Newbie questions
Nick Linker
xlcr@REDACTED
Sat Apr 1 10:09:56 CEST 2006
Richard A. O'Keefe wrote:
>
>I'm aware of (and have used) several programming languages providing
>as-long-as-necessary integer arithmetic. Without consulting manuals
>(everything is being moved out of my office so that new carpet can
>be laid) it's hard to be sure, but from memory, NONE of them provides
>an operation "how many decimal digits in this bignum". Common Lisp
>has HAULONG, which tells you about how many BITS, and Java's
>BigInteger class offers bitLength(), which again tells you how many
>BITS. In Smalltalk, the obvious thing to do would be n printString size,
>and in Scheme it would be (string-length (number->string n)).
>
>
Hm, for example Maple has "ilog10" function, that instantly gets the
result.
(Although Mapple is specialized language.)
>The question remains, WHY? For what kind of problem is it useful to know
>the number of decimal digits but NOT what the digits actually are?
>
>
I had an idea to implement numeric mutable arrays as bignums. (Btw I
know about ets, dictionary and ports).
>I benchmarked the following code against length(integer_to_list(N)),
>for the first however many factorials. The two seemed to be about the
>same speed. Your mileage will of course vary.
>
>integer_digits(N) when integer(N) ->
> if N >= 0 -> natural_digits_loop(N, 0)
> ; N < 0 -> natural_digits_loop(-N, 1)
> end.
>
>natural_digits(N) when integer(N), N >= 0 ->
> natural_digits_loop(N, 0).
>
>natural_digits_loop(N, D) when N >= 10000 ->
> natural_digits_loop(N div 10000, D + 4);
>natural_digits_loop(N, D) when N >= 1000 ->
> D + 4;
>natural_digits_loop(N, D) when N >= 100 ->
> D + 3;
>natural_digits_loop(N, D) when N >= 10 ->
> D + 2;
>natural_digits_loop(_, D) ->
> D + 1.
>
>
Interesting idea - to move by 4 not by 1. I tried to do like that except
I moved by 1 each step, and it wasn't better than
length(integer_to_list(N)).
Your answers are always comprehensive and very useful. Thank you, Richard.
Best regards,
Linker Nick.
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