B2.2.2 Construct programs that apply arrays and Lists.

The Big Idea

Arrays and lists are foundational data structures that allow us to store, organize, and manipulate collections of data in a single variable. Whether we’re dealing with a simple set of values (e.g. test scores) or more complex data grids (e.g. a chessboard), using arrays and lists allows a program to efficiently access, update, and manage multiple elements.

In Java, the term array refers to a fixed-size indexed structure, and ArrayList refers to a resizable, dynamic list. In Python, the term list always refers to a dynamic, resizable array-like structure.

Understanding how to construct, traverse, add to, and remove from these data structures—both in one dimension and two—is essential for solving a wide range of problems in computer science.

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1. One-Dimensional Arrays and Lists

Java: One-Dimensional Arrays

int[] numbers = new int[5];      // Declaration
numbers[0] = 10;                 // Assignment
System.out.println(numbers[0]); // Access

Python: One-Dimensional Lists

numbers = [10, 20, 30]     # Declaration and initialization
numbers[0] = 15            # Modify
print(numbers[0])          # Access

• Indexing starts at 0 in both Java and Python.
• The main difference is mutability and resizing: Java arrays are fixed-size, while Python lists are dynamic.

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2. Two-Dimensional Arrays and Lists

Java: 2D Array (Matrix)

int[][] matrix = new int[3][3];
matrix[0][0] = 1;
System.out.println(matrix[0][0]);

• Java supports arrays of arrays (int[][]), which can be jagged (rows of different lengths).

Python: 2D List

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(matrix[0][0])  # Output: 1

• Python lists can also form "matrices" (lists of lists), and their dimensions are not required to be uniform.

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3. ArrayLists in Java

Dynamic List Declaration and Manipulation

import java.util.ArrayList;
ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.remove("Alice");
System.out.println(names.get(0));  // Outputs: "Bob"

• ArrayList automatically resizes.
• Elements can be added or removed at runtime.

Key Methods:

• .add(element)
• .remove(index or value)
• .get(index)
• .size()

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4. Python Dynamic Lists

Python lists are inherently dynamic and provide built-in methods for flexible manipulation.

names = []
names.append("Alice")
names.append("Bob")
names.remove("Alice")
print(names[0])  # Outputs: "Bob"

Key Methods:

• .append(item) – add to end
• .insert(index, item) – insert at specific index
• .remove(item) – remove by value
• del list[index] – remove by index
• .pop() – remove and return last item
• .len() – get number of elements

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5. Traversing Arrays and Lists

Java: Looping through Arrays

int[] scores = {90, 80, 70};
for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

Java: Looping through an ArrayList

ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
for (String name : names) {
    System.out.println(name);
}

Python: Looping through Lists

scores = [90, 80, 70]
for score in scores:
    print(score)

Python: Looping through a 2D List

matrix = [
    [1, 2],
    [3, 4]
]
for row in matrix:
    for value in row:
        print(value)

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6. Programmatic Example: Dynamic List (Python)

shopping_list = []
# Add items
shopping_list.append("apples")
shopping_list.append("bananas")
shopping_list.insert(1, "oranges")
# Traverse
for item in shopping_list:
    print(item)
# Remove
shopping_list.remove("bananas")
del shopping_list[0]
print("Final list:", shopping_list)

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7. Common Mistakes

Mistake	Explanation
Out-of-bounds access	Accessing an index that doesn’t exist (e.g. list[10] in a list of size 3).
Removing items during traversal	Modifying a list while iterating over it can cause errors.
Confusing 1D with 2D logic	Forgetting to use nested loops when dealing with 2D structures.
Assuming fixed size	Trying to assign to an index that hasn’t been created in a dynamic list.

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Summary Table

Feature	Java Array	Java ArrayList	Python List
Size	Fixed	Dynamic	Dynamic
Syntax	int[] arr	ArrayList<Type>	list = []
Multidimensional	Yes (int[][])	No (use custom classes)	Yes (list of lists)
Add/Remove	Not allowed	.add(), .remove()	.append(), .remove()
Traversal	For loop, index	For-each, .get()	For loop, indexing

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Final Thoughts

Arrays and lists are critical tools for handling collections of data. Arrays provide fast, indexed access with fixed size, while dynamic lists like Java’s ArrayList or Python’s list offer flexibility and ease of use. Mastery of these structures means not only knowing how to declare and access them, but also how to build robust, dynamic programs that can handle real-world input sizes and requirements.