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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?

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    I don't think that is possible without rebind C-u ; according to the documentation : Begin a numeric argument for the following command. Digits or minus sign following C-u make up the numeric argument. C-u following the digits or minus sign ends the argument.
    – flav
    Feb 5 at 8:17
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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.

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  • This is an excellent solution. Yes, I can make a command such as (goto-line (* 3 4)), or elaborate on that idea as much as I like, eg (goto-line (+(* row 4)5)) to offset the start of the blocked text in the file, for example. Thank you. Feb 6 at 16:13

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