IO modes (r, w, a) in Ruby - Time & Space Complexity

Time Complexity: IO modes (r, w, a)

O(n)

Understanding Time Complexity

When working with files in Ruby, we use different modes to read, write, or add data. Understanding how the time to complete these actions grows with file size helps us write better programs.

We want to know: how does the time to read, write, or append change as the file gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

# Open a file in read mode and read all lines
File.open('data.txt', 'r') do |file|
  lines = file.readlines
end

# Open a file in write mode and write lines
File.open('data.txt', 'w') do |file|
  file.puts("Hello World")
end

# Open a file in append mode and add a line
File.open('data.txt', 'a') do |file|
  file.puts("New line")
end
    

This code reads all lines from a file, writes new content replacing old data, and appends a new line at the end.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading all lines from the file (in read mode) involves going through each line one by one.
• How many times: The reading loop runs once for each line in the file.
• Writing and appending write data once, no loops involved.

How Execution Grows With Input

When reading, the time grows as the file gets bigger because each line is read one after another.

Input Size (lines)	Approx. Operations
10	About 10 line reads
100	About 100 line reads
1000	About 1000 line reads

Pattern observation: Reading time grows directly with the number of lines. Writing or appending a single line stays about the same time regardless of file size.

Final Time Complexity

Time Complexity: O(n)

This means reading takes longer as the file grows, because it reads each line once. Writing or appending a small amount is constant time.

Common Mistake

[X] Wrong: "Writing to a file always takes longer as the file gets bigger."

[OK] Correct: Writing with mode 'w' replaces the file, so time depends on what you write, not the old file size. Appending adds data at the end, usually quickly.

Interview Connect

Knowing how file operations scale helps you handle data efficiently in real projects. It shows you understand how programs interact with files and manage time well.

Self-Check

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