CAMBRIDGE IGCSE 0478

Nested Loops & Multidimensional Iteration

Instructional Theory

A Nested Loop is a control structure where one loop resides completely inside the body of another loop.

1. Mechanics of Nested Loops

In a nested loop:

The outer loop begins its first iteration.
The inner loop executes and completes all of its iterations.
Only after the inner loop finishes completely does the outer loop advance to its second iteration.
This cycle repeats until the outer loop reaches its termination bound.

[Outer Iteration 1]
    ├── [Inner 1]
    ├── [Inner 2]
    └── [Inner 3]
[Outer Iteration 2]
    ├── [Inner 1]
    ├── [Inner 2]
    └── [Inner 3]

\text{Total Cycles} = (\text{Outer Iterations}) \times (\text{Inner Iterations})

Independent Identifiers: The outer loop and inner loop must use different counter variables (e.g. Row and Col, or i and j). Re-using the same counter corrupts loop progression.
Proper Nesting Order: The inner loop must be completely closed (NEXT Inner) before the outer loop closes (NEXT Outer). Crossing loop boundaries is a syntax violation.

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Exercise:

• Multiple Choice

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Exam-Style Question

Question

Consider this nested loop structure:

FOR Outer <- 1 TO 4
    FOR Inner <- 1 TO 5
        OUTPUT "*"
    NEXT Inner
NEXT Outer

How many total asterisks ("*") will be printed when this algorithm completes?

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