5): print num

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The for Loop: a Count-Controlled Loop Using the range Function with the for Loop

• First argument, 1, is the starting value for the list
• Second argument, 10, is the ending limit of the list
• Third argument, 2, is the step value for num in range(1, 10, 2): print num

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The for Loop: a Count-Controlled Loop Generating Lists that Range from Highest to Lowest for num in range(5, 0, -1): print num

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The Augmented Assignment Operators The Augmented Assignment Operators Table 5-2 The age variable references the value 25

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Input Validation Loops Concept: Input validation is the process of inspecting data that has been input to a program, to make sure it is valid before it is used in a computation. Input validation is commonly done with a loop that iterates as long as an input variable references bad data.

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Input Validation Loops Error Trap or Error Handler

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Input Validation Loops Priming Read # Get a test score.

Score = input(‘Enter a test score: ‘)

# Make sure it is not less than 0. while score < 0: print ‘ERROR: The score cannot be negative.’

score = input (‘Enter the correct score: ‘)

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Nested Loops Concept: A loop that is inside another loop is called a nested loop.

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Nested Loops About Nested Loops:

• The inner loop goes through all of its iterations for every single iteration of the outer loop
• Inner loops complete their iterations faster than outer loops for hours in range(24): for minutes in range(60): for seconds in range(60): print hours, ‘:’, minutes, ‘:’, seconds

Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

QUESTIONS

? Repetition Structures

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1. Processing Lists with control structure

Processing Lists: Totaling the Values in a List Program 8-16 (total_list.py)

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Processing Lists with control structure Averaging the Values in a List Program 8-17 (average_list.py)

Processing Lists with control structure

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Search a Value in a List

Menu -1

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How the Menu System Works

• The program uses a menu-driven approach to control execution.
• A Main Menu is displayed inside a while loop and repeats until the user selects Exit.
• User input is validated using read_integer_in_range.
• The program uses match / case to decide which function to run.
• A Sub Menu (List Processing Menu) works the same way:
• It has its own loop.
• Each option calls a separate function.
• Choosing “Return to Main Menu” exits the sub menu.
• Input functions are stored in a separate file, keeping the menu logic clean and easy to read. =============================== # Main Menu # =============================== def main(): finished = False while finished == False: print("\nMain Menu:") print("1 List Processing") print("2 Other Feature") print("3 Exit") choice = input_functions.read_integer_in_range("Please enter your choice:", 1, 3) match choice: case 1: list_processing_menu() case 2: other_feature() case 3: finished = True case _: print("Please select again") main()

Menu -2

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import input_functions

# ===============================

# List Processing Sub Menu

# ===============================

def list_processing_menu():

finished = False
while finished == False:

print("\nList Processing Menu:") print("1 Enter a list") print("2 Calculate total") print("3 Calculate average") print("4 Search in list") print("5 Return to Main Menu")

choice =

input_functions.read_integer_in_range("Please enter your choice:", 1, 5) match choice: case 1: enter_list() case 2: calculate_total() case 3: calculate_average() case 4: search_item() case 5:

finished = True

case _: print("Please select again")

# ===============================

# Stub functions (EXPLANATION ONLY)

# ===============================

def enter_list(): print("This is the code for ENTERING a list.") input("Press Enter to continue") def calculate_total(): print("This is the code for CALCULATING the total of a list.") input("Press Enter to continue") def calculate_average(): print("This is the code for CALCULATING the average of a list.") input("Press Enter to continue") def search_item(): print("This is the code for SEARCHING an item in a list.") input("Press Enter to continue")

# ===============================

# Option 2 – Other Feature (Stub)

# ===============================

def other_feature(): print("This is the code for OPTION 2 in the Main Menu.") input("Press Enter to continue")

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# ===============================

# Main Menu (Ruby-style)

# ===============================

def main():

finished = False
while finished == False:

print("\nMain Menu:") print("1 List Processing") print("2 Other Feature") print("3 Exit")

choice = input_functions.read_integer_in_range("Please enter

your choice:", 1, 3) match choice: case 1: list_processing_menu() case 2: other_feature() case 3:

finished = True

case _: print("Please select again") main()

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import input_functions

# ===============================

# Input list function

# ===============================

def input_number_list():

numbers = []
count = input_functions.read_integer_in_range("How many

numbers do you want to enter?", 1, 100) while i < count:

value = float(input(f"Enter number {i + 1}: "))

numbers.append(value) return numbers

# ===============================

# Processing functions # (return only, no print)

# ===============================

def total_list(numbers): while i < len(numbers):

total += numbers[i]

return total def average_list(numbers): return total_list(numbers) / len(numbers) def search_list(numbers, target): while i < len(numbers):

if numbers[i] == target:

return True return False

Process List(full code) with Menu -3

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# ===============================

# List Processing Sub Menu

# ===============================

def list_processing_menu():

finished = False
numbers = []
while finished == False:

print("\nList Processing Menu:") print("1 Enter a list of numbers") print("2 Calculate total of the list") print("3 Calculate average of the list") print("4 Search for a value in the list") print("5 Return to Main Menu")

choice = input_functions.read_integer_in_range("Please

enter your choice:", 1, 5) match choice: case 1:

numbers = input_number_list()

case 2:

if len(numbers) == 0:

print("The list is empty. Please enter a list first.") else: print("The total of the elements is", total_list(numbers)) input("Press Enter to continue") case 3:

if len(numbers) == 0:

print("The list is empty. Please enter a list first.") else: print("The average of the elements is", average_list(numbers)) input("Press Enter to continue") case 4:

if len(numbers) == 0:

print("The list is empty. Please enter a list first.") else:

value = float(input("Enter a value to search for:

")) if search_list(numbers, value): print(value, "was found in the list.") else: print(value, "was not found in the list.") input("Press Enter to continue") case 5:

finished = True

case _: print("Please select again")

# ===============================

# Option 2 – Other Feature (Stub)

# ===============================

def other_feature(): print("You selected Option 2 (Other Feature).") input("Press Enter to return to the Main Menu")

Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

QUESTIONS

?

Recap

• Whatisint h i sl e s s o n?
• The Decision Structure
• The Relational Operators
• The if Statement
• The if-else Statement
• Logical Operators
• The Match Case in Python version 3.0
• Introduction to Repetition Structures
• The while Loop
• The for Loop:
• Processing List with Menu 1-54

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