#define

[Meenu]

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;
}

 

[jacob navia]

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

 

[Chris F.A. Johnson]

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

 

[Martin Ambuhl]

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; }

 

[abhik]

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

 

[Giorgos Keramidas]

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

 

[Peter Pichler]

#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

 

[akarl]

...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