parse error

parse error

[Ankit Jain]

hi

i could not understand why compiler has put semi colon
before else due t owhich this parse error comes....

thanks

  
#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }

However that wouldn't work in some cases. The
following code is meant to be an if-statement with two
branches:

  if(x>y)
    exch(x,y);          // Branch 1
  else  
    do_something();     // Branch 2

But it would be interpreted as an if-statement with
only one branch:

  if(x>y) {                     // Single-branch
if-statement!!!
    int tmp;            // The one and only branch
consists
    tmp = x;            // of the block.
    x = y;
    y = tmp;
  }
  ;                             // empty statement
  else                  // ERROR!!! "parse error
before else"
    do_something();


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Re: parse error

[Elias Athanasopoulos]

On Sun, Sep 26, 2004 at 03:32:52PM +0100, Ankit Jain wrote:
> hi
> 
> i could not understand why compiler has put semi colon
> before else due t owhich this parse error comes....
> 
> thanks
> 
>   
> #define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }

#define exch(x,y) do { int tmp; tmp=x; x=y; y=tmp; } while(0);

Regards,
-- 
University of Athens			I bet the human brain 
Physics Department				is a kludge --Marvin Minsky 

	

Re: parse error

[Jan-Benedict Glaw]

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On Sat, 2004-09-25 09:29:01 +0300, Elias Athanasopoulos <elathan@phys.uoa.gr>
wrote in message <20040925062900.GC1997@velka.phys.uoa.gr>:
> On Sun, Sep 26, 2004 at 03:32:52PM +0100, Ankit Jain wrote:
> > hi
> > 
> > i could not understand why compiler has put semi colon
> > before else due t owhich this parse error comes....
> > 
> > thanks
> > 
> >   
> > #define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
> 
> #define exch(x,y) do { int tmp; tmp=x; x=y; y=tmp; } while(0);

Nearly:-) Omit the last semicolon to make it work...

MfG, JBG

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Re: parse error

[Elias Athanasopoulos]

On Tue, Sep 28, 2004 at 08:14:51AM +0200, Jan-Benedict Glaw wrote:
> On Sat, 2004-09-25 09:29:01 +0300, Elias Athanasopoulos <elathan@phys.uoa.gr>
> wrote in message <20040925062900.GC1997@velka.phys.uoa.gr>:
> > On Sun, Sep 26, 2004 at 03:32:52PM +0100, Ankit Jain wrote:
> > > hi
> > > 
> > > i could not understand why compiler has put semi colon
> > > before else due t owhich this parse error comes....
> > > 
> > > thanks
> > >   
> > > #define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
> > 
> > #define exch(x,y) do { int tmp; tmp=x; x=y; y=tmp; } while(0);
> 
> Nearly:-) Omit the last semicolon to make it work...

Right. :-) Actually after a lot of C Coding it's too difficult to
develop the habit to omit a last semicolon in one line of code. 

Regards,
-- 
University of Athens			I bet the human brain 
Physics Department				is a kludge --Marvin Minsky 

	

RE: parse error

[Huber, George K RDECOM CERDEC STCD SRI]

Ankit wrote:

>i could not understand why compiler has put semi colon
>before else due t owhich this parse error comes....

>#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }

Remember that macro expansion occures early in the compilation process 
(it occures in step 4 if I remember correctly).  In this case a simple
text replacement is performed.

>However that wouldn't work in some cases. The following code is 
>meant to be an if-statement with two branches:

  if(x>y)
    exch(x,y);          // Branch 1
  else  
    do_something();     // Branch 2

As written, assuming exch() is a function you would have a two branch
if statement.  However, because exch is a macro, this is what is passed
from the c preprocessor to the compiler (because exch(..) is replaced with
the body of the defination:

if(x>y)
  { int tmp; tmp=x; x=y; y=tmp;};
else
  do_something();

and you have a one-brance if statement along with a parse error of an 
unassociated elst statement.  There are two fixes to this problem.  The 
first is to remove the semi-colon after the if statement:

if(x>y)
  exch(x,y)
else
  do_something();

This will expand to what you expect.  This is probably the least 
satisfactory fix.  The better fix would be to wrap the entire macro in a 
do-while loop, leaving the semicolon in place (i.e. this construct allows 
a macro call to be treated like a function).

#define exch(x,y)  do{int tmp; tmp=x; x=y; y=tmp;}while(0)

if(x>y)
  do{int tmp; tmp=x; x=y; y=tmp;}while(0);
else
 ...


You should be aware that while macros are powerful, they can be tricky to 
use.  As an example, consifer what would happen with this statement

exch(a, b+4)

George