# #define

Discussion in 'C Programming' started by Meenu, Jul 27, 2005.

1. ### MeenuGuest

the value of a is 11, how??

#define SQR(x) (x*x)
int main()
{
int a,b=3;
a=SQR(b+2);
printf("a=%d\n",a);
return 0;
}

Meenu, Jul 27, 2005

2. ### jacob naviaGuest

Meenu a écrit :
> the value of a is 11, how??
>
> #define SQR(x) (x*x)
> int main()
> {
> int a,b=3;
> a=SQR(b+2);
> printf("a=%d\n",a);
> return 0;
> }
>

a = SQR(b+2) where SQR --> "b+2"
a = b+2*b+2
a = 3+(2*3)+2
a = 3+6+2
a = 11

jacob navia, Jul 27, 2005

3. ### Chris F.A. JohnsonGuest

On 2005-07-27, Meenu wrote:
> the value of a is 11, how??
>
> #define SQR(x) (x*x)

#define SQR(x) ((x)*(x))

Otherwise:

3 + 2 * 3 + 2 = 3 + (2 * 3) + 2 = 11

> int main()
> {
> int a,b=3;
> a=SQR(b+2);
> printf("a=%d\n",a);
> return 0;
> }

--
Chris F.A. Johnson <http://cfaj.freeshell.org>
==================================================================
Shell Scripting Recipes: A Problem-Solution Approach, 2005, Apress
<http://www.torfree.net/~chris/books/cfaj/ssr.html>

Chris F.A. Johnson, Jul 27, 2005
4. ### Martin AmbuhlGuest

Meenu wrote:
> the value of a is 11, how??
>
> #define SQR(x) (x*x)
> int main()
> {
> int a,b=3;
> a=SQR(b+2);

is
a=b+2*b+2;
since the value of b is 3, the value of the expansion is
a=3+6+2;
= 11;
This is why, if you are _sure_ that there are no side effects in the
arguments to SQR() it should be
#define SQR(x) ((x)*(x))
But, of course, you have no such guarantees about no side effects. Both
versions die with
SQR(++b);
for example.
So, what you really want is
inline int square_int(int x) { return x*x; }

> printf("a=%d\n",a);
> return 0;
> }
>

Martin Ambuhl, Jul 27, 2005
5. ### abhikGuest

hi meenu ,

see the problem here is that all the #defines are taken care at
preprocessing level .
So wat the compiler gets is not 5*5 but actually 3 + 2 * 3 + 2
which is 11

Meenu wrote:
> the value of a is 11, how??
>
> #define SQR(x) (x*x)
> int main()
> {
> int a,b=3;
> a=SQR(b+2);
> printf("a=%d\n",a);
> return 0;
> }

abhik, Jul 27, 2005
6. ### Giorgos KeramidasGuest

"abhik" <> writes:
> Meenu wrote:
> > the value of a is 11, how??
> >
> > #define SQR(x) (x*x)
> > int main()
> > {
> > int a,b=3;
> > a=SQR(b+2);
> > printf("a=%d\n",a);
> > return 0;
> > }

>
> see the problem here is that all the #defines are taken care at
> preprocessing level .
> So wat the compiler gets is not 5*5 but actually 3 + 2 * 3 + 2
> which is 11

Which can easily be solved by proper use of parentheses:

#define SQR(x) ((x) * (x))

This macro has yet another potential "bug", which is caused by using the
macro argument twice. If the "x" expression has side-effects they will
be doubled, yielding strange results in invocations like:

SQR(b++, b++)

But that's probably not what the original poster asked for

Giorgos Keramidas, Jul 27, 2005
7. ### Peter PichlerGuest

Giorgos Keramidas wrote:

> #define SQR(x) ((x) * (x))
>
> This macro has yet another potential "bug", which is caused by using the
> macro argument twice. If the "x" expression has side-effects they will
> be doubled, yielding strange results in invocations like:
>
> SQR(b++, b++)

....which would most likely not compile

(SQR takes ONE parameter)

Peter

Peter Pichler, Jul 31, 2005
8. ### akarlGuest

Peter Pichler wrote:
> Giorgos Keramidas wrote:
>
>> #define SQR(x) ((x) * (x))
>>
>> This macro has yet another potential "bug", which is caused by using the
>> macro argument twice. If the "x" expression has side-effects they will
>> be doubled, yielding strange results in invocations like:
>>
>> SQR(b++, b++)

>
>
> ...which would most likely not compile
>
> (SQR takes ONE parameter)

Moreover, even if the function *did* take two parameters it would cause
problems (undefined behavior) even if it was a "true" function
(modification of a variable twice with no intervening sequence point).

August

akarl, Aug 2, 2005