Why does prefix argument translate to '(4) and '(16)
reddit / emacs

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pragmat1c1 · 2022-06-14 08:24

Anyone knows why the prefix argument `C-u` translates to `'(4)` in
elisp code? And the double prefix argument `C-u C-u` translates to
`'(16)`? What is the background of this? Why not `1` and `2`? or
`'(1)` and `'(2)`?

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7890yuiop · 2022-06-14 09:13

> the double prefix argument C-u C-u translates to '(16)?

Because the value is 4^N where N is the number of times C-u is used.
Hence C-u C-u C-u => '(64), etc...

> Why not 1 and 2? or '(1) and '(2)?

I would think because that's not very useful.  Prefix args are often
used for repeating things, and hammering C-u five times instead of
typing C-5 would be silly.  The benefit is in easily generating both
small *and* large numbers.  Small numbers can be generated easily
regardless.  Note that numeric prefix arg values like 16 are generated
from the raw value '(16) so the raw values contain a numeric match for
the numeric values.

> why the prefix argument C-u translates to '(4) in elisp code?

Probably because it's a good value, considering that odd numbers are
likely less desirable, and the even numbers either side are 2 (which
is barely useful on its own, and when squared is only doubled to 4),
and 6 (which is squared to 36 which is rather a large increase for
only two uses of C-u).

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b3n · 2022-06-14 09:50

There's no reason it has to be 4, this just happens to be a convenient
default. Consider scrolling, for example, `C-v` will scroll a little
bit, `C-u C-v` will scroll more, and `C-u C-u C-v` will be a really
big jump.

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Alan_Shutko · 2022-06-14 13:16

I have a tiny theory that it's so that C-u C-x C-i indents four
spaces, as four spaces is a common indent. Realistically, the idea
that the value would be chosen globally for that one use case seems
unlikely, but it's my pet theory.

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pragmat1c1 · 2022-06-14 15:25

Excellent explanation. Thank you!