Why does the loop print 0 at the end?

Question

This is an example I found on a website. The result is really different from what I expected. But there is no further explain.

int num = 0;

for (int i = 0; i < 3; ++i)
{  
    num += num++;
}

System.out.println(num);

Finally, the result will print 0 . I am really confused about the operation num += num++ . Can someone explain this?

Answer 1

num++ increments num after the instruction (it is a post-increment operator).
So num += num++; assigns 0 to num (num = 0 + 0 + 0).

After the instruction num += num++; the post increment of num (that is num++ ) has no effect as num was assigned to another value (that is num += 0 which the result is 0 ) .
So num is valued to 0 . And so on for each iteration.

Replace num += num++ by ++num that is the pre-increment operator, you will get the result : 3 (as you increment 1 by iteration).

Answer 2

This assignment

 num += num++;

invokes two different rules in java: one on the left side, one on the right side.

The right side rule is fairly well known: the value of the post-increment operator is the value of the variable before the increment. This makes the value of 0++ a zero.

The other rule is a bit more obscure (I was not aware of the left side rule).

According to "15.7.1. Evaluate Left-Hand Operand First" in Java Language Specification Java first decides just what gets assigned, and also what is its value . Then the right side of += is computed.

With these two rules in mind, the assignment above is the equivalent of:

temp1 = num // and will assign to num
temp2 = num // before ++
num = num + 1
num = temp1 + temp2

As you can see, the last line adds the original value of num (it happens to be 0 ) to the value of num before ++, which ensures that num does not change. It starts at 0 , it ends with 0 .

Suppose that you're now using a pre-increment operator:

 num += ++num;

Now the situation is sligthly different.

int temp1 = num // and num will be assigned
int temp2 = num = num + 1
num = temp1 + temp2

The value of num used in num += xxx is not the result of ++num - it is the value we had before ++num was executed.

So first iteration we have 0+=1 , second iteration it's 1+=2 and third iteration it's 3+=4 - that's 7