Area 2.6

Actions (Control Structures)

Master the three fundamental control structures: sequence, selection, and iteration. Learn how to control program flow and make decisions in your code.

Control Types

Variations

~2hrs

Study Time

Learning Objectives

Understand the three fundamental control structures in programming
Apply sequence for step-by-step execution of instructions
Use selection (if-else, match-case) for decision making in programs
Implement iteration (for and while loops) for repetitive tasks
Evaluate the benefits and drawbacks of each control structure
Choose appropriate control structures for different programming scenarios

Fundamental Control Structures

Sequence

Instructions executed one after another in a specific order

Benefits:

Easy to read and debug
Predictable execution order
Clear program flow
No complex logic required

Drawbacks:

Cannot make decisions
No flexibility in execution
Cannot handle varying conditions
Limited problem-solving capability

When to Use:

Simple calculations, data input/output, basic program setup, linear processes

Python Examples:

# Sequence - steps executed in order
print("Welcome to the calculator")
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
result = num1 + num2
print(f"The sum is: {result}")
print("Thank you for using the calculator")

# Each line executes after the previous one completes

Selection (if-else)

Choose different paths based on conditions and decision points

Benefits:

Enables decision making
Handles different scenarios
Flexible program behavior
Responds to user input

Drawbacks:

Can become complex with many conditions
May lead to deeply nested code
Harder to test all paths
Potential for logic errors

When to Use:

User input validation, different outcomes based on conditions, menu systems, error handling

Python Examples:

# Selection - if/elif/else statements
age = int(input("Enter your age: "))

if age < 13:
    category = "child"
    price = 5.00
elif age < 65:
    category = "adult"
    price = 10.00
else:
    category = "senior"
    price = 7.50

print(f"Category: {category}, Price: £{price}")

# Match-case (Python 3.10+)
match category:
    case "child":
        print("Special kids menu available!")
    case "adult":
        print("Full menu access")
    case "senior":
        print("Senior discount applied")

Iteration (Loops)

Repeat instructions while conditions are true or for specific counts

Benefits:

Eliminates code duplication
Handles unknown quantities
Processes collections efficiently
Reduces programming effort

Drawbacks:

Risk of infinite loops
Can be complex to debug
Performance concerns with large iterations
Logic errors affect multiple iterations

When to Use:

Processing lists, repetitive calculations, user input validation, counting operations

Python Examples:

# For loop - known number of iterations
print("Countdown:")
for i in range(5, 0, -1):
    print(i)
print("Launch!")

# For loop with collections
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(f"I like {fruit}")

# While loop - condition-based
password = ""
attempts = 0
max_attempts = 3

while password != "secret123" and attempts < max_attempts:
    password = input("Enter password: ")
    attempts += 1

if password == "secret123":
    print("Access granted!")
else:
    print("Too many failed attempts")

Selection Statement Types

Simple if

if condition:

if age >= 18:
    print("Can vote")

Single condition check

if-else

if condition:
    # code
else:
    # code

if score >= 60:
    print("Pass")
else:
    print("Fail")

Two alternative outcomes

if-elif-else

if condition1:
elif condition2:
else:

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

Multiple conditions

match-case

match variable:
    case value1:
    case value2:

match day:
    case "Monday":
        print("Start of week")
    case "Friday":
        print("TGIF!")

Multiple exact value matches

Iteration (Loop) Types

for loop (range)

Known number of iterations

Syntax:

for i in range(start, stop, step):

Example:

for i in range(1, 11):
    print(i)

Termination:

Automatically after specified count

for loop (collection)

Processing each item in a list

Syntax:

for item in collection:

Example:

for name in names:
    print(f"Hello {name}")

Termination:

After processing all items

while loop

Condition-based repetition

Syntax:

while condition:

Example:

while user_input != "quit":
    user_input = input("Command: ")

Termination:

When condition becomes False

do-while (simulation)

Execute at least once

Syntax:

while True:
    # code
    if not condition:
        break

Example:

while True:
    choice = input("Continue? (y/n): ")
    if choice == "n":
        break

Termination:

Break statement when condition met

Learning Activities

Control Flow Design Challenge

Design

45 minutes

Design programs using different combinations of sequence, selection, and iteration

Loop Optimization Workshop

Optimization

40 minutes

Compare for loops vs while loops for different scenarios and optimize performance

Nested Structure Practice

Practice

50 minutes

Create programs with nested loops and conditional statements

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