MATLAB: Using a string shortcut for nested structure

stringstructure array

Hi all,

I tried using the search function but to no avail. Hopefully you can help me with this one.

Suppose I have a structure called 'tmp':

tmp = struct();
tmp.ahoy.matey = 'hello';

Would it be possible to create a sort of an 'alias' or shortcut using a string to get to 'hello' in a direct way, and also reassign it? I tried:

blah = 'ahoy.matey';
tmp.(blah) = 'hi';

But that gets me an error.

Invalid field name: 'ahoy.matey'.

Sure, I can evaluate it:

eval(['tmp.' blah])
ans =
    'hello'

But I cannot really change 'hello' into something different.

Is there a way to do this?

Thank you!

Best regards,

Best Answer

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

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MATLAB: Access Field from Struct

You need to use concatenation brackets [ ]

a=[structname.MeansArea]

Read here: https://www.mathworks.com/help/matlab/ref/double.cat.html

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"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).

Best Answer

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