Writing text files in C Sharp (C#) - Time & Space Complexity

Time Complexity: Writing text files

O(n)

Understanding Time Complexity

When writing text files, it's important to understand how the time it takes grows as the file size grows.

We want to know how the program's work changes when we write more lines or characters.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

using System.IO;

string[] lines = new string[] { "Line1", "Line2", "Line3" };

using StreamWriter writer = new StreamWriter("output.txt");
foreach (string line in lines)
{
    writer.WriteLine(line);
}

This code writes each line from an array into a text file, one line at a time.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Writing each line to the file inside a loop.
• How many times: Once for each line in the input array.

How Execution Grows With Input

As the number of lines increases, the program writes more times, so the work grows steadily.

Input Size (n)	Approx. Operations
10	10 write operations
100	100 write operations
1000	1000 write operations

Pattern observation: The number of write actions grows directly with the number of lines.

Final Time Complexity

Time Complexity: O(n)

This means the time to write grows in a straight line as the number of lines grows.

Common Mistake

[X] Wrong: "Writing a file always takes the same time no matter how big it is."

[OK] Correct: Writing more lines means more work, so time grows with file size.

Interview Connect

Understanding how file writing time grows helps you explain performance in real programs that save data.

Self-Check

"What if we buffered all lines into one big string and wrote once? How would the time complexity change?"

Practice

1. What does the File.WriteAllText method do in C#?

easy

A. It creates a new text file or overwrites an existing one with the specified content.

B. It reads all text from a file and returns it as a string.

C. It appends text to the end of an existing file without overwriting.

D. It deletes a specified text file from the disk.

Solution

1. Step 1: Understand the purpose of File.WriteAllText

   This method is designed to write text to a file, creating it if it doesn't exist or overwriting it if it does.

2. Step 2: Compare with other file methods

   Reading text uses File.ReadAllText, appending uses File.AppendAllText, and deleting uses File.Delete, so these are different methods.

3. Final Answer:

   It creates a new text file or overwrites an existing one with the specified content. -> Option A

4. Quick Check:

   File.WriteAllText writes or overwrites text [OK]

Hint: WriteAllText creates or overwrites files with given text [OK]

Common Mistakes:

• Confusing WriteAllText with ReadAllText
• Thinking it appends instead of overwriting
• Assuming it deletes files

2. Which of the following is the correct syntax to write "Hello World" to a file named "greeting.txt" using File.WriteAllText?

easy

A. File.WriteText("greeting.txt", "Hello World");

B. File.WriteAllText("Hello World", "greeting.txt");

C. WriteAllText.File("greeting.txt", "Hello World");

D. File.WriteAllText("greeting.txt", "Hello World");

Solution

1. Step 1: Check method name and parameters

   The correct method is File.WriteAllText with the first parameter as the file path and the second as the text content.

2. Step 2: Validate syntax correctness

   File.WriteAllText("greeting.txt", "Hello World"); matches the correct method name and parameter order. Options A, B, and D have incorrect method names or parameter order.

3. Final Answer:

   File.WriteAllText("greeting.txt", "Hello World"); -> Option D

4. Quick Check:

   Correct method and parameter order = File.WriteAllText("greeting.txt", "Hello World"); [OK]

Hint: Method name is File.WriteAllText(path, content) [OK]

Common Mistakes:

• Swapping parameters order
• Using incorrect method names like WriteText
• Calling method on WriteAllText instead of File

3. What will be the content of the file "notes.txt" after running this code?

string path = "notes.txt";
File.WriteAllText(path, "Line 1\nLine 2");
File.WriteAllText(path, "New Line");

medium

A. New Line

B. Line 1\nLine 2

C. Empty file

D. Line 1\nLine 2\nNew Line

Solution

1. Step 1: Analyze first File.WriteAllText call

   The first call writes "Line 1\nLine 2" to "notes.txt", creating or overwriting the file.

2. Step 2: Analyze second File.WriteAllText call

   The second call overwrites the entire file content with "New Line", replacing previous text.

3. Final Answer:

   New Line -> Option A

4. Quick Check:

   Second WriteAllText overwrites file content [OK]

Hint: WriteAllText overwrites file, last call wins [OK]

Common Mistakes:

• Assuming text appends instead of overwriting
• Thinking both texts combine in file
• Ignoring the second WriteAllText call

4. Identify the error in this code snippet that tries to write "Data" to "output.txt":

File.WriteAllText(output.txt, "Data");

medium

A. File.WriteAllText cannot write strings

B. Missing quotes around the file name output.txt

C. The method name should be WriteTextAll

D. The file path must be an absolute path

Solution

1. Step 1: Check the file path parameter

   The file path must be a string, so it needs quotes around it. Here, output.txt is unquoted, causing a syntax error.

2. Step 2: Verify method name and parameter type

   The method name is correct, and it accepts strings. Absolute path is not mandatory; relative paths work fine.

3. Final Answer:

   Missing quotes around the file name output.txt -> Option B

4. Quick Check:

   File path must be a string literal [OK]

Hint: File path must be in quotes as a string [OK]

Common Mistakes:

• Forgetting quotes around file path
• Mixing method name order
• Thinking absolute path is required

5. You want to write multiple lines from a string array lines to a file named "log.txt" so that each element appears on its own line. Which code snippet correctly does this using File.WriteAllText?

hard

A. File.WriteAllText("log.txt", lines);

B. File.WriteAllText("log.txt", lines.ToString());

C. File.WriteAllText("log.txt", string.Join("\n", lines));

D. File.WriteAllText("log.txt", string.Concat(lines));

Solution

1. Step 1: Understand the input and desired output

   We have a string array lines and want each element on its own line in the file.

2. Step 2: Check how to convert array to single string with line breaks

   Using string.Join("\n", lines) joins array elements with newline characters, creating the correct multiline string.

3. Step 3: Validate other options

   lines.ToString() returns type name, not content; passing array directly is invalid; string.Concat joins without separators.

4. Final Answer:

   File.WriteAllText("log.txt", string.Join("\n", lines)); -> Option C

5. Quick Check:

   Join array with \n for multiline text [OK]

Hint: Join array with \n before writing text file [OK]

Common Mistakes:

• Passing array directly instead of a string
• Using ToString() on array expecting content
• Concatenating without separators