OpenCL - double is neither <= 300 or >= 200? C++

Question

double test = 0.0;
.... some code
char qwe = ...
.. some code
test += qwe;
if (test >= 200 || test <= 300) {
   test = 7.0;
}
// For some reason, test seems to equal 0?

Why does (test >= 200 || test <= 300) == false?

It's very strange behaviour.

And when I print the output, it's 0?

Have you tried to change the variable test to an int? Or change the comparison >= 200.0 || <= 300.0? — kometen, Nov 21 '17 at 10:42
What is the test supposed to do? And what does it have to do with OpenCL? Perhaps if you showed your *real* code... — Bo Persson, Nov 21 '17 at 10:43
@CaptainGiraffe *mumble mumble* coffee *mumble* brain *mumble mumble*. — Quentin, Nov 21 '17 at 10:47
@CaptainGiraffe: Not necessary. There is only one possibility, and can be quite hard to construct a demo case for that (if you don't know the cause!) — Bathsheba, Nov 21 '17 at 10:47
@Bathsheba That would be true under quite a few assumptions. Not true under others. E.g. test in the condition is not the same `test` as `test=0.0`. — Captain Giraffe, Nov 21 '17 at 10:50
@Bathsheba the if() may also get optimized out as a consequence of UB (invoked somewhere in the '...'; for example, his real code could miss to initialize test) — Massimiliano Janes, Nov 21 '17 at 10:52
@CaptainGiraffe: I still think the OP has done their best in this particular case. It's really bewildering on a first encounter. — Bathsheba, Nov 21 '17 at 10:52
@MassimilianoJanes: Yes, you're technically correct on that point, although I've never observed it. — Bathsheba, Nov 21 '17 at 10:53
@Bathsheba [gcc is famous for doing so](https://godbolt.org/g/79MKsY) :) — Massimiliano Janes, Nov 21 '17 at 11:03

Bathsheba · Answer 1 · 2017-11-21T11:04:26.703

6

Notwithstanding any undefined behaviour in your program, (test >= 200 || test <= 300) == false if, and only if, test is NaN. You can get a NaN (not a number) by some numerical error, such as 0.0 / 0.0 or by calling sqrt with a negative input.

Use std::isnan(test) to check for sure.

edited Nov 21 '17 at 11:04

answered Nov 21 '17 at 10:43

Bathsheba

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May be, it's worth to mention that a NaN value has also another surprising behavior: It's the only value for which `f != f` is true. Because I wasn't sure about it I googled and even found this answer [SO: Checking if a double (or float) is NaN in C++](https://stackoverflow.com/a/570694/7478597) ...which is as short and precise as yours. – Scheff's Cat Nov 21 '17 at 10:54
1

@Scheff: Yes that does work, but compilers are increasingly optimising out that technique. – Bathsheba Nov 21 '17 at 10:55
Just to add: As the question mentions OpenCL in both title and tags, this might be in OpenCL kernel code. In that case, the function for testing for NaN is `isnan()`, as OpenCL doesn't have namespaces and a different set of built-in functions. – pmdj Nov 21 '17 at 13:12
1

Thanks for the answer Bathsheba, it was because of it being NaN. - OP – JamesSin Nov 22 '17 at 11:18
@JamesSin: Glad to be of help. You certainly will never forget the possibility. It's really vexing on first encounter isn't it? – Bathsheba Nov 22 '17 at 11:22
@Bathsheba Yup :D – JamesSin Nov 22 '17 at 12:01

OpenCL - double is neither <= 300 or >= 200? C++

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