How can I pass an expression to a prefix argument?

Question

Sometimes it would be useful to be able to pass an expression to a prefix argument, for example

c-u 3*4 c-n

If I try the command above though it will print three asterisks followed by the '4' character then cursor-down.

Is there something I can do so the prefix argument can be the value of an expression?

Answer 1

There's no convenient interface for that.

For some simple operations, you can make use of two facts:

• Using C-u with no number gives you powers of 4. C-u C-n moves 4 lines down, C-u c-u C-n moves 16 lines down, etc. (There are rare exceptions with commands that treat the no-number case differently.)
• The prefix argument is usually a repeat count, so you can do additions by repeating the command.

If you want a complex calculation, you can use M-: (eval-expression) to call the command. First, if you need to, look up the command name by pressing C-h c (or f1 c) and the keyboard shortcut you meant to invoke. To copy-paste the command name, switch to the *Messages* buffer, or alternatively use C-h k rather than C-h c and switch to the *Help* buffer. Then invoke eval-expression with M-: and specify M-: (the-command-name (the-argument-expression)). This assumes that the prefix argument is the sole argument to the function, which is often but not always the case: check the function's documentation.

Another approach, which works regardless of how the command consumes its prefix argument, is to set prefix-arg explicitly. invoke eval-expression with M-: and specify M-: (setq prefix-arg (the-argument-expression)), then immediately invoke the command. Under the hood, commands read the prefix argument from current-prefix-arg. Between each interactive command, Emacs sets current-prefix-arg to the value of prefix-arg and resets prefix-arg to nil.