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-   -   Why this program outputs 225 istead of 1 ? (http://www.go4expert.com/forums/program-outputs-225-istead-1-t21589/)

lionaneesh 30Mar2010 21:42

Why this program outputs 225 istead of 1 ?
 
Code:

#include<stdio.h>
#define SQR(x) x * x

int main()
{

    printf("%d", 225/SQR(15));

}

why this program outputs 225 instead of 1 (according to calculation).

shabbir 31Mar2010 07:32

Re: Why this program outputs 225 istead of 1 ?
 
Precedence is not working as you expect with the MACROS.

Try
Code:

printf("%d", 225/(SQR(15)));

karthigayan 31Mar2010 09:27

Re: Why this program outputs 225 istead of 1 ?
 
The macro is substituted like the following ,

Code:

#include<stdio.h>
#define SQR(x) x * x
int main()
{

    printf("%d", 225/15*15);

}

Because of the operator precedence 225/15 executed first and have the result 17 .Then the 17 multiplied by 15 .So the result is 225.So use parenthesis .

xpi0t0s 31Mar2010 12:10

Re: Why this program outputs 225 istead of 1 ?
 
Better to use parentheses in the macro so that the macro can be used "normally". You wouldn't normally have to bracket a subexpression so requiring the programmer to remember for SQR that they have to do
Code:

225/(SQR(15))
will lead to bugs because you don't remember stuff like this.

So define the macro as
Code:

#define SQR(x) ( (x) * (x) )
so that it can be used normally, i.e.
Code:

225/SQR(15)
225/SQR(7+8)

etc.
The brackets around the individual x's mean you can put subexpressions into SQR. Without them, the second would be substituted as:
Code:

225/(7+8*7+8)
Of course you always have to be very careful with side effect operators. This won't have the expected result:
Code:

int i=15;
int j=225/SQR(i++);

What's i? 16? Nope. No amount of bracketing will fix this, which is why we make macros upper case. And relying on this is definitely a bad idea, cos if some smart alec rewrites SQR as
Code:

#define SQR(x) (pow((x),2))
then i will be 16 and not 17 as you expected.


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