Data Structure FRQ

Data Structures FRQ: Arrays and ArrayLists

By the end of this lesson, you will be able to write FRQ-quality code manipulating arrays and ArrayLists, avoiding common errors.

FRQ Part 1 or 2: Array/ArrayList Methods

AP Computer Science A FRQs frequently ask you to:

• Traverse an array or ArrayList
• Find/count elements meeting a condition
• Modify elements
• Return a new array/ArrayList filtered from another

Template: Traversal and Accumulation

// Sum all positive elements in an array
public static int sumPositive(int[] arr) {
    int sum = 0;
    for (int val : arr) {
        if (val > 0) {
            sum += val;
        }
    }
    return sum;
}

Template: Find the Maximum (with Index)

public static int indexOfMax(int[] arr) {
    int maxIndex = 0;
    for (int i = 1; i < arr.length; i++) {
        if (arr[i] > arr[maxIndex]) {
            maxIndex = i;
        }
    }
    return maxIndex;
}

Never initialize max to 0 — initialize it to arr[0]. If all values are negative, max = 0 would give the wrong answer.

Template: Build a New ArrayList from a Condition

// Return a new ArrayList of all strings with length > 5
public static ArrayList<String> longWords(ArrayList<String> words) {
    ArrayList<String> result = new ArrayList<String>();
    for (String w : words) {
        if (w.length() > 5) {
            result.add(w);
        }
    }
    return result;
}

Removing While Iterating (Tricky!)

// WRONG:
for (String s : list) {
    if (s.equals("bad")) list.remove(s);  // ConcurrentModificationException
}

// CORRECT: iterate backwards with index
for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i).equals("bad")) list.remove(i);
}

WRONG: removing from an ArrayList while using a for-each loop causes errors.

Template: 2D Array Processing

// Find the sum of each row in a 2D array
public static int[] rowSums(int[][] matrix) {
    int[] sums = new int[matrix.length];
    for (int row = 0; row < matrix.length; row++) {
        int rowSum = 0;
        for (int col = 0; col < matrix[row].length; col++) {
            rowSum += matrix[row][col];
        }
        sums[row] = rowSum;
    }
    return sums;
}

Common FRQ Pitfalls

Mistake	Fix