inc/dec operators with scanf

[shruthi]

Hi everyone! When I give the value for computation in the program it gives me the right answer,but when I use scanf and enter the value,the result is incorrect. I am using gcc compiler.

//program without scanf
#include<stdio>
int main()
{
    int x=2,y;
    y=x++ + ++x;
    printf("x=%d,y=%d\n");
    return 0;
}

Output: x=4,y=6 // correct output
 
//program with scanf
#include<stdio>
int main()
{
    int x,y;
    printf("value for x\n");
     scanf("%d",&x);
    y=x++ + ++x;
    printf("x=%d,y=%d\n");
    return 0;
}
Output: x=3,y=5 //incorrect output

I am not able to find a logical explaination,could anyone help me out here?

[Salem]

Comma Operator - C And C++ | Dream.In.Code
Your code is broken, so getting the "right" answer out of any compiler is a crap shoot.

Learn all about sequence points.
Question 3.8

[grumpy]

Modifying any variable twice with no intervening sequence points gives undefined behaviour (in other words the C standard does not forbid any potential outcome) which means ANY result is acceptable.

When behaviour is undefined, reformatting your hard drive or electrocuting you via your USB port are both alternative outcomes that are just as acceptable as producing the output you expect.

Your code is modifying x twice in one statement, which means there is no sequence point between the two increment operations. Both sets of output you are getting are correct, because all possible results are allowed.

[shruthi]

Thanks for your reply.