29 Jan
2011
29 Jan
'11
12:07 p.m.
Excerpts from fons's message of 2011-01-28 16:11:52 +0100:
log2() suffers from the same problem? I somewhat dislike the idea of
adding a constant.
On Fri, Jan 28, 2011 at 02:02:36PM +0100, Philipp
Überbacher wrote:
Took me a while to figure out that libm is part of glibc :)
Good to know that those functions are available on probably pretty much
all linux systems.
rant_begin
Why can't log mean the same thing everywhere? Why does it need to be
base e here and base 10 there? Why is there no consistency?
And why is there no proper logarithmus dualis function? Because you
can simply do log(n)/log(2)? We've just seen how well this works.
How about:
log() - base 10
ln() - base e - logarithmus naturalis
ld() - base 2 - logarithmus dualis
rant_end
Libm has log(), log10, and log2(). The next
obvious question is: Does the inaccuracy reliably result in
values bigger than 11?
No.
If the input is a power of two, and you expect an integer as
a result, just do
k = (int)(log2(x) + 1e-6) or
k = (int)(log(x)/log(2) + 1e-6)
or
int m, k;
for (k = 0, m = 1; m < x; k++, m <<= 1);
which will round up if x is not a power of 2.
Neat. I thought about it myself yesterday but my ideas weren't exactly
brilliant. One idea was to divide by 2, the other to use a small
lookup table for powers of 2. I don't really know about efficiency, but
I guess bit shifting is as efficient as it gets?
Anyway, it's a neat way to avoid the problem and the rounding properties
of mult/div in case of not power of 2 could be useful as well.
Regards,
Philipp