Matlab "missing" values not clear

Matlab holds some types of "<missing>" values in its string array. For instance

b =

3×3 string array

    "I"          <missing>    "MATLAB" 
    <missing>    "love"       "MATLAB" 
    "I"          "love"       <missing>

when I access each element of the array

b(1)

ans =

string

    "I"

b{1}

ans =

I

b(2)

ans =

string

    <missing>

b{2}

errormsg = Conversion of element 2 from missing to character vector is not supported.

what type of value is "missing", and why can't it simply be an empty character vector?

[Stephen23]

@Temitope Akinpelu: how did you create the variable b ?

[Walter Roberson]

https://www.mathworks.com/help/matlab/ref/missing.html

Answer 1

[Guillaume]

what type of value is "missing"

From the user point of view it's a flag for missing data. How it is implemented should not concern you and is not documented. (And I don't mean that in a negative way. We, as users, should not have to care, it should 'just work')

and why can't it simply be an empty character vector I don't know if it's a deliberate decision from mathworks or an oversight. However, there are plenty of cases where a missing string is not the same at all as an empty string.

Note that missing is brand new as of R2017a. As is documented the curly braces syntax is a compatibility layer for these codes that rely on cell array of char arrays and you should try to avoid it. It is indeed a bit unfortunate that that compatibility layer breaks in case of missing, so if you want that changed create a bug report with mathworks.

Answer 2

[Jan]

"Missing" means, that the string was not defined before. An empty string would be a defined string already. This is something completely different.

With cell arrays an equivalent method was implemented for cell arrays, but not visible from the Matlab layer:

C = cell(1, 5)
C{1}

Now the contents of C{1} is treated as empty matrix [], but internally the element contains a NULL pointer. From the MEX level this must be considered explicitely - with the advantage that it can be distinguished also.