Why is C 's `cin` Significantly Slower than Python's Input, and How Can It Be Improved?

Why Reading Lines from stdin is Much Slower in C than Python

While comparing input reading speeds in Python and C , it was observed that the C code performed an order of magnitude slower.

cin's Lack of Buffering

This discrepancy arises from a fundamental difference in default behavior. cin in C , by default, is synchronized with standard input/output (stdio), causing it to forego input buffering, resulting in the reading of characters one at a time, an expensive process in terms of system calls.

Solution: Disable Synchronization

To improve performance, cin's synchronization with stdio can be disabled by adding the following line at the beginning of the program:

std::ios_base::sync_with_stdio(false);

This allows C streams to buffer I/O independently, significantly enhancing reading speed.

Straw poll to illustrate the difference:

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 (unfair comparison)	54,644,808

This table demonstrates the vast difference in reading speed when cin's synchronization is disabled.

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