Understand and use nested selection and iteration statements

What is a Selection Statement?

Think of a selection statement as a traffic light 🚦 that decides which direction a program should go based on conditions.

• if – choose one path if a condition is true.
• else if – check another condition if the first one fails.
• else – the fallback path when all previous conditions are false.

What is an Iteration Statement?

Iteration is like a repeating chore list 🗒️. The program keeps doing something until a condition changes.

• while – repeat while a condition is true.
• do-while – repeat at least once, then check the condition.
• for – loop a set number of times or over a collection.

Nested Selection Inside a Loop

Imagine you’re sorting a deck of cards. For each card you check its suit and then its rank.

Example (pseudo‑code):

for (int i = 0; i < deckSize; i++) {
    if (deck[i].suit == "Hearts") {
        if (deck[i].rank == "Ace") {
            // Special action for Ace of Hearts
        } else {
            // Other hearts
        }
    } else {
        // Non‑hearts
    }
}
  

Here, the if inside the for loop is a nested selection.

Nested Loops (Iteration Inside Iteration)

Think of a grid of seats in a stadium. You need to visit every seat: first row, then seat, then next row, etc.

Example (pseudo‑code):

for (int row = 1; row <= totalRows; row++) {
    for (int seat = 1; seat <= seatsPerRow; seat++) {
        // Process seat (row, seat)
    }
}
  

This double for loop is a classic nested iteration.

Combining Both: Nested Selection Inside Nested Loops

Example: Find all prime numbers up to 100.

for (int num = 2; num <= 100; num++) {
    bool isPrime = true;
    for (int div = 2; div <= sqrt(num); div++) {
        if (num % div == 0) {
            isPrime = false;
            break; // stop checking further divisors
        }
    }
    if (isPrime) {
        // num is prime
    }
}
  

Here, the if inside the inner for loop is a nested selection, and the inner for itself is inside the outer for loop.

Exam Tips 📚

• Always check the order of conditions in nested if-else chains.
• Remember that a break exits the innermost loop only.
• Use comments to label each level of nesting; it helps during marking.
• When writing for loops, ensure the initialisation, condition, and increment are correctly placed.
• Practice writing nested loops that produce matrices or patterns; they often appear in exam questions.

Quick Recap with Math

Nested loops can calculate sums like:

$$S = \sum_{i=1}^{n} \sum_{j=1}^{m} a_{ij}$$

Here, the outer sum runs over rows and the inner sum over columns.