Matlab: How to properly get a mask equivalent to 2^63-1?

Question

I'm having some problems with MATLAB and 64-bit integers. I want to have a mask equivalent to 2^63-1 (all ones except the MSB), but MATLAB just seems to round everything.

>> mask_fraction = uint64(9223372036854775807)

mask_fraction = 9223372036854775808 % This is 2^63 again, not 2^63-1!

Similarly,

  >> uint64(2^63)

    ans =  9223372036854775808       


    >> uint64(2^63-1)

    ans =  9223372036854775808

Another one of my attempts simply doesn't work:

>> uint64(2^63) - 1
??? Undefined function or method 'minus' for input arguments of type 'uint64'.

Thoughts?

Answer 1

I don't see the problems you report. On my computer (Matlab R2012b on 64-bit Ubuntu12.04):

>> mask_fraction = uint64(9223372036854775807)

mask_fraction =

  9223372036854775807

>> uint64(2^63) - 1

ans =

  9223372036854775807

Do you maybe run an older Matlab version?

I also find the 'undefined function or method minus ...' error a bit suspicious. Do you have uint64 aliased to some other function? Try clear uint64 first ...

Answer 2

@BasSwinckels correctly points out one issue. I'll address another.

The first non-exactly representable double-precision floating point integer is 2^53+1. When you pass in expressions to the uint64 function they are evaluated as doubles before being cast to uint64. If these expressions evaluate to a double precision integer that is not exactly representable, you'll see behavior like you described. This is exactly why uint64(2^63-1) and uint64(2^63) both return 9223372036854775808. All powers of two can be safely represented in double precision, so uint64(2^63)-1 or uint64(2^63)-uint64(1) are what you should use (once you figure out your other issue).

Answer 3

It seems from the other comments that Matlab does not implement the minus method for class uint64 in some older versions. The ugly workaround below works for me in R2011b:

>> bitset(intmax('uint64'), 64, 0)

ans =

  9223372036854775807

Answer 4

I don't know if it is supported by the version of MATLAB you are using but this might help

mask_fraction = uint64(intmax('int64'))

which returns

mask_fraction =
  9223372036854775807

2^63-1 is the maximum value for int64.