Preprocessor log()?

[Richard Cavell]

Hi,

I want to do something like this:

const int a = "283529";
const int b = log to base 2 of a;

and have it *at compile time*.

[Chris Torek]

Re: Preprocessor log()?

In article <cvrcg3$o4b$1@n nrp.waia.asn.au >
Richard Cavell <richardcavell@ mail.com> wrote:[color=blue]
>I want to do something like this:
>
>const int a = "283529";
>const int b = log to base 2 of a;
>
>and have it *at compile time*.[/color]

You cannot even do this:

% cat foo.c
const int a = "283529";
% cc -ansi -pedantic -c foo.c
foo.c:1: warning: initialization makes integer from pointer without a cast
%

Presumably you did not really mean to include the quotes.

If we fix that, so that "a" is a read-only variable initialized to
283529, we still have several problems, the first being that "a"
is a variable. You cannot initialize a second static-storage-duration
variable based on another variable; you can only use constants to
initialize static-duration variables. Remember that, in C, const
means "do not define a constant". (There are other languages in
which const *does* mean "constant"; but if you mean to use one of
those languages, you are in the wrong newsgroup.)

We can repair this by initializing both variables (whether read-only
or not) from a true constant, using either "#define" or "enum". Note,
however, that 283529 is a large enough value that it might exceed
the range for "int", so we really should use "long", and a #define
-- enumeration constants have type int, and if INT_MAX is 32767, the
sequence:

enum { A = 283529 };

should draw a diagnostic. (If INT_MAX is 2147483647, as it is on so
many implementations today, no diagnostic is required.) So now we
have:

#define A 283529
const long a = A; /* or int, if you are sure INT_MAX >= 283529 */
const int b = SOME_MACRO(A);

and we need to find a way to define SOME_MACRO so as to obtain
log2(283529).

Now we have one final problem: log2(283529) is approximately
18.113136779327 283. What integer value do you want to represent
this? Both 18 and 19 are "reasonable " but at most one of them
is correct (although perhaps neither is correct).

Depending on your answer to this last problem, there is an obvious
way to define SOME_MACRO:

#define SOME_MACRO(x) \
((x) > 65536 ? [one set of options] : [other set of options])

where the expansion of one of the sets-of-options is:

(x) > 256 ? [one set of options] : [another set of options]

and these options again expand to something like:

(x) > 16 ? [one set of options] : [another set of options]

and so on, ending with:

(x) > 1 ? 1 : 0

(with an optional test for x < 1 as well, although what value to
use here is again a puzzle).

(This version, with the tests for ">", is designed for rounding
up, so that b will be set to 19, based on A > 262144; if A is
between 131073 and 262144 inclusive, the result would be 18.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
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