Question

As I asked here, I can choose the lisp for running SLIME with prefix argument(C-u), and I is written in here, I see that C-u is a way to insert the output in the current buffer.

I use ‘C-u 10 SOMETHING’, in order to run SOMETHING 10 times.

What’s the usage/purpose of using prefix argument (C-u)? Is C-u the only prefix argument?

Answer 1

Check out the documentation for prefix command arguments.

But, in short, it’s a way to interactively provide more information to the command.

• for alpha-numberic keys, generally bound to 'self-insert, it tells them how many of that character to insert
• for some commands (M-x comment-region) it means to invert the command, i.e. C-u M-x comment-region uncomments the region.
• for some it just tweaks the behavior, C-u C-s does a 'isearch-forward-regexp instead of a regular 'isearch-forward.

So, it all depends on how the command uses the prefix argument.

As far as other “prefix arguments”, there are C--, M--, M-3 (or any other digit), and some others.