String searching and extraction in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the position of the first occurrence of "cat" in the text.

C Sharp (C#)

string text = "The cat is on the roof.";
int position = text.[1]("cat");
Console.WriteLine(position);

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

AReplace

BContains

CSubstring

DIndexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains instead of IndexOf returns a boolean, not a position.

The IndexOf method returns the position of the first occurrence of the specified substring.

2fill in blank

medium

Complete the code to extract the substring "dog" from the text.

C Sharp (C#)

string text = "The dog is barking.";
string animal = text.[1](4, 3);
Console.WriteLine(animal);

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

AIndexOf

BSubstring

CReplace

DContains

Attempts:

3 left

💡 Hint

Common Mistakes

Using IndexOf returns an integer, not a substring.

Substring(startIndex, length) extracts a part of the string starting at startIndex with the given length.

3fill in blank

hard

Fix the error in the code to check if the text contains the word "bird".

C Sharp (C#)

string text = "A bird is flying.";
bool hasBird = text.[1]("bird");
Console.WriteLine(hasBird);

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

AReplace

BIndexOf

CContains

DSubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using IndexOf returns an integer, which needs comparison to check presence.

Contains returns true if the substring exists in the string, which is suitable for this check.

4fill in blank

hard

Fill both blanks to extract the substring after the first space in the text.

C Sharp (C#)

string text = "Hello World";
int spaceIndex = text.[1](' ');
string afterSpace = text.[2](spaceIndex + 1);
Console.WriteLine(afterSpace);

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

AIndexOf

BSubstring

CLastIndexOf

DReplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using LastIndexOf instead of IndexOf finds the last space, not the first.

IndexOf finds the first space position; Substring extracts from that position plus one to the end.

5fill in blank

hard

Fill all three blanks to create a dictionary with words as keys and their lengths as values, only for words longer than 3 characters.

C Sharp (C#)

string[] words = {"apple", "bat", "carrot", "dog"};
var lengths = words.ToDictionary([1] => [1], [2] => [2].Length).Where(kv => kv.Value [3] 3).ToDictionary(kv => kv.Key, kv => kv.Value);
foreach (var item in lengths) Console.WriteLine($"{item.Key}: {item.Value}");

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

Aword

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for key and value selectors causes errors.

Using '<' instead of '>' filters wrong words.

We use 'word' as the key selector and value selector, and filter lengths greater than 3.

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