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PreProcessor issue in C/C++? Why is the answer 7?

+3 votes
413 views 
#include<stdio.h>
#include<conio.h>
#define PROD(x) (x*x)
void main()
{
 clrscr();
 int p=3,k;
 k=PROD(p+1); //here i think value 3+1=4 would be passed to macro
 printf("\n%d",k);
 getch();
}

In my opinion, the output should be 16, but I get 7. Can anyone please help me why?

posted Apr 21, 2014 by Muskan

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1 Answer

+3 votes
 
Best answer

Preprocessor commands are hard replacement before the compilation hence after the replacement your statement will look like this

Before Replacement 

k=PROD(p+1);

After Replacement 

k=(p+1*p+1); // which is 3+1*3+1 = 7

To correct the behavior please change the #define PROD(x) (x*x) to #define PROD(x) ((x)*(x)) by inserting the () around the variable will do the trick.

answer Apr 22, 2014 by Salil Agrawal
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