String searching and extraction in C Sharp (C#) - Mini Project: Build & Apply

String Searching and Extraction

📖 Scenario: You work in a customer service team. You receive messages from customers that include their order IDs inside the text. You want to find and extract these order IDs to help track their orders quickly.

🎯 Goal: Build a simple C# program that searches a message for an order ID and extracts it.

📋 What You'll Learn

Create a string variable with a customer message containing an order ID.

Create a string variable for the order ID prefix to search for.

Use string searching to find the order ID in the message.

Extract the order ID substring from the message.

Print the extracted order ID.

💡 Why This Matters

🌍 Real World

Customer service teams often need to find order IDs or tracking numbers inside messages to help customers quickly.

💼 Career

Knowing how to search and extract parts of text is useful for data processing, automation, and building user-friendly tools.

1

Create the customer message string

Create a string variable called message and set it to the exact text: "Hello, my order ID is ORD12345. Can you check the status?"

C Sharp (C#)

// Your code here

Hint

Use double quotes to create the string and assign it to message.

2

Create the order ID prefix variable

Create a string variable called orderPrefix and set it to the exact value "ORD".

C Sharp (C#)

string message = "Hello, my order ID is ORD12345. Can you check the status?";
// Your code here

Hint

This prefix helps us find where the order ID starts in the message.

3

Find and extract the order ID

Use IndexOf on message with orderPrefix to find the start index. Then use Substring to extract 8 characters starting from that index and save it in a string variable called orderId.

C Sharp (C#)

string message = "Hello, my order ID is ORD12345. Can you check the status?";
string orderPrefix = "ORD";
// Your code here

Hint

Use IndexOf to find where "ORD" starts, then Substring to get the full order ID which has 8 characters.

4

Print the extracted order ID

Write a Console.WriteLine statement to print the orderId variable.

C Sharp (C#)

string message = "Hello, my order ID is ORD12345. Can you check the status?";
string orderPrefix = "ORD";
int startIndex = message.IndexOf(orderPrefix);
string orderId = message.Substring(startIndex, 8);
// Your code here

Hint

This will show the extracted order ID on the screen.

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