Re: [PATCH] fix potential infinite loop given large unsigned integer

From: Jeff Epler <jepler@unpythonic.net>
Date: Aug 11, 2009, 00:55 UTC
Message-ID: <20090811005517.GB32235@unpythonic.net>
In-Reply-To: <40aa078e0908090525h7b4d6efeh658e2edcfbe16c7e@mail.gmail.com>

On Sun, Aug 09, 2009 at 02:25:40PM +0200, Erik Faye-Lund wrote:

> log10() appears to be C99, but can be emulated on earlier C-versions by doing
> #define log10(x) (log(x) / log(10.0))

I don't think you'll like the results of this very much.
    #include <math.h>
    #include <stdio.h>

    int main(void) {
        double n=1;
        int i, j;
        for(i=0; i<10; i++, n*=10) {
            j = (int)(log(n)/log(10));
            if(i != j) printf("%d %d\n", i, (int)j);
        }
        return 0;
    }

(on my system, 3 of the 10 tested cases give the wrong answer due to rounding)

For a tour of some of the difficulties of implementing log10,
    http://www.cs.berkeley.edu/~wkahan/LOG10HAF.TXT

Jeff