String searching and extraction in C Sharp (C#) - Step-by-Step Execution

Concept Flow - String searching and extraction

Start with input string

↓

Search for substring or pattern

↓

Is substring found?

No→Return -1 or empty

Yes↓

Extract substring or index

↓

Use extracted data

↓

End

The program starts with a string, searches for a substring or pattern, checks if found, then extracts or returns the index, and finally uses the extracted data.

Execution Sample

C Sharp (C#)

string text = "Hello, world!";
int index = text.IndexOf("world");
string extracted = index >= 0 ? text.Substring(index, 5) : "";
Console.WriteLine(extracted);

This code searches for "world" in the text, extracts it if found, and prints it.

Execution Table

Step	Action	Evaluation	Result
1	Set text to "Hello, world!"	N/A	text = "Hello, world!"
2	Search for "world" using IndexOf	text.IndexOf("world")	index = 7
3	Check if index >= 0	7 >= 0	True
4	Extract substring from index 7 length 5	text.Substring(7, 5)	extracted = "world"
5	Print extracted substring	Console.WriteLine(extracted)	Output: world

💡 Extraction and printing done; program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
text	"Hello, world!"	"Hello, world!"	"Hello, world!"	"Hello, world!"
index	undefined	7	7	7
extracted	undefined	undefined	"world"	"world"

Key Moments - 2 Insights

Why do we check if index >= 0 before extracting?

Because IndexOf returns -1 if the substring is not found. Checking index >= 0 (see step 3) ensures we only extract when the substring exists.

What happens if the substring is not found?

If not found, index is -1, so the condition in step 3 is false, and extraction is skipped, resulting in an empty string.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'index' after step 2?

A 7

B -1

C 0

D 5

💡 Hint

Check the 'Result' column in step 2 of the execution_table.

At which step does the program decide if it will extract the substring?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Evaluation' column in step 3 where the condition index >= 0 is checked.

If the substring "world" was not in the text, what would 'extracted' be after step 4?

A "world"

B "Hello"

C "" (empty string)

D null

💡 Hint

Refer to the key moment about what happens if substring is not found and the condition in step 3.

Concept Snapshot

String searching uses methods like IndexOf to find substring positions.
If found (index >= 0), use Substring to extract part of the string.
If not found, handle by returning empty or special value.
Always check index before extracting to avoid errors.
Use extracted string as needed (e.g., print or store).

Full Transcript

This example shows how to search for a substring "world" inside a string "Hello, world!" using C# methods. First, the program sets the text variable. Then it uses IndexOf to find the starting position of "world". If the substring is found (index is 7), it extracts the substring starting at that index with length 5. Finally, it prints the extracted substring. If the substring was not found, the program would skip extraction and print an empty string. This step-by-step process helps avoid errors and correctly extracts parts of strings.