>> str2num(char(b(:)))
ans =
-1
0
1
>>
But, not recommended; the underlying content of a categorical variable aren't really numeric even if they are numbers. The representation internally is an ordered set of the integers 1:N as I'm sure you've already discovered via double()
>> categories(b)
ans =
3×1 cell array
{'-1'}
{'0' }
{'1' }
>>
Gives the values of the categories that make up the unique elements but not the corresponding location in the original array.
