Unit 2.1: Algorithms with Selection and Repetition

The three fundamental building blocks of all algorithms are:

Fundamental Algorithm Structures

Sequencing: The default behavior where code runs line-by-line.
Selection: Branching logic that runs code only if a condition is met.
Repetition: Loops that run the same code multiple times.

Sequencing: Executing statements one after another.
Selection: Making decisions using conditional statements like if and switch.
Repetition: Repeating actions using loops like while and for.

This unit focuses on selection and repetition.

Selection

Selection allows a program to follow different paths based on whether a condition is true or false. In programming, we use selection to control the flow of our programs.

Task: Using an if-else statement for simple selection.

// Example of Selection
int temperature = 30;
if (temperature > 25) {
    System.out.println("It's a hot day!");
} else {
    System.out.println("It's a cool day.");
}

You will learn more about selection in the following sections.

Repetition (Iteration)

Repetition, also known as iteration or looping, is when a part of an algorithm is repeated a certain number of times or until a specific condition is met.

Task: Using a for loop for simple repetition.

// Example of Repetition
// This loop prints numbers 1 through 5
for (int i = 1; i <= 5; i++) {
    System.out.println("Count is: " + i);
}

You will learn more about iteration in Part 2 of this unit.

The Importance of Order

The order in which you combine sequencing, selection, and repetition is critical to the correctness of your algorithm.

Structural Priority

The logical flow depends on the nesting of structures. A check before a loop determines if the loop runs at all, while a check inside a loop runs for every iteration.

Performing a check before a loop is different from performing it inside a loop.

Consider an algorithm to find the first even number in a list. You must first check if the list is empty (selection) before you start looping through it (repetition).

Task: Combining selection and repetition in a search algorithm.

import java.util.ArrayList;

public class OrderMatters {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        // Selection first: Check if the list is not empty before looping.
        // If we tried to loop first, we might get an error on an empty list.
        if (!numbers.isEmpty()) {
            // Repetition: Loop through the numbers.
            for (int num : numbers) {
                // Selection: Check if the number is even.
                if (num % 2 == 0) {
                    System.out.println("First even number found: " + num);
                    break; // Exit the loop
                }
            }
        }
    }
}