|  | Increment a number in compile time macro | | |
| | | Bin Chen |  |
|  Posted: Sun Sep 07, 2008 6:54 am Post subject: Increment a number in compile time macro |  |
switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
....
Thanks in advance.
Bin |
| |
| | | Ian Collins | |
| | Posted: Sun Sep 07, 2008 6:54 am Post subject: Re: Increment a number in compile time macro | |
Bin Chen wrote:
| Quote: | switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
Use an enum. Never use magic numbers. |
--
Ian Collins. |
| |
| | | Richard Heathfield | |
| | Posted: Sun Sep 07, 2008 6:54 am Post subject: Re: Increment a number in compile time macro | |
| |  | |
Bin Chen said:
| Quote: | switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
...
|
I scanned Ian's answer ("Use an enum", indeed! Ohhhhhh deary deary me...),
and I immediately thought to myself, "oh, I reckon I can do a lot better
than that - Ian's a nice enough chap but he's really a C++ guy at heart,
and he doesn't understand all the subtle and clever options available to
the real C hacker, does he?" Then, as is only proper, I read the question
through properly, so that I could show off my astounding wizardly C skills
by giving the best possible advice and the most helpful possible insight.
So - are you ready for my astounding pearl of wisdom? Good. Listen
carefully, then...
Use an enum.
--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999 |
| |
| | | Barry Schwarz | |
| | Posted: Sun Sep 07, 2008 6:54 am Post subject: Re: Increment a number in compile time macro | |
| | | |
On Sat, 6 Sep 2008 23:54:08 -0700 (PDT), Bin Chen
<binary.chen@gmail.com> wrote:
| Quote: | switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
|
Macros don't perform calculations. They only perform text
substitution. And a subsequent appearance of a macro in your code
will not have access to the result of a previous appearance.
However, you can do what you want with an enum. Something of the form
enum {CASE1=1, CASE2, ..., CASEN};
and change each case label to
case CASE1:
...
case CASE2:
etc.
Later, when you want to insert a case between CASE1 and CASE2, change
you enum to
enum {CASE1=1, CASE1A, CASE2, ..., CASEN};
and insert the new block of code immediately before the CASE2 label.
To eliminate the rampant confusion after a half-dozen insertions where
CASE6 may be 7 while CASE10 is 13, I would suggest using identifiers
that are suggestive of the function to be performed rather than their
numeric value. Something like
enum {INSERT=1, DELETE, EDIT, COMPUTE};
and then you could later amend it to
enum {INSERT=1, DELETE, EDIT, PRINT, COMPUTE};
--
Remove del for email |
| |
| | | Ben Bacarisse | |
| | Posted: Sun Sep 07, 2008 1:16 pm Post subject: Re: Increment a number in compile time macro | |
| | | |
Nick Keighley <nick_keighley_nospam@hotmail.com> writes:
| Quote: | On Sep 7, 7:54Â am, Bin Chen <binary.c...@gmail.com> wrote:
switch(i) {
case 1:
 break;
case 2:
 break;
[...]
case N:
 break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
switch(i) {
case AUTO_INC_MACRO:
 break;
case AUTO_INC_MACRO:
...
how about
#include <stdio.h
#define INC(N) (N + 1)
int main (void)
{
int i = 2;
switch (i)
{
case INC(1):
printf ("A\n");
break;
case INC(2):
printf ("B\n");
break;
}
return 0;
}
would this work? Though pre-processor magic of this
sort is usually not a good idea.
|
It's not really magic is it? INC(1) is just a funny way to write 2
(as a constant expression). Are you suggesting the OP uses some
search-and-replace to change case x: into case INC(x): for the x
greater then the one they want to add, thus permitting the new case to
be "slotted in"? If so, then the editor is almost certainly capable of
doing the arithmetic itself. If not, then I'm not sure how the INC
macro helps.
Any literal answer almost misses the point, for me. If the OP is
adding cases in the middle of an existing range, surely all sorts of
other parts of the code much change too. Switching to an enum will
automatically fix these as well (once all the constants are replaced by
enum names).
--
Ben. |
| |
| | | Nick Keighley | |
| | Posted: Sun Sep 07, 2008 1:17 pm Post subject: Re: Increment a number in compile time macro | |
On Sep 7, 7:54 am, Bin Chen <binary.c...@gmail.com> wrote:
| Quote: | switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case
2, the result means every case number after case 2 need to be
incremented by 1, it there a macro tricks to make life easier? Such
as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
...
|
how about
#include <stdio.h>
#define INC(N) (N + 1)
int main (void)
{
int i = 2;
switch (i)
{
case INC(1):
printf ("A\n");
break;
case INC(2):
printf ("B\n");
break;
}
return 0;
}
would this work? Though pre-processor magic of this
sort is usually not a good idea.
--
Nick Keighley |
| |
| | | Moi | |
| | Posted: Sun Sep 07, 2008 2:10 pm Post subject: Re: Increment a number in compile time macro | |
On Sat, 06 Sep 2008 23:54:08 -0700, Bin Chen wrote:
| Quote: | switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case 2,
the result means every case number after case 2 need to be incremented
by 1, it there a macro tricks to make life easier? Such as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
...
|
#include <stdio.h>
#define SWITCH(v) switch ((2*(v))+__LINE__-1)
#define CASE case __LINE__
int main(int argc, char *argv[] )
{
int ii;
SWITCH(argc) {
CASE:
printf("One!!\n"); break;
CASE:
printf("two!!\n"); break;
CASE:
printf("three!!\n"); break;
CASE:
printf("four!!\n"); break;
default:
printf("Unknown %d. Please call QA.\n", argc); break;
}
return 0;
}
HTH,
AvK
 |
| |
| | | Barry Schwarz | |
| | Posted: Sun Sep 07, 2008 4:20 pm Post subject: Re: Increment a number in compile time macro | |
| | | |
On Sun, 07 Sep 2008 18:10:32 +0200, Moi <root@invalid.address.org>
wrote:
| Quote: | On Sat, 06 Sep 2008 23:54:08 -0700, Bin Chen wrote:
switch(i) {
case 1:
break;
case 2:
break;
[...]
case N:
break;
}
Now, after some time I want to add a 'case 2' between case 1 and case 2,
the result means every case number after case 2 need to be incremented
by 1, it there a macro tricks to make life easier? Such as:
switch(i) {
case AUTO_INC_MACRO:
break;
case AUTO_INC_MACRO:
...
#include <stdio.h
#define SWITCH(v) switch ((2*(v))+__LINE__-1)
#define CASE case __LINE__
int main(int argc, char *argv[] )
{
int ii;
SWITCH(argc) {
CASE:
printf("One!!\n"); break;
CASE:
printf("two!!\n"); break;
CASE:
printf("three!!\n"); break;
CASE:
printf("four!!\n"); break;
default:
printf("Unknown %d. Please call QA.\n", argc); break;
}
return 0;
}
|
Not bad if the requested flexibility ever becomes a higher priority
than readability. Non-trivial cases could have some pretty long
lines. And let's hope none of the statements within a case contain
any macros that might expand to more than one line.
--
Remove del for email |
| |
| | | Harald van Dijk | |
| | Posted: Sun Sep 07, 2008 5:14 pm Post subject: Re: Increment a number in compile time macro | |
On Sun, 07 Sep 2008 11:20:30 -0700, Barry Schwarz wrote:
| Quote: | And
let's hope none of the statements within a case contain any macros that
might expand to more than one line.
|
A macro can't expand to more than one line. |
| |
| | | Barry Schwarz | |
| | Posted: Sun Sep 07, 2008 10:31 pm Post subject: Re: Increment a number in compile time macro | |
On Sun, 07 Sep 2008 21:14:20 +0200, Harald van D?k <truedfx@gmail.com>
wrote:
| Quote: | On Sun, 07 Sep 2008 11:20:30 -0700, Barry Schwarz wrote:
And
let's hope none of the statements within a case contain any macros that
might expand to more than one line.
A macro can't expand to more than one line.
|
Then the hope is not in vain :-)
For my education, you are saying the preprocessor will merge the lines
in macros like
do {\
many lines of code each ending with \
} while(0);
which is often recommended here? Somehow I had the idea the merging
took place during compilation.
Which raises a new question. As far as the __LINE__ macro is
concerned, is the following code one line or two?
x = 1;\
y = 3;
On my system, the answer is two but n1256 (5.1.1.2-2) seems to imply
it should be one.
--
Remove del for email |
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