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Re: converting numbers to strings in arbitrary base (up to 36)
From: |
Pascal J. Bourguignon |
Subject: |
Re: converting numbers to strings in arbitrary base (up to 36) |
Date: |
Tue, 15 Mar 2011 20:15:23 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) |
Mirko <address@hidden> writes:
> This is an elisp question:
>
> I can specify integers in base 2-36 using #xyr..., and I can read them from a
> file.
>
> But is there a way to write an integer in arbitrary base (again, 2-36) to a
> file?
>
> I looked at string-to-number, format, and calc
>
> In greater detail,
>
> I have a file where I want to keep a counter (in base 36).
> When needed, I want to open the file, read the number.
> Occasionally, I want to increment the number, and write it back out.
>
> I can handle reading and writing :-). It is writing the base 36 that
> I don't know how to handle.
For bases eight, ten, and sixteen, you can use format:
(format "#8r%o #10r%d #16r%x" 42 42 42)
--> "#8r52 #10r42 #16r2a"
For the other bases, you must implement it yourself, or use
integer-to-base (from
http://git.informatimago.com/viewgit/index.php?a=viewblob&p=public/emacs&h=94650b080e8a38e9620687b9abd3fcb3e24c6561&hb=e57067965e5def38d5fa18dab0aa75cff3d049b9&f=pjb-utilities.el
)
(loop for b from 2 to 36
collect (format "#%dr%s" b (integer-to-base 42 b)))
--> ("#2r101010" "#3r1120" "#4r222" "#5r132" "#6r110" "#7r60" "#8r52"
"#9r46" "#10r42" "#11r39" "#12r36" "#13r33" "#14r30" "#15r2C"
"#16r2A" "#17r28" "#18r26" "#19r24" "#20r22" "#21r20" "#22r1K"
"#23r1J" "#24r1I" "#25r1H" "#26r1G" "#27r1F" "#28r1E" "#29r1D"
"#30r1C" "#31r1B" "#32r1A" "#33r19" "#34r18" "#35r17" "#36r16")
(defun integer-to-base (decimal base &optional width padchar
commachar comma-interval)
"
DO: Convert a decimal value into a string contening the
same value expressed into the given base. 1<base<37.
The optional WIDTH specifies the minimum length of the returned
string (0-left-filled), not counting a '-' sign.
SEE-ALSO:float-to-base."
;;TODO: Implement commachar, comma-interval
(cond
((not (integerp base)) (error "Invalid base (%S)." base))
((or (< base 2) (< 36 base)) (error "Invalid base (%d)." base))
((not (integerp decimal))
(error "For now, I only convert integer values.")))
(let ((digits "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ")
(buffer (make-string 32 ?0))
(b 31)
(sign "")
)
(if (< decimal 0)
(setq sign "-"
decimal (abs decimal)))
(while (< 0 decimal)
(let ((digit (% decimal base)))
(aset buffer b (aref digits digit))
(setq decimal (/ (- decimal digit) base)
b (- b 1))))
(if width (if (< 32 width)
(setq sign (concat sign (make-string (- width 32) padchar))
width 32)))
(concat sign
(if (and width (< (- 32 width) (+ 1 b) ))
(substring buffer (- 32 width))
(if (= b 31) "0" (substring buffer (+ 1 b)))))))
--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.