How do I assign a value to a field in a multi-level struct when the field name is contained in a variable?

I have a multi-level struct and I would like to change the value for a field in one of the lower levels. The path to the field name is stored in a char array. For example,

>> fieldToChange = 'levelA.levelB.levelC.myField';

Usually, when the name of a field is stored in a variable, I can access it using parenthetical notation:

>> fieldName = 'levelA';
>> myStruct.(fieldName) = 0;

However, when I use that notation with 'fieldToChange', I see the error below:

>> fieldToChange = 'levelA.levelB.levelC.myField';
>> myStruct.(fieldToChange) = 0;

Reference to non-existent field 'levelA.levelB.levelC.myField'

How do I access the field at the location stored in 'fieldToChange'?

Answer 1

[MathWorks Support Team]

One way to do this is to use the 'subsasgn' function, which can be used to assign a value to a subscripted field. This is demonstrated in the attached example and explained below.

For this case, where there are multiple levels of the structure, create a compound indexing expression, or an array of structures, for the input parameter, 'S'. Each structure in the array must contain the type of subscript and field name. For more information on this parameter, please see the documentation page for 'subsasgn':

https://www.mathworks.com/help/matlab/ref/subsasgn.html

To do this, first split 'fieldToChange' into a cell array using 'strsplit' and splitting on the period delimiter:

>> fieldToChangeCell = strsplit(fieldToChange, '.');

Next, create an anonymous function, 'makeSParam', that takes in a field name, 'x', and places it into a structure for the 'S' parameter format of 'subsasgn':

>> makeSParam = @(x)struct('type','.','subs',x);

You can then apply this function to each cell in 'fieldToChangeCell' using 'cellfun', resulting in the compound indexing expression, 'idx':

>> idx = cellfun(makeSParam, fieldToChangeCell);

Finally, use 'subsasgn' to set the field to the desired value of 0:

>> myStruct = subsasgn(myStruct, idx, 0);

[Stephen23]

"create an anonymous function, 'makeSParam', that takes in a field name, 'x', and places it into a structure for the 'S' parameter format of 'subsasgn':"

The standard MATLAB approach is to simply call the inbuilt SUBSTRUCT() function:

https://www.mathworks.com/help/matlab/ref/substruct.html

Answer 2

[Stephen23]

You do not need any obfuscated fiddling around with SUBSASGN().

The standard MATLAB approach is to use SETFIELD() / GETFIELD():

F = 'levelA.levelB.levelC.myField';
V = pi; % value or array
S = struct()

S = struct with no fields.

C = split(F,'.');
S = setfield(S,C{:},V)

S = struct with fields:
    levelA: [1×1 struct]

Checking:

S.levelA.levelB.levelC.myField

ans = 3.1416

And for completeness, the reverse operation:

W = getfield(S,C{:})

W = 3.1416