Count Words with Given Prefix in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of CountWordsWithPrefix Method

What is the output of the following C# code that counts words starting with a given prefix?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static int CountWordsWithPrefix(List<string> words, string prefix) {
        int count = 0;
        foreach (var word in words) {
            if (word.StartsWith(prefix)) {
                count++;
            }
        }
        return count;
    }

    static void Main() {
        var words = new List<string> {"apple", "app", "application", "banana", "apply", "ape"};
        Console.WriteLine(CountWordsWithPrefix(words, "app"));
    }
}

A 5

B 3

C 4

D 2

💡 Hint

Count how many words start exactly with "app".

The words starting with "app" are: "apple", "app", "application", and "apply". That makes 4 words.

🧠 Conceptual

intermediate

Understanding Prefix Matching

Which of the following best describes what the method CountWordsWithPrefix does?

A Counts how many words start exactly with the given prefix.

B Counts how many words contain the prefix anywhere inside the word.

C Counts how many words end with the given prefix.

D Counts how many words have the prefix as a suffix.

💡 Hint

Look at the method StartsWith used in the code.

The method uses StartsWith which checks if the word begins with the prefix, so it counts words starting exactly with the prefix.

🔧 Debug

advanced

Identify the Error in Prefix Counting Code

What error will occur when running this code snippet?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static int CountWordsWithPrefix(List<string> words, string prefix) {
        int count = 0;
        foreach (var word in words) {
            if (word.StartsWith(prefix))
                count++;
            else
                count = count + 1;
        }
        return count;
    }

    static void Main() {
        var words = new List<string> {"cat", "car", "dog"};
        Console.WriteLine(CountWordsWithPrefix(words, "ca"));
    }
}

A The method throws a NullReferenceException.

B The method returns the total number of words instead of counting only those with the prefix.

C The method returns zero always.

D The method causes a compile-time error due to missing braces.

💡 Hint

Check what happens in the else block inside the loop.

The else block increments count for words that do not start with the prefix, so count ends up being the total number of words.

❓ Predict Output

advanced

Output with Empty Prefix

What is the output of the following code when counting words with an empty prefix?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static int CountWordsWithPrefix(List<string> words, string prefix) {
        int count = 0;
        foreach (var word in words) {
            if (word.StartsWith(prefix)) {
                count++;
            }
        }
        return count;
    }

    static void Main() {
        var words = new List<string> {"dog", "deer", "deal"};
        Console.WriteLine(CountWordsWithPrefix(words, ""));
    }
}

A 3

B 0

C Throws an ArgumentException

D 1

💡 Hint

What does StartsWith return when prefix is empty?

In C#, StartsWith with an empty string returns true for every word, so all words are counted.

🚀 Application

expert

Optimizing Prefix Counting for Large Data

You have a large list of words and need to count how many start with a given prefix many times efficiently. Which data structure is best to optimize repeated prefix count queries?

A Use a Stack to store words and pop words starting with prefix.

B Use a HashSet to store words and check prefix counts by iteration.

C Use a Queue to enqueue words and dequeue those with prefix.

D Use a Trie (prefix tree) to store words and count prefixes quickly.

💡 Hint

Think about a data structure designed for prefix searches.

A Trie stores words by their prefixes, allowing fast prefix count queries without scanning all words each time.

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. Sort words alphabetically

D. Replace the prefix in all words with another string

Solution

1. Step 1: Understand the problem statement

   The problem asks to count words that begin with a given prefix, not to sort or replace words.

2. Step 2: Identify the key action

   Counting words starting with the prefix is the main task.

3. Final Answer:

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

4. Quick Check:

   Count words with prefix = Count how many words in a list start with a specific prefix [OK]

Hint: Focus on words starting with prefix, not other operations [OK]

Common Mistakes:

• Confusing counting with sorting
• Thinking the task is to replace prefixes
• Ignoring the prefix condition

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

easy

A. if (word == prefix) { /* do something */ }

B. if (word.Contains(prefix)) { /* do something */ }

C. if (word.EndsWith(prefix)) { /* do something */ }

D. if (word.StartsWith(prefix)) { /* do something */ }

Solution

1. Step 1: Recall string methods in C#

   The method StartsWith checks if a string begins with a given substring.

2. Step 2: Compare options

   Contains checks anywhere in the string, EndsWith checks the end, and == checks exact equality.

3. Final Answer:

   if (word.StartsWith(prefix)) { /* do something */ } -> Option D

4. Quick Check:

   Use StartsWith to check prefix [OK]

Hint: Use StartsWith method to check prefix in C# strings [OK]

Common Mistakes:

• Using Contains instead of StartsWith
• Checking equality instead of prefix
• Using EndsWith which checks suffix

3. What is the output of the following C# code?

string[] words = {"apple", "app", "application", "banana"};
string prefix = "app";
int count = 0;
foreach (var word in words) {
    if (word.StartsWith(prefix)) {
        count++;
    }
}
Console.WriteLine(count);

medium

A. 2

B. 1

C. 3

D. 4

Solution

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

   Words: "apple" (yes), "app" (yes), "application" (yes), "banana" (no).

2. Step 2: Count matches

   Three words start with "app", so count = 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Words starting with "app" = 3 [OK]

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

Common Mistakes:

• Counting only exact matches
• Including words that contain prefix but don't start with it
• Off-by-one counting errors

4. The following code is intended to count words starting with a prefix but gives wrong output. What is the error?

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++;
    }
}
Console.WriteLine(count);

medium

A. The loop condition should be i < words.Length instead of <=.

B. The prefix variable is incorrect.

C. The StartsWith method is used wrongly.

D. The count variable should be initialized inside the loop.

Solution

1. Step 1: Analyze the for loop condition

   The loop runs from i = 0 to i <= words.Length, which causes an index out of range error because valid indices are 0 to words.Length - 1.

2. Step 2: Correct the loop condition

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

3. Final Answer:

   The loop condition should be i < words.Length instead of <=. -> Option A

4. Quick Check:

   Loop index must be less than array length [OK]

Hint: Use i < array.Length to avoid index errors in loops [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Reinitializing count inside loop
• Misusing StartsWith method

5. Given a list of words ["tree", "trie", "trigger", "trip", "try", "matrix"] and prefix "tri", which C# code snippet correctly counts the words starting with the prefix and prints the count?

hard

A. int count = 0; foreach(var word in words) { if(word.Contains(prefix)) count++; } Console.WriteLine(count);

B. int count = words.Count(word => word.StartsWith(prefix)); Console.WriteLine(count);

C. int count = 0; for(int i=0; i<=words.Length; i++) { if(words[i].StartsWith(prefix)) count++; } Console.WriteLine(count);

D. int count = words.Length; Console.WriteLine(count);

Solution

1. Step 1: Understand the requirement

   We need to count words starting exactly with prefix "tri".

2. Step 2: Evaluate each option

   int count = words.Count(word => word.StartsWith(prefix)); Console.WriteLine(count); uses LINQ Count with StartsWith correctly.
   int count = 0; foreach(var word in words) { if(word.Contains(prefix)) count++; } Console.WriteLine(count); uses Contains which counts words containing prefix anywhere.
   int count = 0; for(int i=0; i<=words.Length; i++) { if(words[i].StartsWith(prefix)) count++; } Console.WriteLine(count); has loop with <= causing index error.
   int count = words.Length; Console.WriteLine(count); counts all words ignoring prefix.

3. Final Answer:

   int count = words.Count(word => word.StartsWith(prefix)); Console.WriteLine(count); -> Option B

4. Quick Check:

   Use LINQ Count with StartsWith for prefix count [OK]

Hint: Use LINQ Count with StartsWith for concise prefix counting [OK]

Common Mistakes:

• Using Contains instead of StartsWith
• Loop index out of range errors
• Counting all words without prefix check