MATLAB: Did the default rounding behaviour of num2str change between R2016b and R2018b

MATLABnum2strrounding

I have some code that produces a different result when run in R2018b than it did in R2016b, and I've identified that num2str (which is used to create a text file to exchange data with an external tool) is giving different output for some values. For example, in R2016b:
>> num2str(1.51535)
ans =
1.5154
but in R2018b:
>> num2str(1.51535)
ans =
    '1.5153'
I can work around this difference, but I can't find anything about this change in the MATLAB release notes, so before I commit any code changes I want to be sure that this is actually a change in MATLAB's default behaviour and not some setting I've overlooked that might be different between my R2016b and R2018b installations. Is this the case?

Best Answer

The change appears to have occurred in R2017b. E.g., on Windows:
>> num2strexact(1.51535)
ans =
    '1.5153499999999999747757328805164434015750885009765625'
R2016b:
>> num2str(1.51535)
ans =
1.5154
R2017a:
>> num2str(1.51535)
ans =
    '1.5154'
R2017b:
>> num2str(1.51535)
ans =
    '1.5153'
R2018a:
>> num2str(1.51535)
ans =
    '1.5153'
R2018b:
>> num2str(1.51535)
ans =
    '1.5153'
Looking closer, it appears they have switched the library functions they use behind the decimal conversion functions (maybe to make them more like their UNIX counterparts that have behaved this way for years?). E.g., again on Windows:
R2017a:
>> sprintf('%70.60f',1.51535)
ans =
    '        1.515350000000000000000000000000000000000000000000000000000000'
R2017b:
>> sprintf('%70.60f',1.51535)
ans =
    '        1.515349999999999974775732880516443401575088500976562500000000'
UNIX versions of MATLAB have been doing the full decimal digit conversion for years, and it appears that now the Windows versions of MATLAB do the same as of R2017b. So my guess is that MATLAB has changed some of the underlying library code they use for this (and possibly other) stuff in the background.
IMO this change is a good thing and an improvement, as it gives a more correct result to decimal conversions and also makes MATLAB results more consistent across different platforms.
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