Loop Structures: FOR, WHILE and REPEAT Loops

By the end of this article, you should be able to:

• Explain why loops are used in algorithms and programming
• Distinguish between FOR, WHILE, and REPEAT loop structures
• Write looping structures using CSEC-style pseudocode
• Trace loop execution step-by-step using trace tables
• Answer CSEC exam questions involving loops with confidence

Why Do We Need Repetition in Algorithms?

Imagine you need to calculate the total marks for 30 students in a class. Without loops, you would need to write 30 separate input statements and 30 separate addition operations. Loops allow us to write the code once and repeat it automatically for each student.

The Problem Without Loops

Consider calculating the average of 100 numbers without using a loop:

// Calculating average WITHOUT loops (tedious!)
INPUT Num1
INPUT Num2
INPUT Num3
... (97 more input statements!)
Total = Num1 + Num2 + Num3 + ... (100 variables!)
Average = Total / 100

This approach is impractical! Loops solve this problem elegantly.

Benefits of Using Loops

// INFINITE LOOP - DO NOT RUN!
Counter = 1
WHILE Counter > 0 DO
    OUTPUT Counter
    // Counter never decreases, so condition always TRUE
ENDWHILE

When to Use a FOR Loop

Use a FOR loop when:

• You know the exact number of iterations needed
• You need to count through a range of values
• The loop depends on a numeric counter

FOR Loop Syntax

FOR counter ← start TO end DO
    statement(s) to repeat
ENDFOR

Example 1: Counting from 1 to 5

Let's trace through a simple FOR loop that counts from 1 to 5:

FOR Count ← 1 TO 5 DO
    OUTPUT Count
ENDFOR

Expected Output:

1
2
3
4
5

Step-by-Step Trace Table

Example 2: Summing Five Numbers

A practical example calculating the sum of five numbers:

// Calculate the sum of 5 numbers
Total = 0

FOR Num ← 1 TO 5 DO
    INPUT Number
    Total = Total + Number
ENDFOR

OUTPUT "The sum is: ", Total

Sample Execution: If user enters 10, 20, 30, 40, 50, the output will be "The sum is: 150"

FOR Loop with STEP

You can skip values using the STEP keyword:

// Counting by 2s (even numbers only)
FOR Num ← 2 TO 10 STEP 2 DO
    OUTPUT Num
ENDFOR

Output: 2, 4, 6, 8, 10

When to Use a WHILE Loop

Use a WHILE loop when:

• You don't know how many iterations are needed
• The loop depends on user input or external conditions
• The loop should only run if a condition is true

WHILE Loop Syntax

WHILE condition DO
    statement(s) to repeat
ENDWHILE

Example 1: Password Validation

A login system that keeps asking for the password until correct:

// Password validation loop
CorrectPassword = "CSEC2024"
Attempt = 0

WHILE Attempt < 3 DO
    INPUT UserPassword
    IF UserPassword = CorrectPassword THEN
        OUTPUT "Access Granted!"
        EXIT  // Exit the loop immediately
    ELSE
        OUTPUT "Incorrect password. Try again."
    ENDIF
    Attempt = Attempt + 1
ENDWHILE

OUTPUT "Account locked. Too many attempts."

Counter = 5
WHILE Counter < 3 DO  // FALSE immediately!
    OUTPUT "This never prints"
ENDWHILE
OUTPUT "Done"  // This prints instead

Example 2: Reading Until Zero

Processing numbers until the user enters 0:

// Read numbers until zero is entered
INPUT Number
Sum = 0
Count = 0

WHILE Number <> 0 DO
    Sum = Sum + Number
    Count = Count + 1
    INPUT Number
ENDWHILE

OUTPUT "Count: ", Count, ", Sum: ", Sum

Trace Table Example

When to Use a REPEAT-UNTIL Loop

Use a REPEAT-UNTIL loop when:

• You need the loop to run at least once
• You're asking for user input that must be collected before validation
• Creating menus or dialogs that should show before asking to continue

REPEAT-UNTIL Syntax

REPEAT
    statement(s) to repeat
UNTIL condition

Example 1: Menu System

A restaurant menu that displays options until the user makes a selection:

// Restaurant menu system
REPEAT
    OUTPUT "=== MAIN MENU ==="
    OUTPUT "1. Chicken Curry"
    OUTPUT "2. Jerk Pork"
    OUTPUT "3. Fried Chicken"
    OUTPUT "4. Exit"
    INPUT "Enter your choice (1-4): ", Choice
UNTIL Choice >= 1 AND Choice <= 3

OUTPUT "You selected option ", Choice

Example 2: Validating Age Input

Asking for age until a valid value (0-120) is entered:

// Validate age input - always asks at least once
REPEAT
    INPUT "Enter your age: ", Age
    IF Age < 0 OR Age > 120 THEN
        OUTPUT "Invalid age! Please try again."
    ENDIF
UNTIL Age >= 0 AND Age <= 120

OUTPUT "Valid age entered: ", Age

Counter = 10
REPEAT
    OUTPUT "Counter is ", Counter  // This WILL print!
    Counter = Counter - 1
UNTIL Counter < 5  // Condition checked AFTER

REPEAT-UNTIL vs WHILE

Loop Type Comparison Table

When to Use Each Loop

How Loops Are Represented

All loops in flowcharts use the decision diamond to represent the loop condition. The flow returns to the decision point after each iteration.

FOR Loop Flowchart

WHILE Loop Flowchart

REPEAT-UNTIL Flowchart

Example: FOR Loop Trace Table

Algorithm:

// Calculate the square of numbers 1 to 4
FOR Num ← 1 TO 4 DO
    Square = Num * Num
    OUTPUT Square
ENDFOR

Example: WHILE Loop Trace Table

Algorithm:

Counter = 1
Total = 0

WHILE Counter <= 3 DO
    Total = Total + Counter
    Counter = Counter + 1
ENDWHILE

OUTPUT Total

Example: REPEAT-UNTIL Trace Table

Algorithm:

Number = 0

REPEAT
    INPUT Number
    OUTPUT Number
UNTIL Number > 10

Assume user inputs: 5, 15

When to Use Nested Loops

• Processing 2D data (tables, grids, matrices)
• Creating patterns and shapes
• Comparing every pair of items in a list
• Multiplication tables, timetables

Example: Multiplication Table

Creating a multiplication table (1 to 3):

// Outer loop: rows (1 to 3)
FOR Row ← 1 TO 3 DO
    // Inner loop: columns (1 to 5)
    FOR Col ← 1 TO 5 DO
        Product = Row * Col
        OUTPUT Row, "*", Col, "=", Product
    ENDFOR
ENDFOR

How Many Times Does the Inner Loop Execute?

If the outer loop runs M times and the inner loop runs N times, the total iterations = M × N

In our example: 3 (rows) × 5 (columns) = 15 total iterations

Step-by-Step Execution

1. Multiple-Choice Questions (Paper 1)

• Identifying how many times a loop executes
• Selecting the correct loop type for a scenario
• Recognizing infinite loops
• Understanding loop terminology

2. Trace Tables (Paper 2)

• Completing trace tables for loop algorithms
• Tracking variable values through iterations
• Determining final output

3. Pseudocode Questions (Paper 2)

• Writing loop-based algorithms
• Converting between loop types
• Fixing loop errors
• Completing partially-written code

4. Flowchart Questions (Paper 2)

• Interpreting loop flowcharts
• Drawing flowcharts with loops
• Tracing execution paths