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?"