Count Words with Given Prefix in DSA C# - Build from Scratch

Count Words with Given Prefix

📖 Scenario: You are working on a simple text search feature. You have a list of words from a document, and you want to find out how many words start with a certain prefix typed by a user.

🎯 Goal: Build a program that counts how many words in a list start with a given 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 matching words

💡 Why This Matters

🌍 Real World

Counting words with a given prefix is useful in search features, autocomplete suggestions, and filtering lists in apps.

💼 Career

This skill helps in building efficient text search and filtering functions in software development roles.

1

Create the list of words

Create a List<string> called words with these exact entries: "apple", "application", "banana", "apricot", "berry".

DSA C#

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

Hint

Use new List<string> { ... } to create the list with the exact words.

2

Create the prefix variable

Create a string variable called prefix and set it to "app".

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List words = new List { "apple", "application", "banana", "apricot", "berry" };
        // Create the string variable 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 foreach loop with variable word to go through words. Inside the loop, use word.StartsWith(prefix) to check if the word starts with the prefix. If yes, increase count by 1.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List words = new List { "apple", "application", "banana", "apricot", "berry" };
        string prefix = "app";
        // Create count variable and set to 0
        // Use foreach loop to check each word
        // Increase count if word starts with prefix
        // Your code here
    }
}

Hint

Use foreach (string word in words) and inside it check if (word.StartsWith(prefix)).

4

Print the count

Use Console.WriteLine to print the value of count.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List words = new List { "apple", "application", "banana", "apricot", "berry" };
        string prefix = "app";
        int count = 0;
        foreach (string word in words)
        {
            if (word.StartsWith(prefix))
            {
                count++;
            }
        }
        // Print the count variable
        // Your code here
    }
}

Hint

Use Console.WriteLine(count); to show the result.

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