Vim search for a pattern and if NOT occurs delete line

Question

I've read this question, but I want to know how can I delete the line when the pattern DOESN'T occur.

Answer 1

:v/pattern/s/.*//

Answer 2

Deleting the rest of the line when a pattern does not occur is ... hard for me to understand.

Did you mean, delete the whole line if it doesn't have a pattern?

:v/pattern/d

If you meant, preserve the line but clobber the characters:

:v/pattern/s/.*//

Answer 3

Equivalently:

:g!/pattern/d

Easier to remember in my opinion, because! is ingrained as "not" in my brain.