MATLAB: How to modify field any levels deep in a structure with a string

fieldfieldname selectionstructstructure

Ive been trying to dynamically modify the value of a field in a struct that can be any number of levels deep and was hoping to build a string that led to it, for example str = "x.y.z" and then use app.(str) to change its value. But it seems that matlab only lets you use strings to go into a maximum of 1 field deep, so app.(str) would work for str = "x" , but not for str = 'x.y.z'. Adding more periods to go deeper gives errors.

Because i cant know how deep any field will be, i cant break the string into its fields and use something like app.(str1).(str2).str(3), hence the need to have been able to build the string and call it at once with app.(longStr)

Any suggestions?

Best Answer

"i cant break the string into its fields and use something like app.(str1).(str2).str(3)..."

You cannot use dynamic fieldnames like that, but you can easily use getfield to do exactly what you want:

>> S.A.B.C = 123;
>> str = 'A.B.C';
>> spl = regexp(str,'\.','split');
>> val = getfield(S,spl{:})
val =  123

And of course setfield does the same for assigning a value. If you have control over the data design, it might be better to store the fieldnames separately anyway (i.e. like spl above).

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MATLAB: Downsizing a struct with different data type fields

nfield = length(myfields);
newStruct = cell2stuct(cell(nfield,1), myfields);
for fieldCtr = 1 : nfield
    thisfield = myfields{fieldCtr};
    sample = oldstruct(1).(thisfield);
    if isrow(sample)
        newStruct.(thisfield) = vertcat(oldstruct.(thisfield));
    elseif iscolumn(sample)
        newStruct.(thisfield) = horzcat(oldstruct.(thisfield));
    else
        nd = ndims(sample);
        newStruct.(thisfield) = cat(nd+1, oldstruct.(thisfield));
    end
end

This code tries to be intelligent about how to arrange the results. It turns rows into 2D arrays by vertical concatenation; it turns columns into 2D arrays by horizontal concatenation; it turns other arrays into one higher dimension by concatenating one dimension higher than it already is (so for example, two 32 x 64 arrays would not turn into 64 x 64 or 32 x 128, and would instead turn into 32 x 64 x 2)

You might want to make an exception for character vectors: instead of creating a character array, you might want to create a cell array of character vectors, so as to allow for the possibility that the character vectors are different lengths.

MATLAB: Using a string shortcut for nested structure

For accessing a field of one specific structure (which can be nested) you should use dynamic fieldnames:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

For accessing multiple arbitrary fields of arbitrarily nested structures the most robust way is to use setfield and getfield, for which you will need to split the string into the separate field names (because each field belongs to a different structure, they cannot be provided as one string) which can be provided as a comma-separated list, for example:

>> tmp = struct();
>> tmp.ahoy.matey = 'hello';
>> str = 'ahoy.matey';
>> spl = regexp(str,'\.','split');
>> getfield(tmp,spl{:})
ans = hello
>> tmp = setfield(tmp,spl{:},'world');
>> getfield(tmp,spl{:})
ans = world

Best Answer

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MATLAB: Downsizing a struct with different data type fields

MATLAB: Using a string shortcut for nested structure

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