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"fun" C++ quiz time. what will this program print? (and why?)

Guess 

3. The array only contains 3 vecs because (a,b,c) is just an expression that evals to c. -1[p].x is arr[0].x, which is the first value in the array, which is 3.

Guess 

@jpab close, but not quite! That minus is subject to operator precedence rules...

Guess 

@aras oh well, there had to be one more thing I was missing!

Guess 

@aras I assume it must be -9. The bracket and comma initialisers just cancel down, p is pointing to the 2nd element, 1[p] is same as p[1] (ie. Now the 3rd element in arr) because magic, operator precedence puts the negate after the resolve, so x of the 3rd is 9, which is then negated?

Guess correction 

@aras arg, looked it up, comma takes left side so -7

Guess correction 

@mdiluz the thinking is correct, the answer nope :)

Guess correction 

@aras oh. Oh no.
If *both* sets of commas cancel, and comma takes right side (got it right first time...) , then arr is actually (3,6,9),(12,15,18)... etc. Meaning we should have -21?

Guess correction 

@aras or, well, not both "cancelling", the second set should aggregate? Don't know how to describe that, but C let's you initialize struct members in an array sequentially, so I'm sticking with -21

Guess correction 

@aras I couldn't take the insanity and wrote it out. Looks right. Oh man. What a hellscape this language is.

:thinking: 

@aras

Shouldn't it print 1 ?
arr[idx] is syntax sugar for *(arr + idx) so -1[p] == p[-1], and since p starts at the second element you should have the first then.

But does it compiles everywhere ?
Years ago, I had a similar situation with compiler warning about arithmetic operation on pointers, like "trying to access the *p value of the -1 array", but I don't remember well so I'll say 1, .x value of the 0th element 🙂

🤔 

@Xipiryon hint 1: it does not index by -1, due to operation precedence.

Guess 2 

@aras

Then -7 ? Since p already points to arr[1], then it's -((p+1)->x), so the 3rd element of the array 🙂

At least I hope operator precedence doesn't imply (-(p+1))->x, because it lools like an undefined behaviour then 😅

Oh and btw it's super nice of you to take the time to answer everyone !

Guess 2 

@Xipiryon third element of the array, bingo! but that's not 7 or -7 :P

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