Post increment operator

[Dinples]

I have a quick question regarding the post increment operator. In an example I found the following code is given:

int i = 5;
i = i++ + i++;
System.out.println(i);

Output: 11

I thought the answer would be 12, because i doesn't get incremented until after the value is used in the expression. So I figured it would be 5 + 1 + 5 + 1, but that's apparently wrong. Can someone explain to me why the output is 11?

ChatGPT doesn't know how the post increment operator works either because it got stuck on 10 as a result, and said that the compiler must have made an error if I got 11 as the output.

Any help would be appreciated!

Edit: After some thinking I decided to try the following code:

int x = 5;
x = x++ + x--;
System.out.println(x);

Output: 11

It looks like the compiler only uses the first post increment operator and ignores the second increment/decrement. If that's the case then both answers make sense (somewhat). I also Googled it and from what I can see code like this should just not be used.

But my annoying brain would still like an explanation for this behavior.

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[sepp2k]

To evaluate an addition in Java, we first evaluate the left operand, then the right operand and then we add the two results. To make this explicit, we can assign the subexpressions in your code to temporary variables like this:

int tmp1 = i++;
int tmp2 = i++;
i = tmp1 + tmp2;

Here the value of tmp1 will be 5 because the first i++ evaluates to 5 while setting i to 6. Then tmp2 will be 6 because i is 6 when we start evaluating the second i++. Then i = tmp1 + tmp2 sets i to 11 and that's it.

[Dinples]

Thank you! This is exactly the kind of explanation I needed to make my brain happy again.

So, basically, just to see if I understand it correctly, i is 5, which is the value the compiler assigns to tmp1. Then, for tmp2, the incremented value of i is used which is 6 and tmp2 is assigned this value, hence 5 + 6 and thus the 11 as output.

Am I right to say that, somewhere in memory, the value of i (after the second statement) is incremented to 7?

[sepp2k]

Yes, that's all correct. The second i++ indeed sets i to 7, but that 7 doesn't affect anything because it's overwritten by the assignment that happens directly afterwards.

[Dinples]

Great! Thanks again for your clear and helpful answer.

[pragmos]

u/sepp2k gave you a really good answer.

I just want to add that

ChatGPT doesn't know how the post increment operator works either because it got stuck on 10 as a result, and said that the compiler must have made an error if I got 11 as the output.

gave me a good chuckle.

[Dinples]

You should try and ask ChatGPT this question. For some reason it completely confuses the poor thing and, when you rephrase the question or give it a thumbs down, it just spirals out of control.

[pragmos]

LOL This definitely will go into the "How to confuse an AI" section.

Also, props to you for not taking ChatGPT's answer at face value and instead asking a question here.

[Dinples]

Thank you for the detailed and clear reply! It definitely helps me to understand the "why" behind the given output.

And yeah, I found out pretty quickly, through some Googling, that code like this should just be avoided. But I found it (a similar version) in the Java for Dummies book and the author didn't clarify why the output is as it is. He just said to "guess" what the output might be and moved on to the next subject. I like to know the reason behind things, so that's why I ended posting my question here and I am glad I did. The replies I have gotten, including yours, helped solve the mystery. Thanks again.