Output confusion

Question

as while outputting x it goes from right to left following stack but in y output it doesnt. is this because of operator precedence
https://code.sololearn.com/cMX8qEhxBWQw/?ref=app

Answer 1

I also find it interesting that if I change
justFill(a, x++, x, x); // 101111
to
justFill(a, x++, (x), x); // 101010
The output changes on my compiler, but again not on sololearn

Answer 2

@Dennis: SoloLearn: GNUC (gcc) 4.8.1
https://code.sololearn.com/cu8tmoP4h80H/?ref=app

@Dennis and @anoneon....

I got this warning: "Operation on 'x' may be undefined" while testing other gcc's (and I thought it was just mixing ++x and x++ in the same statement). It's still a "sequence point" problem, because there are no sequence points between function parameters....and this "<<" is a function in the cout template system.

Using the "operator<<" expansion from the links (below + additional reading):
cout << x << x++ << endl; // expands to, I believe:

operator<< ( operator<< (operator<< (cout, x), x++), endl);

If I'm reading the links below properly, the 'unspecified' interaction is between the sequenced function, the unsequenced parameters and the post-increment (operating at a sequence point):

https://stackoverflow.com/questions/25466285/c-cout-side-effect-sequencing
https://stackoverflow.com/questions/10782863/what-is-the-correct-answer-for-cout-c-c

Behavior is (as @Dennis originally indicated) 'undefined' / 'unspecified'.

* SoloLearn does not appear to warn like the others.

Answer 3

Of all the tasks I've done, working as an intrusion analyst honed: "yeah...but how/why?" into a bit of a trigger for me. :)

Answer 4

This is because of undefined behaviour.
Sololearn outputs:
1110
1011

While my compiler outputs
1010
1011

It's kinda the same issue as with:
arr[x] = x++; //Don't do this ^^

The compiler desides in which order it is evaluated.

Answer 5

@Dennis...would you mind checking your output on this? Here I see a traveling 10 but I suspect for you it will fill in a triangle

main snippet, fills an array parameter (a, b or c):

justFill(a, x++, x, x);
justFill(b, y, y++, y);
justFill(c, z, z, z++);

SoloLearn output (the above, and):
a : 101111
b : 111011
c : 111110
xyz: 111111

https://code.sololearn.com/cFt8yu3krN5R/?ref=app

Answer 6

@Dennis: That's interesting. This seems to imply an internal inconsistency in cout vs functions (and it feels like a compiler bug). Because of this I added a set of cout's with the same pattern as the function. SoloLearn produces the same output for both (which makes me wonder again if you'll now get a triangle).

Answer 7

Damn, you sure put alot of research into this. Kudos to you. :)

Answer 8

Actually I get the same output as on sololearn, with the exception of the first line again

1110 -> 1010

Answer 9

Last 2 lines on my compiler are different

Sololearn:
101111
111011
111110

GCC 7.1.0 (should you be interested)
101111
101011
101010

Answer 10

thanks all