jacob@cadvax (Jacobo Bulaevsky) (11/02/84)
I've started functions in the following way:
function ()
{
static char first_time_in = TRUE;
if (first_time_in) {
first_time_in = FALSE;
bla, bla, bla...
};
etc...
}
One contruct that I've always wanted to have is:
function ()
{
on1 {
bla, bla, bla...
};
etc...
}
I've been trying to define a macro to do this but haven't been succesful.
What I've come up with looks like:
#define on1(X) static char first_time_in = TRUE; if (first_time_in) {first_time_in = FALSE; X;}
But unfortunately this macro has to be used like:
on1 (
bla, bla, bla...
);
which is VERY inappropriate. Can anybody out there think of a better
macro definition, comments, suggestions?
Thanks in advance.
Jacobo Bulaevsky
--
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PHONE: (408)998-7110 x 3171olson@fortune.UUCP (Dave Olson) (11/06/84)
In article <124@cadvax> jacob@cadvax.UUCP (Jacobo Bulaevsky) writes: >What I've come up with looks like: > >#define on1(X) static char first_time_in = TRUE; if (first_time_in) {first_time_in = FALSE; X;} > >But unfortunately this macro has to be used like: > > on1 ( > bla, bla, bla... > ); > >which is VERY inappropriate. Can anybody out there think of a better >macro definition, comments, suggestions? The macro you propose won't even work with most versions of cpp. I presume you meant that it must be used as: on1 ( bla; bla; bla... ); (Note the ';' instead of ','. Most versions of cpp will NOT allow macro functions with variable #'s of arguments.) If your version of cpp accepts multi-line macros and invocations (as most, and apparently yours, do), you can simply use: on1( bla1; bla2; bla3; ... ); which looks *somewhat* natural if you read the parens as braces. (Of course, if you use a ',' that cpp parses as an argument separator (e.g. if(z) y=3,z=4) you are going to get cpp error messages... Another potential problem is too long an argument list. Since first-time only code is usually fairly short, this won't be much of a problem.) Dave Olson, Fortune Systems
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (11/07/84)
> I've started functions in the following way: > > function () > { > static char first_time_in = TRUE; > > if (first_time_in) { > first_time_in = FALSE; > bla, bla, bla... > }; > > etc... > } > > One contruct that I've always wanted to have is: > > function () > { > on1 { > bla, bla, bla... > }; > > etc... > } > > I've been trying to define a macro to do this but haven't been succesful. Try: #define on1 static long on1_flag = 0L; if ( on1_flag++ == 0L ) There are other tricks if you don't like this one. Remember, the condition inside the "if" parentheses can accomplish as many side actions as desired.
brad@sdcrdcf.UUCP (Brad Spear) (11/07/84)
In article <124@cadvax> jacob@cadvax.UUCP (Jacobo Bulaevsky) writes: >I've started functions in the following way: > >function () >{ > static char first_time_in = TRUE; > > if (first_time_in) { > first_time_in = FALSE; > bla, bla, bla... > }; > > etc... >} Your letter got me to thinking, I have to start some procedures that way also. The following is what I came up with. I tested it with the simple program that follows, and it appeared to work, so we'll see. #define TRUE 1 #define FALSE 0 #define ON_FIRST_TIME static char _done = FALSE; if (!_done && (_done=TRUE)) main() { int j; for (j = 0; j < 3; j++) sub(); } sub() { ON_FIRST_TIME printf ("inside init\n"); printf("routine call\n"); } Note that ON_FIRST_TIME would be treated like any 'if' statement, so just put in some '{}', and off you go. The only restriction is that it must be the first expression in a block, because of the declaration of done. Note that if the expression "!_done" evaluates to FALSE, which it does after the first time, the expression "(_done=TRUE)" will not be re-evaluated. If you want an expression that doesn't use a hidden variable, you might try this one, although I haven't tested it. #define ON_FIRST_TIME(x) if (!(x) && ((x)=TRUE)) In this case, the variable used for x would already have to be declared, but it also allows the macro to be used anywhere. Brad Spear sdcrdcf!brad
jerry@oliveb.UUCP (Jerry Aguirre) (11/07/84)
How about:
#define on1(X) static char first_time_in = TRUE; if ((first_time_in) && (!(first_time_in = FALSE)))
function ()
{
on1() {
bla;
bla;
...
};
etc...
}
The evaluation of the && will reset the flag the first time thru.
Because the reseting is done as part of the test the braced part of
the code is not affected.
Jerry Aguirre @ Olivetti ATC
{hplabs|fortune|idi|ihnp4|ios|tolerant|allegra|tymix}!oliveb!jerrybrad@sdcrdcf.UUCP (Brad Spear) (11/07/84)
Silly me. If I had thought about it for 10 more minutes, I would have had a
better version the first time. In this case, the _f doesn't have to be
negated every time. I thought I had ruled this case out, but I made a mistake
somewhere originally, and it does work. Note that it depends on the fact
that "_f" is evaluated before "!(_f=FALSE)", which is what C is supposed to
guarantee.
#define ON_FIRST_TIME static char _f = TRUE; if (_f && !(_f=FALSE))
This expression can be used like:
{ declarations ...
ON_FIRST_TIME
{
blah1, blah1, blah1 ...
}
blah2, blah2, blah2 ...
}
It will cause "blah1, blah1, blah1 ..." to be executed the first time only.
It would be nice if someone could figure out how to make a non-function block
return a value, then the expression could go anywhere, and the _f could be
totally invisible.
Brad Spear
sdcrdcf!bradron@brl-tgr.ARPA (Ron Natalie <ron>) (11/08/84)
This message is empty.
prl@murdu.OZ (Peter Lamb) (11/08/84)
[]
A nice way to put on1 in is to avoid the text of the on1 being part
of the macro. This involves a pretty grotty-looking if (with side-effects)
so if you want to keep pure, read no further:
#define on1 static int _not1_ = 0; if(!_not1_ && (_not1_ = 1))
then on1 has EXACTLY the same behavior as the if, except that it must be
the first statement unless the whole lot is covered by {}'s.
eg:
on1 do_this();
else do_that();
or:
on1 {
blah, blech; blah;
}
If you don't want the on1 code as the first `executable' statement then
{
on1 do_this();
else do_that();
}
will get you out of trouble, albeit inelegantly.
Peter Lamb, ......!decvax!mulga!prl
University of Melbourne
Parkville 3052
Australiastorm@diku.UUCP (Kim Fabricius Storm) (11/09/84)
[dette er en test] >From: jacob@cadvax (Jacobo Bulaevsky) <124@cadvax> 2-Nov-84 03:22:23 > > One contruct that I've always wanted to have is: > > function () > { > on1 { > bla, bla, bla... > }; > > etc... > } Try one of these: #define on1 \ static char FIRST_CALL = 1; \ if (FIRST_CALL && FIRST_CALL--) #define on1 \ static char FIRST_CALL = 1; \ for (; FIRST_CALL; FIRST_CALL = 0) --------------------- [Datalogy, what the H... is that?] Kim F. Storm @ Institute of Datalogy, University of Copenhagen, Denmark UUCP: ..mcvax!diku!storm
dwd@ccice6.UUCP (David W. Donald) (11/09/84)
> One contruct that I've always wanted to have is: > function () > { > on1 { > bla, bla, bla... > }; > etc... > } Try: #define on1 static char x = 1; if ( x ? (--x,1) : 0 ) f() { on1 { printf( "first time only\n" ); printf( "even more\n" ); } printf( "first and subsequent times\n" ); } On a VAX with 4.2BSD I get unbeatable code with cc -O. Dave Donald
karsh@geowhiz.UUCP (Bruce Karsh) (11/09/84)
> One contruct that I've always wanted to have is: > > function () > { > on1 { > bla, bla, bla... > }; > > etc... > } It's a little cryptic, but here is one way to do it: -- on1.c ------------- Cut Here ----------------------------------------- #define on1() static int notfirst= -1; if(!(++notfirst ? notfirst=1 : 0)) main() { sub(); sub(); sub(); } sub() { on1() printf("first time\n"); else printf("not first time\n"); } ---------------------- Cut Here ----------------------------------------- -- Bruce Karsh --------------------------- Univ. Wisconsin Dept. of Geology and Geophysics | | 1215 W Dayton, Madison, WI 53706 | THIS SPACE FOR RENT | (608) 262-1697 | | {ihnp4,seismo}!uwvax!geowhiz!karsh ---------------------------