Why is Reading Lines from stdin Slower in C than in Python, and How Can It Be Improved?

Performance Penalty of Reading lines from stdin in C Compared to Python

One might be surprised to discover that reading lines of text from stdin in C is significantly slower than the equivalent operation in Python. To address this performance disparity, let's delve into why it occurs and provide solutions to mitigate it.

Default Settings in C

By default, C 's cin stream is synchronized with standard input and output (stdio), resulting in disabled input buffering. This means that cin performs one character read at a time, resulting in numerous system calls, which is a performance bottleneck.

Python's Optimized Input Handling

In contrast, Python employs optimized input handling mechanisms that buffer input in chunks, reducing system calls and improving performance.

Improving C Read Performance

To enhance the reading performance of C , two approaches can be adopted:

1. Disable cin's Synchronization with stdio:

   std::cin.sync_with_stdio(false);

2. Utilize fgets Instead:

   char buffer[1024];
   while (fgets(buffer, sizeof(buffer), stdin)) {
       // process line
   }

Comparison of Approaches

The following table compares the performance of various implementations:

Implementation	Lines per Second
Python (default)	3,571,428
cin (default/naive)	819,672
cin (no sync)	12,500,000
fgets	14,285,714
wc (not fair comparison)	54,644,808

As evident, disabling cin synchronization or using fgets significantly improves the performance of C , making it comparable to Python in terms of reading lines from stdin.

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