Thread: program ouputs wierd results

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h> 

#define CUBE(x) (x*x*x)


int main(void)
{
int x, y;
x = 5;
y = CUBE(++x);
printf("y is %d\n", y);
}

following code snipet:

Ouputs : 392

changing macro to

Code:

#define CUBE(x) (x*x)

Ouputs : 49

shouldn't this be 42......

Using VC++ IDE

Instead of ++x if you write x++ then it is giving 125 as a correct answer in the case of CUBE(x*x*x).
And also in case of CUBE(x*x) it is giving the result as 25

You can use the macro expansion, If you have cc compiler.
you can use -E option to expand a macro.

It will give like,

Code:

y = (++x*++x); //for y=CUBE(x);

I think now you can understand clearly.

In your program,your macro CUBE(x*x*x) will be expanded as follows.

CUBE((x*x)*x)

So,when your program passes (++x) as the input to this function.It will be expanded as follows.

((++x*++x)*++x)

First,the value of x is incremented to 6 and then incremented to 7 because ++ operator has higher precedence than *.When the multiplication operation performs both the values will be 7*7.So,the result of it is 49 and then when the final increment of x.The value of x will be incremented to 8 and then it multiplied with 49.That's why,the result is 392.

You run your code by removing one x in your CUBE(x*X),then it will give the result as 49.

Thanks Salem.......