Reading from files in C - Time & Space Complexity

Time Complexity: Reading from files

O(n)

Understanding Time Complexity

When reading from files, we want to know how the time needed grows as the file size grows.

We ask: How does reading more data affect how long the program runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

FILE *file = fopen("data.txt", "r");
char buffer[256];

while (fgets(buffer, sizeof(buffer), file)) {
    // process the line
}
fclose(file);
    

This code reads a file line by line until the end, processing each line.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading each line from the file using fgets inside a while loop.
• How many times: Once for every line in the file, repeating until the file ends.

How Execution Grows With Input

As the file gets bigger, the number of lines grows, so the loop runs more times.

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

Pattern observation: The time grows directly with the number of lines; double the lines, double the reads.

Final Time Complexity

Time Complexity: O(n)

This means the time to read the file grows in a straight line with the number of lines in the file.

Common Mistake

[X] Wrong: "Reading a file always takes the same time no matter the size."

[OK] Correct: The program reads each line one by one, so more lines mean more work and more time.

Interview Connect

Understanding how file reading time grows helps you write efficient programs and explain your code clearly in interviews.

Self-Check

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