why C compiler gets wrong when a[5]==5[a] [closed]

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Closed 8 years ago.

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Why does this happpens

double zero = 0.0;    
double a[] = { 0,0,0,0,0, zero/zero}; // NaN
cout << (a[5] == 5[a] ? "true" : "false") << endl;

prints

false

Answer 1

The issue is not a[5] vs. 5[a]. It is that NaN is never equal to anything, including itself. This gnu.org page has more details on comparing NaN and Infinity.

Answer 2

The compiler isn't getting it wrong, and it has nothing to do with a[5] == 5[a]. zero/zero is a nonsense expression and gets assigned NaN as a result but NaN != NaN because it can be produced in multiple non-equal ways.

Mind you, checking it out I think this is actually strictly undefined behaviour because the standard doesn't actually require the use of IEEE 754, or equivalent, floating point standards and can actually not support NaN. Where it does support NaN, section 7.2.14 (in the 2011 draft standard) requires that NaN comparisons return false.