CSA 2024 FRQ 3 Homework

Challenge Part A

import java.util.ArrayList;
import java.util.Arrays;

public class WordChecker {
    private ArrayList<String> wordList;
    
    public WordChecker(ArrayList<String> words) {
        wordList = words;
    }
    
    public boolean isWordChain() {
        for (int i = 1; i < wordList.size(); i++) {
            if (!wordList.get(i).contains(wordList.get(i - 1))) {
                return false;
            }
        }
        return true;
    }
    
    public static void main(String[] args) {
        ArrayList<String> test1 = new ArrayList<>(Arrays.asList("an", "band", "band", "abandon"));
        WordChecker checker1 = new WordChecker(test1);
        System.out.println("Test 1: " + test1);
        System.out.println("Result: " + checker1.isWordChain());
        
        ArrayList<String> test2 = new ArrayList<>(Arrays.asList("to", "too", "stool", "tools"));
        WordChecker checker2 = new WordChecker(test2);
        System.out.println("Test 2: " + test2);
        System.out.println("Result: " + checker2.isWordChain());
    }
}

WordChecker.main(null);

Test 1: [an, band, band, abandon]
Result: true
Test 2: [to, too, stool, tools]
Result: false

Challenge Part B

import java.util.ArrayList;
import java.util.Arrays;

public class WordChecker {
    private ArrayList<String> wordList;
    
    public WordChecker(ArrayList<String> words) {
        wordList = words;
    }
    
    public ArrayList<String> createList(String target) {
        ArrayList<String> result = new ArrayList<String>();
        for (String word : wordList) {
            if (word.startsWith(target)) {
                result.add(word.substring(target.length()));
            }
        }
        return result;
    }
    
    public static void main(String[] args) {
        ArrayList<String> words = new ArrayList<>(Arrays.asList("catch", "bobcat", "catchacat", "cat", "at"));
        WordChecker checker = new WordChecker(words);
        
        System.out.println("Original wordList: " + words);
        System.out.println();
        
        ArrayList<String> result1 = checker.createList("cat");
        System.out.println("createList('cat'): " + result1);
        
        ArrayList<String> result2 = checker.createList("catch");
        System.out.println("createList('catch'): " + result2);
        
        ArrayList<String> result3 = checker.createList("dog");
        System.out.println("createList('dog'): " + result3);
    }
}

WordChecker.main(null);

Original wordList: [catch, bobcat, catchacat, cat, at]

createList('cat'): [ch, chacat, ]
createList('catch'): [, acat]
createList('dog'): []

How I Solved It

Part (a) was about checking if a list of words forms a “chain” where each word contains the previous one as a substring. I used a simple for loop starting at index 1 to iterate through the wordList. For each word, I used the contains() method to check if it contains the previous word as a substring. If any word didn’t contain its predecessor, I immediately returned false. If the loop completed without finding any violations, I returned true.

Part (b) required creating a new ArrayList based on words that start with a target string, with that target removed. I created a new ArrayList and used an enhanced for loop to iterate through wordList. For each word, I checked if it started with the target using startsWith(). If it did, I added the word with the target prefix removed using substring(target.length()). This preserved the original order while filtering and modifying the words.

Area of Struggle

Part (a) was straightforward once I understood the substring logic. Part (b) required careful attention to string manipulation. The key was remembering that substring(target.length()) removes exactly the characters at the beginning, which works perfectly even when the target is completely removed (leaving an empty string). I made sure to create a new ArrayList rather than modifying the original wordList, which the postcondition required.

Coderunner proof