x *= x++ + ++x;

I want to understand clearly how pre and post increment ops work.

Code:

---

int main()
{
  x = 1;
  x *= x ++ + ++ x;
 printf("\n %d \n ",x);
}

---

The highlighted expression above can be seen as follows:

x op1 x op2 op3 op4 x

The C Operator Precedence Chart gives the following order of precedence:

op4 followed by op2 followed by op3 followed op1 (?)
... which means the following happens:

1. 2 is returned to the expression after op4 happens. Now, the value of x in memory becomes 2 (?)

2. 2 is returned to the expression after op2 happens. Now the value of x in memory becomes 3 (?)

3. 4 is returned to the expression after op3 happens. No change to x.

4. op1 evaluates to 3 * 4 which means x becomes 12.

But the output of the program is 8. Which means that in step 4, op1 still reads the value of x as 2 not 3 ? why?

Anything could happen, the expression is undefined because you modify x more than once between sequence points.

http://www.eskimo.com/~scs/C-faq/q3.2.html

when i run the code with tc3,the result is 9,why?

Read the link!

the pre/post in/de-crement operators are just a matter of when the increment occurs. if it's pre, then then increment is performed before evaluation. if it's post, the increment is peformed after evaluation. run the following:

Code:

---

#include <iostream>
using std::cout;
using std::endl;
int main()
{
    char hello[] = "Hello World";
    char *p;
 
    //POST
    p = hello;
    cout << *p++ << endl;
    cout << *p << endl << endl;
 
    //PRE
    p = hello;
    cout << *++p << endl;
    cout << *p << endl;
    return 0;
}

---

Quote:

---

Originally Posted by misplaced

the pre/post in/de-crement operators are just a matter of when the increment occurs. if it's pre, then then increment is performed before evaluation. if it's post, the increment is peformed after evaluation.

---

One more time! Everybody, all together now: "READ THE LINK!"

Quzah.

We get this question often enough that you would think everyone with more than 20 posts would know the answer by now. :rolleyes:

Even worse than that, however, is the fact that some teachers quiz students on innane stuff like this -- and they're wrong!

http://www.eskimo.com/~scs/C-faq/q3.2.html
Yeah I know Dave said it, and everyone else said it as well, so I just thought I'd say it again just incase some noobs simply don't get it.

The last noob who tried to argue their way out of the pits of hell.
http://cboard.cprogramming.com/showt...ight=undefined