参数“单”的八度nan函数

Question

The code I'm reading has a line:

someData = nan(3, 4, 5, "single")

I can't find the documentation for the "nan" function, but this code appears to generate a 3x4x5 matrix of NaN. However, I don't know what "single" does. Replacing it with a random other string gets "error: NaN: invalid data type specified", and replacing it with "double" appears to give the same result. What is the function of the "single"?

Answer 1

It makes the resulting matrix of nan a matrix that is of the single-precision data type which contains single-precision floating-point numbers. If you want single precision, you need to specify this explicitly, otherwise Octave and MATLAB will use double-precision by default.

You can check the class of the output using class .

class(nan(3, 4, 5, 'single'))
%  'single'

class(nan(3, 4, 5))
%  'double'

As far as looking the same, they will look the same until you start trying to store numbers that exceed the range of numbers that can be represented with single-precision floating point representation. This is because single precision numbers use half the amount of memory that double precision numbers do.

a = nan(1, 1, 'single');
a(1) = 1e-64
% 0

b = nan(1, 1);
b(1) = 1e-64
% 1.000e-64

Also if we inspect the variables with whos we can confirm the size difference.

a = nan(1,1,'single');
b = nan(1,1)
whos('a', 'b')

%    Variables in the current scope:
%   
%      Attr Name        Size                     Bytes  Class
%      ==== ====        ====                     =====  =====
%           a           1x1                          4  single
%           b           1x1                          8  double