@himanshu:
preprocessor is a program which is called by the compiler before the
actual compilation.
see, for the preprocessor directive #if there is a condition that it
should be followed by a constant because it is evaluated before
compiling the program.
Now these constants can be constant say 1,8, etc or it should be
define using another preprocessor directive #define.
In ur program, #if i==0, the preprocessor first sees if i here is
defined using #define because its not a constant. Since u haven't, it
takes the by default value for constant i as 0. This "i" is not ur int
i, but it is seen as a #define stuff.
see this program for example
#include<stdio.h>
#include<stdlib.h>
#define p 1
int main()
{
#if i==1
printf("doom");
#endif
int i=1;
#if j==0
printf("1234");
#endif
#if k==5
printf("chaffeur");
#endif
#if p==1
printf("srikant");
#endif
return 0;
}
it prints: 1234 and srikant.
the statement int i=1 has no role in preprocessing.
I have not defined j or k which is used by #if so it will take it as
value 0 and checks for the condition which evaluates to true and false
respectively.
whereas i have defined constant p as 1 using #define, so this
condition will evaluate to true printing srikant.
In case u want to see the output just after preprocessor runs i.e.
before compilation use gcc -E filename.
Hope this clears ur doubt.
Regards
Doom
On Jul 2, 12:08 am, himanshu kansal <himanshukansal...@gmail.com>
wrote:
> If i do #if i==0 thn it vl cmpl it..means its tkng it as 0..bt why...
>
> On 7/1/11, Dumanshu <duman...@gmail.com> wrote:
>
>
>
>
>
>
>
>
>
> > i think it will take a garbage value for i because these are
> > preprocessor directives.
>
> > On Jun 30, 4:46 pm, himanshu kansal <himanshukansal...@gmail.com>
> > wrote:
> >> 1 more thng....if its possible to eval. variable expression lyk abv...
> >> thn...
> >> int i=2;
> >> #if i==2
> >> printf("hi");
> >> #else
> >> printf(""bye); //prints bye....i think it shld print hi...
> >> #endif
>
> >> even this code prints bye.....
>
> >> int i=3;
> >> #if i==2
> >> printf("hi");
> >> #else
> >> printf(""bye);
> >> #endif
>
> >> On Thu, Jun 30, 2011 at 5:13 PM, himanshu kansal <
>
> >> himanshukansal...@gmail.com> wrote:
> >> > somewhere i read dt conditional compilation such as #if and #elif etc
> >> > cn only use constant expressions and previously defined macros....they
> >> > cant use variables cz preprocessng is done b4 compilation...
> >> > bt whn i try it on gcc and vc both r compiling it with variables abs.
> >> > fyn....even disabling extensions dint work....
> >> > i.e int i;
> >> > #if i+1
> >> > printf("hi"); vl print hi...no error occurs
> >> > #endif
>
> >> > thn i read it on net dt it cn eval. varables also....
> >> > nw m confused who is ryt...
> >> > whtr it cn eval var also or nt...
> >> > nd if it cn thn hws it possible dt preprocessor knows d val. of var.
> >> > b4 compiling d prog.......
>
> >> --
>
> >> Regards
> >> Himanshu Kansal
> >> Msc Comp. sc.
> >> (University of Delhi)
>
> > --
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>
> Regards
> Himanshu Kansal
> Msc Comp. sc.
> (University of Delhi)
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