How to analyze this type of multidimensional array?

Question

I'm working on Computing Olympiads in Turkey right now. This question was asked in 2016. I scanned lots of C documentation and unfortunately I couldn't find any understandable information.

How can we write "d[-1][1][2]" in a C program? How can we put a negative integer in square brackets? Isn't this a run-time error?

int main()
{
   int d[][3][2]={4,5,6,7,8,9,10,11,12,13,14,15,16};
   int i=-1;
   int j;
   j=d[i++][++i][++i];
   printf("%d",j);
   return 0;
}

Output of this code?

The output is "4". How could that be? Also if you know some good documentations about multi-dimensional arrays in C, please share with me. Thanks in advance.

The behaviour of `j=d[i++][++i][++i];` is undefined, as the 3 increments are unsequenced w.r.t. each other. Otherwise using `-1` is well-defined for indexing *if* it leads to a proper index being calculated. Anyhow, the **question** as it is now is meaningless and the answer certainly isn't "4". — Antti Haapala -- Слава Україні, Mar 23 '19 at 18:22
Agree with @AnttiHaapala Even a good compiler could have warned like _warning: multiple unsequenced modifications to 'i' [-Wunsequenced]_ — Achal, Mar 23 '19 at 18:23
Given the array and the definition, `d[-1][1][2]` will have *undefined behaviour* because the `-1` indexes array out of bounds. But if `d` was e.g. a pointer to the *2nd* subarray of some multidimensional array `x`, then it could be well-defined. — Antti Haapala -- Слава Україні, Mar 23 '19 at 18:26
@AnttiHaapala You can copy-paste this code and run in your own PC. The answer is "4". — , Mar 23 '19 at 18:27
@YusufTalha **incorrect**. The code does not compile on my computer because of the switches I give to the compiler. — Antti Haapala -- Слава Україні, Mar 23 '19 at 18:27
As for the other compiler, `clang` the first run output `-1242862408`, and the second run `-1328830152` and then `1646703160` so clearly you're wrong, and so is the olympiad organization in Turkey. — Antti Haapala -- Слава Україні, Mar 23 '19 at 18:30
It looks like the code in your question was buggy, but you really wanted to ask about negative array indices. If so, your question was answered in a comment. If the answer is still unclear, please post a separate question without this `[i++][++i][++i]` nonsense. — anatolyg, Mar 23 '19 at 19:59

How to analyze this type of multidimensional array?

0 Answers0