String searching and extraction in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of IndexOf and Substring

What is the output of this C# code snippet?

C Sharp (C#)

string text = "Hello, world!";
int index = text.IndexOf('o');
string result = text.Substring(index, 5);
Console.WriteLine(result);

A ow ,o

B o, worl

C o, wo

D lrow ,o

💡 Hint

Remember that Substring's second parameter is length, not end index.

IndexOf('o') returns 4 (the first 'o' in "Hello"). Substring(4,5) extracts 5 characters starting at index 4: 'o, wo'.

❓ Predict Output

intermediate

Extracting domain from email

What is the output of this C# code?

C Sharp (C#)

string email = "[e-mail]";
int atPos = email.IndexOf('@');
string domain = email.Substring(atPos + 1);
Console.WriteLine(domain);

A user@

B [e-mail]

C @example.com

D example.com

💡 Hint

Substring starting after '@' extracts the domain part.

IndexOf('@') finds position of '@'. Substring(atPos + 1) extracts everything after '@', which is 'example.com'.

❓ Predict Output

advanced

Using Regex to extract numbers

What does this C# code print?

C Sharp (C#)

using System.Text.RegularExpressions;

string input = "Order1234: 56 items";
var match = Regex.Match(input, @"\d+");
Console.WriteLine(match.Value);

A 1234

B 56

C Order1234

D items

💡 Hint

Regex.Match finds the first sequence of digits.

The pattern \d+ matches one or more digits. The first such sequence in the string is '1234'.

❓ Predict Output

advanced

Extracting substring between two characters

What is the output of this C# code?

C Sharp (C#)

string data = "[start]middle[end]";
int startIndex = data.IndexOf('[') + 1;
int endIndex = data.IndexOf(']');
string extracted = data.Substring(startIndex, endIndex - startIndex);
Console.WriteLine(extracted);

A start]middle

B start

C start]middle[end

D start]middle[end]

💡 Hint

Substring extracts from after '[' up to before ']'.

IndexOf('[') returns 0, so startIndex is 1. IndexOf(']') returns 6. Substring(1, 5) extracts 'start'.

🧠 Conceptual

expert

Behavior of String.IndexOf with missing substring

What is the value of variable 'pos' after running this C# code?

C Sharp (C#)

string text = "hello world";
int pos = text.IndexOf("xyz");

A -1

B 0

C throws an exception

D length of the string

💡 Hint

IndexOf returns -1 if substring is not found.

When the substring is not found, IndexOf returns -1 instead of throwing an error or returning 0.

Practice

1. What does the IndexOf method return if the searched substring is not found in a string?

easy

A. 0

B. -1

C. null

D. An exception is thrown

Solution

1. Step 1: Understand IndexOf behavior

   The IndexOf method returns the zero-based index of the first occurrence of the substring if found.

2. Step 2: Check return value when substring is missing

   If the substring is not found, IndexOf returns -1 to indicate absence.

3. Final Answer:

   -1 -> Option B

4. Quick Check:

   IndexOf returns -1 if substring missing [OK]

Hint: Remember: Not found means -1 from IndexOf [OK]

Common Mistakes:

• Thinking it returns 0 when not found
• Expecting null instead of -1
• Assuming it throws an error if missing

2. Which of the following is the correct syntax to extract a substring starting at index 3 with length 5 from a string text?

easy

A. text.Substring(3, 5);

B. text.SubString(5, 3);

C. text.Substring(5);

D. text.Substr(3, 5);

Solution

1. Step 1: Recall Substring method signature

   Substring takes start index first, then length: Substring(int startIndex, int length).

2. Step 2: Match correct syntax

   text.Substring(3, 5); uses correct method name and parameter order: start at 3, length 5.

3. Final Answer:

   text.Substring(3, 5); -> Option A

4. Quick Check:

   Correct method and parameters = text.Substring(3, 5); [OK]

Hint: Remember: Substring(startIndex, length) with capital S [OK]

Common Mistakes:

• Using wrong method name like Substr or SubString
• Swapping start index and length parameters
• Using only one parameter when two are needed

3. What is the output of this code?

string s = "hello world";
int pos = s.IndexOf("world");
string part = s.Substring(pos, 5);
Console.WriteLine(part);

medium

A. hello

B. worldd

C. world

D. error

Solution

1. Step 1: Find index of "world" in string

   "world" starts at index 6 in "hello world".

2. Step 2: Extract substring from index 6 with length 5

   Substring(6, 5) extracts "world" exactly.

3. Final Answer:

   world -> Option C

4. Quick Check:

   IndexOf + Substring extracts "world" [OK]

Hint: IndexOf finds start, Substring extracts exact length [OK]

Common Mistakes:

• Using wrong start index for Substring
• Confusing length parameter
• Expecting output to include extra characters

4. The following code throws an exception. What is the main cause?

string s = "example";
int pos = s.IndexOf("z");
string part = s.Substring(pos, 3);
Console.WriteLine(part);

medium

A. Substring called with negative start index

B. IndexOf throws exception if not found

C. Length parameter is too large

D. Console.WriteLine cannot print substrings

Solution

1. Step 1: Check IndexOf result for "z"

   "z" is not in "example", so IndexOf returns -1.

2. Step 2: Substring called with start index -1 causes exception

   Substring cannot start at negative index, so it throws ArgumentOutOfRangeException.

3. Final Answer:

   Substring called with negative start index -> Option A

4. Quick Check:

   Negative index in Substring causes error [OK]

Hint: Check if IndexOf is -1 before using Substring [OK]

Common Mistakes:

• Assuming IndexOf throws exception when not found
• Ignoring negative index causes Substring error
• Blaming Console.WriteLine for error

5. You want to extract the first word from a sentence stored in string sentence. Which code correctly extracts the first word assuming words are separated by spaces?

hard

A. string firstWord = sentence.Substring(sentence.IndexOf(' ') + 1);

B. string firstWord = sentence.Substring(0, sentence.IndexOf(' '));

C. int spacePos = sentence.IndexOf(' ');
string firstWord = sentence.Substring(spacePos);

D. int spacePos = sentence.IndexOf(' ');
string firstWord = spacePos == -1 ? sentence : sentence.Substring(0, spacePos);

Solution

1. Step 1: Find position of first space

   Use IndexOf(' ') to find where the first space is.

2. Step 2: Extract substring from start to space or whole string if no space

   If no space found (-1), the whole sentence is one word; else extract from 0 to space position.

3. Final Answer:

   int spacePos = sentence.IndexOf(' '); string firstWord = spacePos == -1 ? sentence : sentence.Substring(0, spacePos); -> Option D

4. Quick Check:

   Check for space, then substring from start [OK]

Hint: Check if space exists before substring to avoid errors [OK]

Common Mistakes:

• Not handling case when no space exists
• Extracting substring after space instead of before
• Using wrong substring parameters