File pointers in C - Time & Space Complexity

Time Complexity: File pointers

O(n)

Understanding Time Complexity

When working with file pointers in C, it's important to understand how the time to read or write data grows as the file size changes.

We want to know how the program's running time changes when it processes larger files.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

FILE *fp = fopen("data.txt", "r");
char buffer[100];

while (fgets(buffer, sizeof(buffer), fp) != NULL) {
    // process the line
}
fclose(fp);
    

This code reads a file line by line until it reaches the end.

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 until the end is reached.

How Execution Grows With Input

As the number of lines in the file grows, the number of times the loop runs grows roughly the same.

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

Pattern observation: The number of operations grows directly with the number of lines in the file.

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.

Common Mistake

[X] Wrong: "Reading a file always takes the same time no matter its 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 that handle large data smoothly.

Self-Check

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