Count Words with Given Prefix in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a word starts with the given prefix.

DSA Java

if (word.[1](prefix)) {
    count++;
}

Drag options to blanks, or click blank then click option'

Aequals

Bcontains

CstartsWith

DendsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals instead of startsWith

Using contains which checks anywhere in the word

The method startsWith checks if the word begins with the given prefix.

2fill in blank

medium

Complete the code to loop through all words in the list.

DSA Java

for (String [1] : words) {
    if ([1].startsWith(prefix)) {
        count++;
    }
}

Drag options to blanks, or click blank then click option'

Aword

Bprefix

Ccount

Dwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using the list name 'words' as the loop variable

Using 'prefix' which is the search term, not the loop variable

The loop variable should be a single word from the list, so 'word' is appropriate.

3fill in blank

hard

Fix the error in the method signature to return the correct type.

DSA Java

public static [1] countWordsWithPrefix(List<String> words, String prefix) {
    int count = 0;
    for (String word : words) {
        if (word.startsWith(prefix)) {
            count++;
        }
    }
    return count;
}

Drag options to blanks, or click blank then click option'

Avoid

BString

Cboolean

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using void which means no return value

Using String which is incorrect for a number

The method returns the count of words, which is an integer, so the return type should be int.

4fill in blank

hard

Fill both blanks to create a map of words and their lengths for words starting with the prefix.

DSA Java

Map<String, Integer> wordLengths = words.stream()
    .filter(word -> word.[1](prefix))
    .collect(Collectors.toMap(word -> word, word -> word.[2]()));

Drag options to blanks, or click blank then click option'

AstartsWith

Blength

Csize

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals instead of startsWith in filter

Using size() which is not a String method

Use startsWith to filter words by prefix and length() to get the word length.

5fill in blank

hard

Fill all three blanks to count words with prefix using streams and lambda.

DSA Java

long count = words.stream()
    .filter([1] -> [2].[3](prefix))
    .count();

Drag options to blanks, or click blank then click option'

Aw

CstartsWith

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in lambda

Using equals instead of startsWith

Use 'w' as the lambda variable and call startsWith on it to filter words by prefix.

Practice

1. What does the problem "Count Words with Given Prefix" ask you to do?

easy

A. Count how many words in a list start with a specific prefix

B. Find the longest word in a list

C. Count the total number of characters in all words

D. Sort the words alphabetically

Solution

1. Step 1: Understand the prefix concept

   The prefix is the starting part of a word. We need to check if words start with this prefix.

2. Step 2: Count matching words

   We count all words that begin with the given prefix from the list.

3. Final Answer:

   Count how many words in a list start with a specific prefix -> Option A

4. Quick Check:

   Prefix means start of word = Count matching words [OK]

Hint: Focus on words starting exactly with the prefix [OK]

Common Mistakes:

• Counting words containing prefix anywhere, not just start
• Confusing prefix with suffix
• Counting characters instead of words

2. Which of the following Java code snippets correctly checks if a string word starts with a prefix pref?

easy

A. word.endsWith(pref)

B. word.contains(pref)

C. word.equals(pref)

D. word.startsWith(pref)

Solution

1. Step 1: Recall Java String methods

   Java's startsWith method checks if a string begins with a given prefix.

2. Step 2: Match method to requirement

   Since we want to check the start, word.startsWith(pref) is correct.

3. Final Answer:

   word.startsWith(pref) -> Option D

4. Quick Check:

   startsWith checks prefix = word.startsWith(pref) [OK]

Hint: Use startsWith() to check prefix in Java strings [OK]

Common Mistakes:

• Using contains() which checks anywhere in string
• Using equals() which checks full string equality
• Using endsWith() which checks suffix

3. What is the output of the following Java code?

String[] words = {"apple", "app", "application", "banana"};
String prefix = "app";
int count = 0;
for (String w : words) {
    if (w.startsWith(prefix)) {
        count++;
    }
}
System.out.println(count);

medium

A. 3

B. 1

C. 2

D. 4

Solution

1. Step 1: Check each word against prefix "app"

   Words starting with "app" are "apple", "app", and "application" (3 words).

2. Step 2: Count matching words

   Count increments 3 times for these words.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 words start with "app" = 3 [OK]

Hint: Count words starting with prefix by checking each word [OK]

Common Mistakes:

• Counting only exact matches
• Counting words containing prefix anywhere
• Off-by-one errors in counting

4. The following code is intended to count words starting with a prefix but has a bug. What is the bug?

String[] words = {"cat", "car", "dog", "cart"};
String prefix = "car";
int count = 0;
for (int i = 0; i <= words.length; i++) {
    if (words[i].startsWith(prefix)) {
        count++;
    }
}
System.out.println(count);

medium

A. Prefix variable is not initialized

B. startsWith method is used incorrectly

C. Loop condition should be i < words.length, not i <= words.length

D. Array words is empty

Solution

1. Step 1: Analyze loop condition

   The loop runs from i = 0 to i <= words.length, which causes an IndexOutOfBoundsException because last index is words.length - 1.

2. Step 2: Correct loop condition

   Change condition to i < words.length to avoid accessing invalid index.

3. Final Answer:

   Loop condition should be i < words.length, not i <= words.length -> Option C

4. Quick Check:

   Array index max is length-1 = fix loop condition [OK]

Hint: Use i < array.length to avoid index errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Ignoring array index limits
• Misusing startsWith method

5. Given a list of words ["tree", "trie", "trigger", "trip", "trick"] and prefix "tri", which Java code snippet correctly counts the words starting with the prefix using streams?

hard

A. int count = Arrays.stream(words).map(w -> w.startsWith(prefix)).count();

B. long count = Arrays.stream(words).filter(w -> w.startsWith(prefix)).count();

C. long count = Arrays.stream(words).filter(w -> w.contains(prefix)).count();

D. int count = Arrays.stream(words).filter(w -> w.endsWith(prefix)).count();

Solution

1. Step 1: Use stream filter with startsWith

   The filter should keep words starting with the prefix using w.startsWith(prefix).

2. Step 2: Count filtered words

   Use count() to get the number of matching words. The result is a long type.

3. Final Answer:

   long count = Arrays.stream(words).filter(w -> w.startsWith(prefix)).count(); -> Option B

4. Quick Check:

   Filter with startsWith + count() = long count = Arrays.stream(words).filter(w -> w.startsWith(prefix)).count(); [OK]

Hint: Use stream filter with startsWith and count() for prefix count [OK]

Common Mistakes:

• Using map instead of filter
• Using contains instead of startsWith
• Using endsWith instead of startsWith