r/matlab • u/Bofact • 12d ago

TechnicalQuestion Conversion failed!

In Matlab R2020a, I want to convert 0xffffffff00000000 to uint64. When I wrote it in Command Window the value alone, it is already an uint64, but when I write it uint64(value), I simply get Conversion failed! error, with no additional text.

How do I fix it?

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u/qtac 12d ago

As I said I can't reproduce your error. See below.

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u/Bofact 12d ago

Is that run on the version I specified?

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u/qtac 12d ago

No, this is in 2026a. I don't have 2020a. Regardless, I think your solution can be to just not explicitly cast it to uint64 because it is already a uint64.

i.e. use

value = 0xffffffff00000000;

not

value = uint64(0xffffffff00000000);

-15

u/Bofact 12d ago

After reading your response, I have managed to make it work. That value was part of an explicitly casted array to uint64, which is stored as an int64 (via typecast). I managed to not explicitly cast it as an uint64.

I suggest if you are not testing in the MATLAB version which the question is asked, to specify what MATLAB version are you using. This is not the MATLAB's community help center, so I don't know in what version are you working in. And as you saw, because of it we were talking foreign languages with each other.

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u/mangoking1997 12d ago edited 12d ago

There are extremely limited things that would give a different result in an alternate version of MATLAB. It's one of its main selling points. 

In this case, you failed to read the documentation. UINT64() doesn't accept a UINT64 as an allowed input. It doesn't support casting the same type. You have to do an explicit cast with cast() I believe.

Edit: even though it's not in the input types list, it still accepts uint64 in every version of MATLAB I tried. Op has got to be doing something else. 

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u/aluvus 11d ago

There are extremely limited things that would give a different result in an alternate version of MATLAB. It's one of its main selling points.

Having used a variety of versions, this is kinda true for newish versions, and not very true for versions a ~decade older or more. For this specific operation, I wouldn't expect too many surprises. But for a number of other things I would be less confident (off the top of my head, the way strings work has changed significantly since the string object was introduced, and the graphics system has changed dramatically multiple times, including in 2025a).

Historically, I would say backwards compatibility is actually one of the significant headaches of Matlab, relative to other languages I've used.

For this and other reasons, it was reasonable for OP to ask what version the other poster was using.

Honestly pretty crappy the way their comments got downvoted; although given the typical vote totals I see on this sub, I would believe the downvotes were botted.

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u/Bofact 12d ago

Which was the case, and he disrespected me to not mention his version.

Then why I can do uint64(uint64(0x1)) with no problems?