Why is the return value not what I expected in this C program with macro? [duplicate]

Answer 1

Needs more parentheses. This:

#define SQR(a) a*a

expands to this:

i+2*i+2

which is:

3+2*3+2

which is 11 because * has precedence over +.

You need to define your macro like this:

#define SQR(a) ((a)*(a))

to ensure that this kind of thing doesn't happen.

Answer 2

Macros are not the same as regular functions.

During the reprocessing all macros are replaced exactly by what they are define. In your case, the line:

 printf("%d",SQR(i+2));

is replaced by the line:

 printf("%d", i+2*i+2);

So, you see the unexpected result there.

The correct way is:

#define SQR(a) ((a)*(a))

The preprocessor result will be:

printf("%d", ((i+2)*(i+2)));

Try to learn on this mistake. Issues of this kind are quite hard to debug.