Is it possible to check for existence of fields in nested structures with isfield in MATLAB 8.1 (R2013a)?

I have the following structure

a.b.c = 1;

I know it is possible to search for the nested

isfield(a.b,'c')

But I would like to check for the existence of the field 'c' even when I am not sure that the field 'b' exists, e.g.

isfield(a,'b.c');

Answer 1

[MathWorks Support Team]

There is no MATLAB function to determine the existence of fields in nested structures. The only workaround is to check separately for the existence of 'b' and of 'c':

isfield(a, 'b') && isfield(a.b, 'c')

[Benoît Béranger]

Let's say you have the following structure :

a.b.c.d.e.f.g.h.i.j.K = 1;

This solution is not elegant :

isfield(a, 'b') && isfield(a.b, 'c') && isfield(a.b.c, 'd') ...

You need another strategy, like something recursive.

This function does the work : https://github.com/fieldtrip/fieldtrip/blob/master/inverse/private/issubfield.m

Answer 2

[Tom]

Or if, like me, you don't want to do a ton of isfield()s on a deeply nested structure, you can shortcut to testing the last one like this:

eval('isfield(a.b.c.d.e.f,''g'')','0')

Of course, in the example I tried, this took 5 times longer than the multiple isfield method, so you have to decide if you need the speed or easier to read code. (Also, the deeper the nesting gets, the more efficient this version gets relative to multiple calls to isfield.) Furthermore, the eval() statement might also allow you to skip the 'if' entirely if you're just wanting to get the value of your nested field/assign a default value, e.g.

MyData = eval('a.b.c.d.e.f.g','NaN');

[Matt J]

Might be faster if you do

 try
  eval('a.b.c.d.e.f.g');
  tf=true;

 catch
   tf=false;
 end

Answer 3

[Geoff McVittie]

For later releases of MATLAB, another strategy is to create your own issubfield function. The issubfield has the advantage of not calling eval and can handle any depth. Enjoy!

function tf = issubfield(S,FIELD)
%ISSUBFIELD Determine if FIELD is valid in struct S
%   Determine if the specified FIELD or nested FIELD is present in the
%   given structure.
%
%   A.b.c.d = 1;
%   issubfield(A,"b.c.d")       % TRUE
%   issubfield(A,"b")           % TRUE
%   issubfield(A,"b.c.d.e")     % FALSE
%   issubfield(A,"f")           % FALSE

arguments
    S (1,1) struct
    FIELD (1,1) string
end

SUBFIELD = strsplit(FIELD,'.');

if numel(SUBFIELD) == 1
    tf = isfield(S,FIELD);
    return;    
end

tf = true;
for i = 2:numel(SUBFIELD)    
    S = S.(SUBFIELD(i-1));
    if ~isfield(S,SUBFIELD(i))
        tf = false;
        break;
    end
end

end

[Martijn Kortenhoeven]

Very nice function! One remark though, when the first item of a nested field does not exist, you get an error. Using the example:

A.b.c.d = 1;
issubfield(A,"f.c.d.e")     % ERROR

Answer 4

[Steven Lord]

Another approach that uses neither eval nor repeated calls to isfield is to use getfield.

a.b.c = 1;
isNestedField(a, 'b.c')

ans = logical
   1

isNestedField(a, 'b.d')

ans = logical
   0

isNestedField(1:10, 'a')

ans = logical
   0

function tf = isNestedField(s, thefields)
fieldlist = split(thefields, '.');
try
    getfield(s, fieldlist{:});
    tf = true;
catch
    tf = false;
end
end