3. Copy once, check twice

The fragment is taken from Audacity project. The error is detected by the following PVS-Studio diagnostic: V501 There are identical sub-expressions to the left and to the right of the '-' operator.

sampleCount VoiceKey::OnBackward (....) {
  ...
  int atrend = sgn(buffer[samplesleft - 2]-
                   buffer[samplesleft - 1]);                          
  int ztrend = sgn(buffer[samplesleft - WindowSizeInt-2]-
                   buffer[samplesleft - WindowSizeInt-2]);
  ...
}

Explanation

The "buffer[samplesleft - WindowSizeInt-2]" expression is subtracted from itself. This error appeared because of copying a code fragment (Copy-Paste): the programmer copied a code string but forgot to replace 2 with 1.

This a really banal error, but still it is a mistake. Errors like this are a harsh reality for programmers, and that's why there will speak about them several times here. I am declaring war on them.

Correct code

int ztrend = sgn(buffer[samplesleft - WindowSizeInt-2]-
                 buffer[samplesleft - WindowSizeInt-1]);

Recommendation

Be very careful when duplicating code fragments.

It wouldn't make sense to recommend rejecting the copy-paste method altogether. It's too convenient, and too useful to get rid of such an editor functionality.

Instead, just be careful, and don't hurry - forewarned is forearmed.

Remember that copying code may cause many errors. Here, take a look at some examples of bugs detected with the V501 diagnostic. Half of these errors are caused by using Copy-Paste.

If you copy the code and then edit it - check what you've got! Don't be lazy!

We'll talk more about Copy-Paste later. The problem actually goes deeper than it may seem, and I won't let you forget about it.