Beware of using cerr on Codeforces

Many beginners hear this advice:

"Use cerr for debugging, it is ignored by the judge."

This statement is only partially true and can be misleading.

This blog explains when cerr is safe, when it is dangerous, and best practices used by experienced Codeforces users.

What is true about cerr

Output sent to cerr is not checked by the judge

It does not affect correctness

You will not get Wrong Answer because of cerr output

This is why many people recommend using cerr for debugging.

What is often missed (important)

cerr is unbuffered.

This means:

Every cerr << flushes immediately

Flushing is slow

Frequent usage consumes real execution time

So while the judge ignores the output, time is NOT ignored.

How cerr can cause TLE

Using cerr inside loops or for large input sizes can easily lead to Time Limit Exceeded.

Example of dangerous usage:

for (int i = 0; i < n; i++) { cerr << a[i] << " "; }

If n is large, this can be extremely slow.

When cerr is safe to use

Safe cases:

Very small input

Printing a few values

Printing once or twice

Local debugging only

Example (usually safe):

cerr << "Test case started\n";

When cerr should NOT be used

Avoid cerr:

Inside large loops

While printing arrays

During final submission

In performance-critical code

If you forget to remove such logs, you may get TLE even with a correct solution.

Best practice: conditional debugging (recommended)

Use a debug macro so that debug output is completely removed in submissions.

ifdef LOCAL

define debug(x) cerr << x << "\n";

else

define debug(x)

endif

Usage:

debug("current value = " << x);

On Codeforces:

LOCAL is not defined

All debug statements disappear

Zero runtime overhead

How experienced CF users debug

Debug locally

Print only summary values

Remove all debug output before submitting

Avoid printing inside loops

Use logic checks instead of heavy logs

Final takeaway

cerr output is ignored by the judge

cerr execution time is NOT ignored

Excessive cerr usage can cause TLE

Always remove or disable debug logs before submission

Use cerr carefully, especially in contests.