String searching and extraction in C Sharp (C#)

Introduction

We use string searching and extraction to find specific words or parts inside a sentence or text. This helps us get useful information from bigger text.

Finding a word in a sentence to check if it exists.

Getting a person's name from a full address string.

Extracting a date from a message.

Checking if a password contains a special character.

Cutting out a part of a text to show as a preview.

Syntax

C Sharp (C#)

string text = "your text here";
int index = text.IndexOf("searchTerm");
string part = text.Substring(startIndex, length);

IndexOf returns the position of the first match or -1 if not found.

Substring extracts part of the string starting at startIndex for length characters.

Examples

Finds the position of "world" in the text. Result is 6.

C Sharp (C#)

string text = "Hello world!";
int pos = text.IndexOf("world");

Extracts 5 characters starting at position 6, which is "world".

C Sharp (C#)

string text = "Hello world!";
string part = text.Substring(6, 5);

Searches for "moon" which is not in the text, so result is -1.

C Sharp (C#)

string text = "Hello world!";
int pos = text.IndexOf("moon");

Sample Program

This program looks for the word "fox" in the sentence. If found, it shows the position and extracts the word from the sentence.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        string sentence = "The quick brown fox jumps over the lazy dog.";
        string wordToFind = "fox";

        int position = sentence.IndexOf(wordToFind);

        if (position != -1)
        {
            string extracted = sentence.Substring(position, wordToFind.Length);
            Console.WriteLine($"Found '{wordToFind}' at position {position}.");
            Console.WriteLine($"Extracted word: {extracted}");
        }
        else
        {
            Console.WriteLine($"'{wordToFind}' not found in the sentence.");
        }
    }
}

Important Notes

IndexOf is case-sensitive. Use ToLower() on both strings to search ignoring case.

Substring will throw an error if startIndex or length is out of range. Always check string length first.

Summary

Use IndexOf to find where a word or part starts in a string.

Use Substring to get a piece of the string by position and length.

Check if IndexOf returns -1 to know if the word is missing.

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