Programming 101 chevron_right Repetition and the Iteration Control Structure

Repetition and the Iteration Control Structure

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Learning Objectives: By the end of this lesson, you will be able to:

  • Use WHILE, FOR, and REPEAT loops in pseudocode to represent repetition
  • Identify and use loop control variables
  • Write clear, structured pseudocode for iterative problems

1. Welcome and Introduction (5 minutes)

Icebreaker Activity

Have you ever had to do something over and over again, like counting steps, practicing a skill, or searching for a friend in a crowd? How do you keep track of what you’ve already done?

Lesson Overview

  • Duration: 90 minutes
  • Prerequisites: Day 1–3 lessons (algorithms, pseudocode basics, selection logic)
  • Materials Needed: Pen and paper or a text editor for writing pseudocode

2. Learning Objectives and Session Outline (5 minutes)

What We’ll Cover Today:

  1. Loop structures: WHILE, FOR, REPEAT
  2. Loop control variables and avoiding infinite loops
  3. Writing pseudocode for repetitive tasks
  4. Hands-on practice and exercises
  5. Common pitfalls and troubleshooting

Success Criteria:

By the end of this session, you should be able to write pseudocode that repeats actions using loops and manages loop variables correctly.

3. Foundation Knowledge (15-20 minutes)

Core Concepts

Loop Structures in Pseudocode

  • WHILE…DO…ENDWHILE: Repeat as long as a condition is true
  • FOR…TO…DO…ENDFOR: Repeat a set number of times
  • REPEAT…UNTIL: Repeat until a condition becomes true (always runs at least once)

Real-World Analogy: Think of practicing a song until you get it right (REPEAT…UNTIL), or counting out 10 push-ups (FOR loop).

Why This Matters: Loops are essential for processing lists, automating repetitive tasks, and handling large amounts of data efficiently.

Loop Control Variables

A loop control variable keeps track of the number of repetitions or the current position in a list. For example, a counter in a FOR loop.

Avoiding Infinite Loops

An infinite loop happens when the loop’s condition never becomes false. Always make sure your loop will eventually stop!

Key Terminology

  • Iteration: Repeating a set of instructions
  • Loop: A structure that enables iteration
  • Control Variable: A variable that determines when the loop stops

4. Detailed Explanation and Examples (25-30 minutes)

Understanding Iteration in Pseudocode

Iteration allows you to automate repetitive tasks, making your algorithms more powerful and efficient.

Practical Applications

  • Searching for an item in a list
  • Summing numbers
  • Printing messages multiple times

Common Misconceptions

  1. Misconception: Loops are only for advanced problems. Reality: Many simple tasks require repetition.
  2. Misconception: You don’t need to update the control variable. Reality: Failing to update the control variable causes infinite loops.

5. Live Coding Demonstration (20-25 minutes)

Example 1: Searching for a Name in a List (FOR Loop)

START
  INPUT nameToFind
  SET found TO FALSE
  FOR i FROM 0 TO LENGTH(namesList) - 1 DO
    IF namesList[i] == nameToFind THEN
      SET found TO TRUE
      OUTPUT "Found!"
      BREAK
    ENDIF
  ENDFOR
  IF found == FALSE THEN
    OUTPUT "Not found."
  ENDIF
END

Explanation:

  • The FOR loop goes through each name
  • The control variable i tracks the current position
  • The loop stops early if the name is found

Example 2: Print “Hello” 5 Times (FOR Loop)

START
  FOR count FROM 1 TO 5 DO
    OUTPUT "Hello"
  ENDFOR
END

Key Points:

  • FOR loops are ideal for a known number of repetitions

Example 3: Sum Numbers from 1 to 100 (WHILE Loop)

START
  SET sum TO 0
  SET number TO 1
  WHILE number <= 100 DO
    SET sum TO sum + number
    SET number TO number + 1
  ENDWHILE
  OUTPUT sum
END

Example 4: REPEAT…UNTIL Loop

START
  SET guess TO 0
  REPEAT
    INPUT guess
  UNTIL guess == 7
  OUTPUT "You guessed the secret number!"
END

Debugging Demonstration

Common mistake: Forgetting to update the control variable.

// Incorrect
START
  SET number TO 1
  WHILE number <= 5 DO
    OUTPUT number
  ENDWHILE
END

// Corrected version
START
  SET number TO 1
  WHILE number <= 5 DO
    OUTPUT number
    SET number TO number + 1
  ENDWHILE
END

6. Hands-On Practice (30-40 minutes)

Exercise 1: Basic Practice (10 minutes)

Objective: Write pseudocode to print numbers 1 to 10.

Instructions:

  1. Use a FOR or WHILE loop
  2. Output each number

Starter Code:

START
  // Your steps here
END

Solution:

START
  FOR i FROM 1 TO 10 DO
    OUTPUT i
  ENDFOR
END

Exercise 2: Intermediate Challenge (15 minutes)

Objective: Write pseudocode to find the sum of all even numbers from 1 to 50.

Requirements:

  • Use a loop
  • Use an IF statement to check for even numbers
  • Output the final sum

Hints:

  • Use number % 2 == 0 to check for even
  • Initialize sum to 0

Exercise 3: Creative Application (15 minutes)

Objective: Write pseudocode for a simple password guessing game.

Challenge:

  • Allow the user to keep guessing until they enter the correct password
  • Output a message for each incorrect guess
  • Output a success message when correct

Extension Ideas:

  • Limit the number of attempts
  • Give hints after each guess

7. Common Pitfalls and Troubleshooting (10 minutes)

Frequent Mistakes

  1. Problem: Infinite loops Cause: Not updating the control variable or incorrect loop condition Solution: Always update the control variable and check your condition
  2. Problem: Off-by-one errors Cause: Loop starts or ends at the wrong value Solution: Double-check your loop boundaries

Debugging Tips

  • Trace your loop with sample values
  • Print the control variable each iteration to check progress
  • Make sure your loop will eventually stop

8. Best Practices and Professional Tips (5 minutes)

Industry Standards

  • Use clear, consistent loop structures
  • Name control variables meaningfully
  • Avoid deeply nested loops when possible

Next Steps in Your Learning Journey

  • Practice writing loops for different scenarios
  • Learn about nested loops and more complex patterns
  • Explore how loops are implemented in programming languages

9. Summary and Review (5 minutes)

Key Takeaways

  1. Loops automate repetitive tasks in algorithms
  2. Control variables are essential for managing loops
  3. Always ensure your loops will stop (no infinite loops)

Quick Review Questions

  1. What is the difference between a FOR and a WHILE loop?
  2. How do you avoid infinite loops?
  3. Why are control variables important?

For Instructors

Teaching Tips

  • Use real-world repetition examples (e.g., counting, searching)
  • Encourage students to trace loops by hand
  • Discuss common loop errors and how to fix them

Assessment Ideas

  • Have students write pseudocode for a new repetitive task
  • Peer review: students check each other’s loops for correctness
  • Short quiz on loop types and control variables

Adaptation Notes

  • For advanced students, introduce nested loops or early exits (BREAK)
  • For online delivery, use interactive loop tracing tools
  • Adjust timing based on student engagement and questions