Can you explain the output!

Question

https://code.sololearn.com/cn1iwBrZJl1Z/?ref=app

Answer 1

The operations in the printf 'slots' are evaluated from right to left.

Only in case of a postfix, the value in that printf slot is determined right then.

So ++x increases x to 3, but doesn't settle the last slot because it's prefix.

Next, x++ sets the value to 4, but before (postfix), the slot is set to the former value 3.

This happens again in the first slot, so 4 is left in the slot, then x incremented.

Finally, now that all postfixes are settled, the prefix expression in the last slot gets the final value of x.

Questions like these come up a lot here, but the most important thing to remember about this is that we shouldn't write code like that, because depending on the compiler, the result might be different (undefined behaviour).

If you change the value of a variable in the middle of a statement, this variable should occur only this one time in that statement.

Answer 2

There is nothing to explain, the output is indefinite, output in one compiler will be different with that of another, as HonFu said, undefined behaviour.

You might consider this article about sequence points: I'm still struggling to understand it so I shouldn't try to explain, I'd only make things worse : )

http://c-faq.com/expr/seqpoints.html

Answer 3

I found this phenomenon interesting so I tested it and can confirm HonFu's explanation.
But I had to realy play the scenario through to grasp the concept completely.
So the Statement is evaluated from right to left where the post fix increment puts out x's current value and then increments x and the prefix increments x but waits with outputing x's current and final value (reference type) until last. Thus it must go something like this:

0. x++,++x, ++x // x=2
1. x++,++x, ++(2)=x // x=3
2. x++, ++(3)=x, x // x=4
3. (4)++=4, x, x // x=5
4. 4, 5, 5

0. x++,x++, ++x // x=2
1. x++,x++, ++(2)=x // x=3
2. x++, (3)++=3, x // x=4
3. (4)++=4, 3, x // x=5
4. 4, 3, 5

++x,++x,++x // 555
++x,++x,x++ // 552
++x,x++,++x // 535
++x,x++,x++ // 532
x++,++x,++x // 455
x++,++x,x++ // 452
x++,x++,++x // 435
x++,x++,x++ // 432

Answer 4

Is it 235?

Answer 5

@Robin My head is spinning but thank you. :)
As far as i could read the output, CLANG would put out something quite different: 2,3,5 instead of 4,3,5?
So my conclusion for now will be that I ought not touch that stuff even with a long stick. ;)

Answer 6

Thank you Robin. I will try to remember to try and to ask when I hit a wall while doing so. :)