Basic doubt

[Manoj Gundawar]

System.out.println("Hi all");
I have a basic doubt.
int c = 10;
c = c++;
System.out.println(c);
Here c prints the value 10, instead of 11. I know that, in this case ++ is a postfix operator. But once the expression is evaluated, it should complete the postfix operation and thus should have the value 11. This is what I was thinking.
Thanks,
System.exit(0);

[chi Lin]

The confusion happens because you assigned it back to c again.
better way & for it to work as you expected use c++;

Originally posted by Jmannu gundawar:
System.out.println("Hi all");
I have a basic doubt.
int c = 10;
c = c++;
System.out.println(c);
Here c prints the value 10, instead of 11. I know that, in this case ++ is a postfix operator. But once the expression is evaluated, it should complete the postfix operation and thus should have the value 11. This is what I was thinking.
Thanks,
System.exit(0);

[Marlene Miller]

c = c++;
Rule: Evaluate the operands before performing the operation. We must evaluate c and c++ before performing the assignment =.
1. Evaluate the left operand.
The result of the left operand is the variable c.
2. Evaluate the right operand.
Perform the postfix operator ++. Add 1 to the value of c and store it in c. Now c holds the value 11.
The result of the postfix expression is value of the variable before the new value was stored. The result of the expression c++ is 10.
3. Perform the assignment operation =.
Store the result of the postfix expression in c. Store 10 in c.

[Marlene Miller]

Notice the difference between the value of an expression and a side effect that may occur when an expression is evaluated:
Assume int c = 10; boolean b = false;
1. Here is an expression: c + 1
The result of the expression is 11. There is no side effect.
2. Here is another expression: c++
The result of the expression is 10. There is also a side effect.
The side effect is that 1 is added to 10 and stored in c.
3. Here is another expression inside the parentheses: if (b = true) {}
The result of the expression is true. There is also a side effect.
The side effect is that b is assigned the value true.

[Manoj Gundawar]

Hi Merelene,
Thanks for the detail explanation. Unfortunately I am not fully convinced. Although your try was really good.
Here is my doubt.

from the steps you have written:
2. Evaluate the right operand.
Perform the postfix operator ++. Add 1 to the value of c and store it in c. Now c holds the value 11.
The result of the postfix expression is value of the variable before the new value was stored. The result of the expression c++ is 10.
3. Perform the assignment operation =.
Store the result of the postfix expression in c. Store 10 in c.
====> here the assignment happens first and c gets value 10. But due to side effect c's value also get incremented at the end of the entire expression and gets stored in c in the same was as c++.
When next line is executed, the prior line is evaluated completely. So when println is executed, the value of C should be 11, as per your own explanation of the side effect in second example.
In this case where is that side effect happening?
I am still doubtfull about this.
ANyway...
Thanks for your reply.
Mannu

[Anupam Sinha]

Hi Jmannu
I guess you misunderstood Marlene's post.

here the assignment happens first and c gets value 10. But due to side effect c's value also get incremented at the end of the entire expression and gets stored in c in the same was as c++.

2. Evaluate the right operand.
Perform the postfix operator ++. Add 1 to the value of c and store it in c. Now c holds the value 11.

This means that you need to evaluate c++. So now c is 11.

The result of the postfix expression is value of the variable before the new value was stored. The result of the expression c++ is 10.

This means that when you do c++ on c which intially contains 10 then c would be incremented but the value returned by c++ will be the intial value of c. Example System.out.println(c++ + c++ + c++) the result would be (10 + 11 + 12) because the intial c++ causes c to become 11 and returns 10 so the equation's result is 33.

3. Perform the assignment operation =.
Store the result of the postfix expression in c. Store 10 in c.

The = operator has the last precedence. So the assignment of the value 10 will happen at the end when c has been incremented.

[Marlene Miller]

Yes, Anupam. That's it. Thank you.

[Marlene Miller]

This is an experiment. Sometimes code is better than words. It�s real.

class Test {
  int m(int c) {
    c = c++;
    return c;
  }
  public static void main(String[] args) {
    int c = new Test().m(10);
    System.out.println(c);
  }
} 

Method int m(int)
0 iload_1
1 iinc 1 1
4 istore_1
5 iload_1
6 ireturn
iload_1 : The value (10) of the local variable at index 1 is pushed onto the operand stack.
iinc 1 1 : The local variable at index 1 is incremented by 1 (c = 11).
istore_1 : The value (10) on top of the operand stack is popped from the operand stack and the value of the local variable at index 1 is set to the value (c = 10).
Oh dear, out of scope again.

[Manoj Gundawar]

Hi Merelene, Anupam,
Now it's pretty clear to me. Sorry for being so dumb to not understand the earlier explanation
But example and the further details made it very clear. Now I have no doubts.
I appreciate it a lot. You people are making this place a better platform to understand Java.

Manoj
[ June 30, 2003: Message edited by: Jmannu gundawar ]