davidsen@sungod.crd.ge.com (William Davidsen) (08/02/89)
I recently discovered the following technique. While I can't swear
that it's new and unique to me, I can claim independent discovery. I
make no claims that it is suited to any problem you have, will have, or
would admit to having.
I was calculating a function and had two algorithms, one of which was
13 times faster than the other but unreliable for input values less than
1.0, the other very accurate but slow. I had to evaluate the function
6E8 times on a slow machine.
Here's the code:
double hacs1(), hacs2();
double asect;
(*(asect < 1.0 ? hacs2 : hacs1)(asect);
Since the name of a function is typed "pointer to function
returning..." this was treated as "(*ptr)()" and generated the desired
code. It was slightly faster than the obvious:
if (asect < 1.0) hasc2(asect);
else hasc1(asect);
bill davidsen (davidsen@crdos1.crd.GE.COM)
{uunet | philabs}!crdgw1!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -menet@tub.UUCP (Oliver Laumann) (08/04/89)
In article <1430@crdgw1.crd.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes: > I recently discovered the following technique. While I can't swear > that it's new and unique to me, I can claim independent discovery. I > make no claims that it is suited to any problem you have, will have, or > would admit to having. > > (*(asect < 1.0 ? hacs2 : hacs1)(asect); This is quite a common idiom, I have used it lots of times (without the syntax error in your example, of course). You can even omit the * since it's superfluous. Or is it? Our K&R (the old edition) doesn't seem to say anything about this, but "gnucc -pedantic -ansi" doesn't complain about the following program (a good heuristic for checking whether it's valid C): int g() { return 5; } int foo() { int (*f)() = g; return f(); } -- Oliver Laumann net@TUB.BITNET net@tub.UUCP
scjones@sdrc.UUCP (Larry Jones) (08/08/89)
In article <888@tub.UUCP>, net@tub.UUCP (Oliver Laumann) writes: > In article <1430@crdgw1.crd.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes: > > (*(asect < 1.0 ? hacs2 : hacs1)(asect); > > You can even omit the * since it's superfluous. > > Or is it? Our K&R (the old edition) doesn't seem to say anything about > this, but "gnucc -pedantic -ansi" doesn't complain about the following > program (a good heuristic for checking whether it's valid C): dpANS C allows the star to be omitted, K&R C requires it. ---- Larry Jones UUCP: uunet!sdrc!scjones SDRC scjones@SDRC.UU.NET 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 "You can't get a body like mine in a bottle -- unless you push REAL HARD." - Judy Tenuta / Dr. Pepper
karl@haddock.ima.isc.com (Karl Heuer) (08/08/89)
In article <888@tub.UUCP> net@tub.UUCP (Oliver Laumann) writes: >You can even omit the * since it's superfluous. Or is it? In ANSI C, it is. (I consider this unfortunate; C already has too much DWIM.) But in pre-ANSI C, you do need the `*' when invoking a function via a pointer (though some compilers silently did what ANSI now requires). Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
davidsen@sungod.crd.ge.com (ody) (08/08/89)
In article <888@tub.UUCP> net@tub.UUCP (Oliver Laumann) writes: | This is quite a common idiom, I have used it lots of times (without the | syntax error in your example, of course). I'm not sure that everyone would agree with that statement... the mail I got runs about 7:1 with people who found it new and disgusting as opposed to those who use it all the time (that's scarey, I wouldn't use it unless there was a good reason). I never claimed more than independent discovery, so I can ffel hurt that it's not a totally new idea. There aren't many of those. | | You can even omit the * since it's superfluous. Not in portable code it isn't. ANSI made it optional, K&R 1st ed required it. I personally (opinion here) feel that it makes the code more readable to include it, so that the reader knows what's going on. The omission will also break a lot of tools which scan the code and provide cross index, etc. bill davidsen (davidsen@crdos1.crd.GE.COM) {uunet | philabs}!crdgw1!crdos1!davidsen "Stupidity, like virtue, is its own reward" -me