Check for existence of nested fields

I have a struct with some nested fields, such as cfg.accel.filename, cfg.accel.calibration, cfg.gyro.filename, etc. Sometimes the fields exist, sometimes they don't, at different levels. In the above example, the 'gyro' field could be missing, or it could just be cfg.accel.calibration that could be missing.

Is there a way to check for deeply nested fields? Currently I'm doing something like:

    topstruct = struct(whatever...)
    if isfield(topstruct, 'f1')
        if isfield(topstruct.f1, 'f2')
            if isfield(topstruct.f2, 'fleaf')
                do_something(topstruct.f1.f2.fleaf)
            end
        end
    end

Ideally I'd like to do some sort of thing like doifget(mystruct, action, field1, field2,...) where action is not actually evaluated unless mystruct.field1.field2 exists. Maybe action can be a lambda/anonymous function.

Probably just something like getif(mystruct, field1, field2, ...) returns [] if the field doesn't exist, and the value if it does.

[per isakson]

What's wrong with

try
    do_something(topstruct.f1.f2.fleaf)
catch me
    % find out what happened
end

[Michael Schwager]

I think that would probably work, but I guess using try/catch for non-errors is something I come to understand as a questionable practice. I can try it and see how the code ends up looking.

[Stephen23]

See:

https://www.mathworks.com/matlabcentral/answers/95923-is-there-a-matlab-function-that-can-check-if-a-field-exists-in-a-matlab-structure#answer_105275

https://www.mathworks.com/matlabcentral/answers/94122-is-there-a-function-that-will-display-all-the-fields-of-a-nested-structure-in-matlab#answer_103473

Answer 1

[Thorsten]

You can use my function isnestedfield:

function yn = isnestedfield(s, fields)
%ISNESTEDFIELD True if nested fields are in a structure.
%   ISNESTEDFIELD(S,FIELDs) returns true if the string FIELDs is containes
%   nested fields of S in the format of a single string of fieldnames 
%   separated by '.'
%
%Example: 
%   Check if s.f1.f11.f111 is a field:
%   isfield(s, 'f1.f11.f111')
%
%   [e-mail]  2016-11-10

% extract fields
field = textscan(fields, '%s', 'delimiter', '.');
field = field{1};

yn = true;
structstr = 's'; % name of first input parameter
i = 1;
while yn && i <= numel(field)
  % disp(['yn = isstruct(' structstr ')'])
  eval(['yn = isstruct(' structstr ');']);
  if yn
    % disp(['yn = isfield(' structstr ', ' field{i} ')'])
    eval(['yn = isfield(' structstr ', field{i});']);
  end
  if yn
    structstr = strcat(structstr, '.', field{i});
    i = i + 1;
  end
end