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

Time Complexity: Reading text files

O(n)

Understanding Time Complexity

When reading text files, it's important to know how the time it takes grows as the file gets bigger.

We want to understand how the program's work changes when the file size changes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

using System.IO;

string[] lines = File.ReadAllLines("file.txt");
foreach (string line in lines)
{
    Console.WriteLine(line);
}
    

This code reads all lines from a text file into an array, then prints each line.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading each line from the file and printing it.
• How many times: Once for each line in the file.

How Execution Grows With Input

As the number of lines grows, the program does more work roughly in direct proportion.

Input Size (n)	Approx. Operations
10	About 10 reads and prints
100	About 100 reads and prints
1000	About 1000 reads and prints

Pattern observation: The work grows evenly as the file gets bigger.

Final Time Complexity

Time Complexity: O(n)

This means the time to read and print grows directly with the number of lines in the file.

Common Mistake

[X] Wrong: "Reading a file is always instant, so time doesn't grow with file size."

[OK] Correct: Reading each line takes time, so bigger files take longer because more lines must be processed.

Interview Connect

Understanding how file reading time grows helps you write efficient programs and answer questions about handling large data.

Self-Check

"What if we read the file line by line using a stream instead of reading all lines at once? How would the time complexity change?"

Practice

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

easy

A. Reads the entire file content as a single string.

B. Reads all lines from a text file and returns them as a string array.

C. Writes lines to a text file.

D. Deletes the specified text file.

Solution

1. Step 1: Understand the method purpose

   File.ReadAllLines reads a text file and returns each line as an element in a string array.

2. Step 2: Compare with other methods

   File.ReadAllText returns the whole file as one string, not an array of lines.

3. Final Answer:

   Reads all lines from a text file and returns them as a string array. -> Option B

4. Quick Check:

   ReadAllLines = string array [OK]

Hint: ReadAllLines returns array of lines, not one big string [OK]

Common Mistakes:

• Confusing ReadAllLines with ReadAllText
• Thinking it writes to a file
• Assuming it deletes files

2. Which of the following is the correct syntax to read all text from a file named "data.txt" using File.ReadAllText?

easy

A. string content = File.ReadAllText("data.txt");

B. string content = File.ReadAllLines("data.txt");

C. string[] content = File.ReadAllText("data.txt");

D. string[] content = File.ReadAllLines("data.txt");

Solution

1. Step 1: Identify method return types

   File.ReadAllText returns a single string, so the variable must be string.

2. Step 2: Match syntax with variable type

   string content = File.ReadAllText("data.txt"); uses string variable with ReadAllText correctly. Options B and D mismatch method and variable types. string[] content = File.ReadAllText("data.txt"); tries to assign string to string array.

3. Final Answer:

   string content = File.ReadAllText("data.txt"); -> Option A

4. Quick Check:

   ReadAllText returns string, so variable is string [OK]

Hint: Match method return type with variable type [OK]

Common Mistakes:

• Assigning ReadAllText to string array
• Using ReadAllLines but expecting string
• Wrong variable type for method

3. What will be the output of this C# code if the file "example.txt" contains three lines: "Hello", "World", "!"?

string[] lines = File.ReadAllLines("example.txt");
Console.WriteLine(lines.Length);

medium

A. 3

B. Hello World !

C. 1

D. Error at runtime

Solution

1. Step 1: Understand ReadAllLines output

   The method returns an array with one element per line in the file. Here, 3 lines means array length is 3.

2. Step 2: Analyze Console.WriteLine output

   Printing lines.Length outputs the number of lines, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Array length = number of lines = 3 [OK]

Hint: ReadAllLines length equals number of lines in file [OK]

Common Mistakes:

• Expecting content printed instead of length
• Confusing ReadAllText with ReadAllLines
• Assuming runtime error without cause

4. Identify the error in this code snippet that reads all lines from "notes.txt":

string[] lines = File.ReadAllLines(notes.txt);
foreach (string line in lines)
{
    Console.WriteLine(line);
}

medium

A. Console.WriteLine cannot print strings.

B. File.ReadAllLines returns a string, not string array.

C. foreach loop syntax is incorrect.

D. Missing quotes around the file name in ReadAllLines.

Solution

1. Step 1: Check file path syntax

   The file name must be a string literal, so it needs quotes: "notes.txt".

2. Step 2: Verify other code parts

   File.ReadAllLines returns string array, foreach syntax is correct, and Console.WriteLine can print strings.

3. Final Answer:

   Missing quotes around the file name in ReadAllLines. -> Option D

4. Quick Check:

   File path must be in quotes [OK]

Hint: File names must be in quotes in method calls [OK]

Common Mistakes:

• Forgetting quotes around file path
• Thinking ReadAllLines returns string
• Misunderstanding foreach syntax

5. You want to read a text file and count how many lines contain the word "error" (case insensitive). Which code snippet correctly does this?

hard

A. var lines = File.ReadAllLines("log.txt");
int count = 0;
foreach(var line in lines) {
if(line.Contains("error")) count++;
}

B. string content = File.ReadAllText("log.txt");
int count = content.Split('\n').Count(line => line.Contains("error"));

C. var lines = File.ReadAllLines("log.txt");
int count = lines.Count(line => line.ToLower().Contains("error"));

D. string[] lines = File.ReadAllText("log.txt").Split('\n');
int count = lines.Count(line => line.Contains("error"));

Solution

1. Step 1: Choose method to read lines

   File.ReadAllLines returns an array of lines, perfect for line-by-line processing.

2. Step 2: Count lines with "error" case-insensitive

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); converts each line to lowercase and checks if it contains "error", then counts matches using LINQ.

3. Step 3: Check other options

   string content = File.ReadAllText("log.txt"); int count = content.Split('\n').Count(line => line.Contains("error")); uses ReadAllText but misses case-insensitive check. var lines = File.ReadAllLines("log.txt"); int count = 0; foreach(var line in lines) { if(line.Contains("error")) count++; } misses case-insensitive check. string[] lines = File.ReadAllText("log.txt").Split('\n'); int count = lines.Count(line => line.Contains("error")); incorrectly assigns ReadAllText to string array without ToLower.

4. Final Answer:

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); -> Option C

5. Quick Check:

   Use ReadAllLines + ToLower + Count for case-insensitive search [OK]

Hint: Use ReadAllLines and ToLower for case-insensitive line checks [OK]

Common Mistakes:

• Ignoring case sensitivity
• Using ReadAllText but treating as array
• Not converting lines to lowercase