StringBuilder methods and performance in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of StringBuilder Append and Insert

What is the output of this C# code snippet?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder("Hello");
        sb.Append(" World");
        sb.Insert(5, ",");
        Console.WriteLine(sb.ToString());
    }
}

A Hello, World

B Hello World,

C Hello World

D Hello,World

💡 Hint

Remember that Insert adds characters at the specified index without removing existing ones.

The StringBuilder starts with "Hello". Append adds " World" making "Hello World". Insert at index 5 adds a comma after "Hello", resulting in "Hello, World".

❓ Predict Output

intermediate

StringBuilder Replace method effect

What will be printed by this code?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder("abracadabra");
        sb.Replace("a", "o", 3, 5);
        Console.WriteLine(sb.ToString());
    }
}

A abrocodobro

B abrocodobra

C obrocodobro

D obrocadobra

💡 Hint

Replace only affects the substring starting at index 3 for length 5.

Replace("a", "o", 3, 5) changes 'a' to 'o' only in substring from index 3 to 7. The original string is "abracadabra". The substring "acada" (index 3 to 7) has 'a's replaced with 'o's resulting in "ocodo". So the final string is "abrocodobra".

❓ Predict Output

advanced

Performance difference between string concatenation and StringBuilder

What is the expected output of this code snippet?

C Sharp (C#)

using System;
using System.Text;
using System.Diagnostics;

class Program {
    static void Main() {
        int n = 10000;
        Stopwatch sw = new Stopwatch();

        sw.Start();
        string s = "";
        for (int i = 0; i < n; i++) {
            s += i.ToString();
        }
        sw.Stop();
        Console.WriteLine("String concat time: " + sw.ElapsedMilliseconds + "ms");

        sw.Reset();
        sw.Start();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.Append(i.ToString());
        }
        string result = sb.ToString();
        sw.Stop();
        Console.WriteLine("StringBuilder time: " + sw.ElapsedMilliseconds + "ms");
    }
}

A String concat time: much greater than StringBuilder time

B String concat time: much less than StringBuilder time

C Both times are about the same

D Code throws an exception due to memory overflow

💡 Hint

String concatenation creates many intermediate strings, while StringBuilder modifies in place.

String concatenation inside a loop is slow because it creates new strings each time. StringBuilder is optimized for many appends and is much faster.

❓ Predict Output

advanced

Effect of Capacity and Length on StringBuilder

What will be the output of this code?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder(5);
        Console.WriteLine(sb.Capacity);
        sb.Append("HelloWorld");
        Console.WriteLine(sb.Capacity);
        sb.Length = 5;
        Console.WriteLine(sb.ToString());
    }
}

A

5
20
HelloWorld

B

5
10
Hello

C

5
16
Hello

D

5
16
HelloWorld

💡 Hint

Capacity grows automatically when exceeded. Length truncates the string.

Initial capacity is 5. After appending "HelloWorld" (10 chars), capacity grows to 16 (at least double the previous capacity). Setting Length to 5 truncates to "Hello".

❓ Predict Output

expert

StringBuilder Thread Safety and Exception

What happens when this code runs?

C Sharp (C#)

using System;
using System.Text;
using System.Threading.Tasks;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder();
        Parallel.For(0, 1000, i => {
            sb.Append(i);
        });
        Console.WriteLine(sb.Length);
    }
}

A Prints a number close to 2893 without error

B Throws a System.ArgumentOutOfRangeException

C Throws a System.IndexOutOfRangeException

D Throws a System.InvalidOperationException

💡 Hint

StringBuilder is not thread-safe for concurrent writes.

StringBuilder is not safe to use from multiple threads without synchronization. Concurrent Append calls cause InvalidOperationException.

Practice

1. What is the main advantage of using StringBuilder over regular string concatenation in C#?

easy

A. It modifies the string without creating new string copies, improving performance.

B. It automatically sorts the characters in the string.

C. It encrypts the string for security.

D. It converts strings to uppercase by default.

Solution

1. Step 1: Understand string immutability in C#

   Regular strings cannot be changed once created, so concatenation creates new strings each time.

2. Step 2: How StringBuilder works

   StringBuilder changes the text in place without making new copies, which is faster for many changes.

3. Final Answer:

   It modifies the string without creating new string copies, improving performance. -> Option A

4. Quick Check:

   StringBuilder avoids new copies [OK]

Hint: StringBuilder changes text without copying strings [OK]

Common Mistakes:

• Thinking StringBuilder sorts or encrypts text
• Believing StringBuilder changes case automatically
• Confusing StringBuilder with regular string methods

2. Which of the following is the correct way to append text to a StringBuilder named sb?

easy

A. sb.Append("Hello");

B. sb.Add("Hello");

C. sb.Insert("Hello");

D. sb.Concat("Hello");

Solution

1. Step 1: Recall StringBuilder methods

   Append is the method used to add text at the end of the current content.

2. Step 2: Check method names

   Add and Concat are not valid StringBuilder methods; Insert adds text at a specific position, not at the end.

3. Final Answer:

   sb.Append("Hello"); -> Option A

4. Quick Check:

   Append adds text at end [OK]

Hint: Use Append() to add text at the end [OK]

Common Mistakes:

• Using Add() which does not exist
• Confusing Insert() with Append()
• Trying to use Concat() on StringBuilder

3. What will be the output of the following C# code?

var sb = new System.Text.StringBuilder("Hi");
sb.Append(" there");
sb.Replace("Hi", "Hello");
sb.Remove(5, 1);
Console.WriteLine(sb.ToString());

medium

A. Hello here

B. Hello there

C. Hellothere

D. Hi there

Solution

1. Step 1: Trace Append and Replace

   Start with "Hi", Append adds " there" -> "Hi there". Replace "Hi" with "Hello" -> "Hello there".

2. Step 2: Apply Remove

   Remove(5,1) removes 1 character at index 5 (0-based). Index 5 is the space between "Hello" and "there", so removing it joins words -> "Hellothere".

3. Final Answer:

   Hellothere -> Option C

4. Quick Check:

   Remove space at index 5 = "Hellothere" [OK]

Hint: Remember Remove(index, count) deletes characters at index [OK]

Common Mistakes:

• Forgetting zero-based index in Remove
• Assuming Replace changes all occurrences incorrectly
• Not converting StringBuilder to string before printing

4. Identify the error in this code snippet:

var sb = new System.Text.StringBuilder();
sb.Append("Start");
sb.Remove(10, 3);
Console.WriteLine(sb.ToString());

medium

A. ToString method is missing parentheses.

B. Append method is used incorrectly.

C. StringBuilder cannot be empty when created.

D. Remove method call will throw an ArgumentOutOfRangeException.

Solution

1. Step 1: Check Remove parameters

   Remove(10, 3) tries to remove 3 characters starting at index 10, but current string length is 5 ("Start").

2. Step 2: Understand exception

   Removing beyond string length causes ArgumentOutOfRangeException at runtime.

3. Final Answer:

   Remove method call will throw an ArgumentOutOfRangeException. -> Option D

4. Quick Check:

   Remove index out of range = Exception [OK]

Hint: Check Remove index is within current length [OK]

Common Mistakes:

• Assuming Remove silently ignores invalid indexes
• Thinking Append is incorrect here
• Believing ToString needs no parentheses

5. You want to build a comma-separated list of numbers from 1 to 5 using StringBuilder. Which code snippet is the most efficient and correct?

hard

A. var sb = new StringBuilder();
for(int i=1; i<=5; i++) {
sb.Append(i).Append(",");
}
sb.Remove(sb.Length - 2, 1);
Console.WriteLine(sb.ToString());

B. var sb = new StringBuilder();
for(int i=1; i<=5; i++) {
sb.Append(i);
if(i < 5) sb.Append(",");
}
Console.WriteLine(sb.ToString());

C. var sb = new StringBuilder();
sb.Append("1,2,3,4,5,");
Console.WriteLine(sb.ToString());

D. var sb = new StringBuilder();
for(int i=1; i<=5; i++) {
sb.Append(i + ",");
}
Console.WriteLine(sb.ToString());

Solution

1. Step 1: Appending comma after each number then removing trailing

   Appends number and comma each time, then removes last comma. Remove call adds overhead.

2. Step 2: Conditional comma append

   Appends number, then comma only if not last number. Avoids extra Remove call, more efficient and clear.

3. Step 3: Analyze options B and C

   B appends comma after last number, no removal, so extra comma remains. C hardcodes string, no loop, less flexible.

4. Final Answer:

   sb.Append(i); if(i < 5) sb.Append(","); -> Option B

5. Quick Check:

   Append comma conditionally [OK]

Hint: Add comma only between items, not after last [OK]

Common Mistakes:

• Leaving trailing comma without removal
• Hardcoding string instead of looping
• Removing characters unnecessarily