How to get the "zero/blank" element of a given variable?

I have an array VAR of unknown type. When the array is extended, Matlab knowns how to fill in blank ("zero") values of the correct type. How do I get this default blank value for any given Var type?

One solution is simply to extend an array and see what value gets padded:

BLANK(2) = VAR(1) ; BLANK(2) = [] ;

This returns a scalar BLANK with the correct "0" value and works for any VAR type.

BUT: It doesn't work with empty VAR.

Any thoughts of a general way to get the Blank value for any given array even if empty?

Answer 1

[Rik]

You can abuse repmat for this:

a=uint8(8);
repmat(a,0,0)
b='foo_bar';
repmat(b,0,0)

Edit:

On second read, you might actually mean this instead:

a=uint8(8);b='foo_bar';c={'1',2};
get_blank_val=@(x) eval([class(x) '(0)']);
get_blank_val(a)
get_blank_val(b)
get_blank_val(c)

[royk]

thanks. this is an elegant way to get an empty array of a given type. but what i am looking for is getting a scalar "zero" of the given type.

Say, for a=uint8(8) I would like to get BLANK = uint8(0) ;

for a='foo_bar', BLANK = ' ' ; (single space)

for a={10,20}, BLANK = {[]} ;

etc

[Rik]

While you comment suggests the blank for a char is a space, a quick test on R2015a shows it is a char(0). That might have changed in later releases though. A quick way to check is this:

bb='foo';
bb(8)='y';
disp(double(bb))

[royk]

yes, you are right it is char(0) indeed

But, what is then the general way of getting this Blank value for any variable type? Especially how do I get the blank/zero value corresponding to a given empty variable?

[Rik]

My edited answer should give you that. I admit the use of eval is suboptimal, but this strategy supports a wider range of options than the zeros function (e.g. the cell class) and it should also support custom classes. It also works on more releases than zeros(1,'like',x) would.

[royk]

very nice! much thanks!

it works quite well, but for instance, won't work on structs. (it throws an error).

Yet, the “0” for struct seems well defined within Matlab:

>> Var = struct('a',3,'b',4) ;
>> Var(3) = Var(1) ;
>> Var(2)
ans = 
  struct with fields:

    a: []
    b: []
    

[Rik]

You could put in a special condition for a struct, but then you can't make it an anonymous function anymore.

function blank=get_blank_val(x)
if ~isa(x,'struct')
    blank=eval([class(x) '(0)']);
else
    fn=fieldnames(x);
    tmp=[fn repmat({[]},size(fn))]';
    blank=struct(tmp{:});
end
end

[royk]

that will do the job

there are still other exceptions. for example, for an array of MyClass objects, Matlab determines Blank for array extension by calling empty(MyClass).

It thereby feels like there got to be a generic Matlab function that returns the Blank object for array extension for ANY type.

Anyway, your kind suggestion above surely does the job for my needs.

thanks!