Count Words with Given Prefix in DSA Java - Build from Scratch

Count Words with Given Prefix

📖 Scenario: You are working on a simple text analysis tool. You want to count how many words in a list start with a certain prefix. This helps in searching and filtering words quickly.

🎯 Goal: Build a Java program that counts how many words in a given list start with a specific prefix.

📋 What You'll Learn

Create a list of words with exact values

Create a string variable for the prefix

Use a loop to count words starting with the prefix

Print the count of words with the prefix

💡 Why This Matters

🌍 Real World

Counting words with a specific prefix is useful in search engines, autocomplete features, and text analysis tools.

💼 Career

This skill helps in roles involving data processing, software development, and building search or filtering features.

1

Create the list of words

Create a List<String> called words with these exact entries: "apple", "application", "banana", "appetite", "grape"

DSA Java

// Create the list of words called words with the given entries
// Your code here

Hint

Use List.of(...) to create an immutable list with the exact words.

2

Create the prefix variable

Create a String variable called prefix and set it to "app"

DSA Java

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List words = List.of("apple", "application", "banana", "appetite", "grape");
        // Create a String variable called prefix and set it to "app"
        // Your code here
    }
}

Hint

Use String prefix = "app"; to create the prefix variable.

3

Count words starting with the prefix

Create an int variable called count and set it to 0. Use a for loop with variable word to iterate over words. Inside the loop, if word.startsWith(prefix) is true, increase count by 1.

DSA Java

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List words = List.of("apple", "application", "banana", "appetite", "grape");
        String prefix = "app";
        // Create int count = 0
        // Use for loop with word to iterate over words
        // If word.startsWith(prefix), increment count
        // Your code here
    }
}

Hint

Use for (String word : words) to loop through the list and word.startsWith(prefix) to check the prefix.

4

Print the count

Use System.out.println to print the value of count

DSA Java

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List words = List.of("apple", "application", "banana", "appetite", "grape");
        String prefix = "app";
        int count = 0;
        for (String word : words) {
            if (word.startsWith(prefix)) {
                count++;
            }
        }
        // Print the count
        // Your code here
    }
}

Hint

Use System.out.println(count); to show the final count.

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