Is there a MATLAB function that can check if a field exists in a MATLAB structure?

The 'exist' function returns true if a structure with a particular name exists. The 'isfield' function returns true if a field is in a structure array. However, I would like a function that determines if a field exists anywhere in a structure of structures. For example, in the following code, the 'isfield' function does not identify that "c" is a field of "a".

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

results in:

ans =
 0

Answer 1

[MathWorks Support Team]

There is no MATLAB function that examines every level of a structure of structures, or nested structure, to determine if a field exists. The 'isfield' function examines only the top level of a nested structure. To determine if a field exists at any other level, you can use either of the following methods.

1. To determine if a field exists in a particular substructure, use 'isfield' on that substructure instead of the top level. In the example, the value of a.b is itself a structure, and you can call 'isfield' on it.

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

ans =
logical
1

Note: If the first input argument is not a structure array, then 'isfield' returns 0 (false). While there are other MATLAB data types that have properties you can access with dot notation, those other data types are not structure arrays.

2. To determine if a field exists at any level in a nested structure, create a new function that examines all levels of the structure. Open the MATLAB Editor and paste the following function into it. Save the function as a MATLAB file, named 'myIsField.m'.

function isFieldResult = myIsField (inStruct, fieldName)
% inStruct is the name of the structure or an array of structures to search
% fieldName is the name of the field for which the function searches
isFieldResult = 0;
f = fieldnames(inStruct(1));
for i=1:length(f)
if(strcmp(f{i},strtrim(fieldName)))
isFieldResult = 1;
return;
elseif isstruct(inStruct(1).(f{i}))
isFieldResult = myIsField(inStruct(1).(f{i}), fieldName);
if isFieldResult
return;
end
end
end

Call 'myIsField' on a nested structure.

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

ans = 
logical 
1

[broken_arrow]

I modified it a little to also work with multilevel fieldName inputs like 'b.c'. I too second extending the built in isfield accordingly.

function out_isfieldresult = anyisfield(in_rootstruct,in_field)
%ANYISFIELD: Extension of built in isfield function. Added features:
%1. Multilevel inputs (e.g. "b.c.d")
%2. Identification of matches at any level of nested structs

in_field = string(in_field);
rootfieldnames = string(fieldnames(in_rootstruct(1)));

out_isfieldresult = false;

if contains(in_field,".")
    current_searchfieldname = extractBefore(in_field,".");
else
    current_searchfieldname = in_field;
end

remaining_searchfieldnames = extractAfter(in_field,".");

for i=1:length(rootfieldnames)
    if strcmp(rootfieldnames(i),current_searchfieldname)
        if ismissing(remaining_searchfieldnames)
            out_isfieldresult = true;
            return
        else
            out_isfieldresult = anyisfield(in_rootstruct(1).(current_searchfieldname),remaining_searchfieldnames);
        end
    elseif isstruct(in_rootstruct(1).(rootfieldnames(i)))
        out_isfieldresult = anyisfield(in_rootstruct(1).(rootfieldnames(i)),current_searchfieldname);
        if out_isfieldresult == true
            return;
        end
    end
end

end