StringBuilder methods and performance in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to append a string to the StringBuilder.

C Sharp (C#)

var sb = new StringBuilder();
sb.[1]("Hello");
Console.WriteLine(sb.ToString());

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

AAdd

BInsert

CConcat

DAppend

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add which does not exist for StringBuilder.

Using Concat which is a string method, not StringBuilder.

Using Insert which adds text at a specific position, not at the end.

The Append method adds the specified string to the end of the StringBuilder.

2fill in blank

medium

Complete the code to insert a string at the beginning of the StringBuilder.

C Sharp (C#)

var sb = new StringBuilder("World");
sb.[1](0, "Hello ");
Console.WriteLine(sb.ToString());

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

AInsert

BAppend

CReplace

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Append which adds text only at the end.

Using Replace which changes existing text.

Using Remove which deletes text.

The Insert method inserts a string at the specified index, here at position 0 (the start).

3fill in blank

hard

Fix the error in the code to remove 5 characters starting at index 6.

C Sharp (C#)

var sb = new StringBuilder("Hello, World!");
sb.[1](6, 5);
Console.WriteLine(sb.ToString());

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

ADelete

BRemove

CCut

DErase

Attempts:

3 left

💡 Hint

Common Mistakes

Using Delete which does not exist in StringBuilder.

Using Cut or Erase which are not valid methods.

The Remove method deletes a specified number of characters starting at a given index.

4fill in blank

hard

Fill both blanks to replace 'World' with 'C#' in the StringBuilder.

C Sharp (C#)

var sb = new StringBuilder("Hello, World!");
sb.[1]("World", [2]);
Console.WriteLine(sb.ToString());

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

A"C#"

BReplace

C"Java"

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove which deletes text but does not replace.

Swapping the order of method and replacement string.

The Replace method replaces all occurrences of a string with another string. Here, 'World' is replaced with 'C#'.

5fill in blank

hard

Fill all three blanks to create a StringBuilder, append 'Hello', insert 'C# ', and remove 1 character at index 5.

C Sharp (C#)

var sb = new [1]();
sb.[2]("Hello");
sb.[3](0, "C# ");
sb.Remove(5, 1);
Console.WriteLine(sb.ToString());

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

AStringBuilder

BAppend

CInsert

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of StringBuilder for the object.

Mixing up Append and Insert methods.

First, create a StringBuilder object. Then Append adds 'Hello' at the end. Next, Insert adds 'C# ' at the start. Finally, Remove deletes one character at index 5.

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