MATLAB: Creating a structure from a cell array with nested structure fields

cell arraysMATLABstructstructures

I have a structure with a cell array with structure fields.

The structure is a.fields. where a.fields{1,1} = 'b.c', a.fields{2,1}='c.d'

The real info is longer with even more nested structures, but I need a way that takes those fields to create the actual structure. I am using a for loop to go through the list, and so far I have:

    structure_name = a.fields{i;1};
    name.structure_name = {some info};

with the final hope that name.b.c = {some info} and so on and so forth.

I considered a loop by separating the cell array strings by period and looping through as well but figured I would ask about the (hopefully) more simple way. Obviously Matlab doesnt like that the names have periods and its telling me that it's invalid.

I would appreciate any insight.

Best Answer

Nope, that won't work.

Best approach: split on the period characters, then use setfield and getfield as appropriate:

S.field1.field2.field3 = 42;
P = 'field1.field2.field3';
pathPieces = split(P, '.');
theAnswer = getfield(S, pathPieces{:})

This example is taken from Steven Lord's answer here:

https://www.mathworks.com/matlabcentral/answers/514322-dot-notation-from-string#answer_423141

Related Solutions

MATLAB: Dot notation from string

DON'T use eval. There's no need to use it in this case anyway. split the text representation of the nested fields into pieces then pass those pieces into getfield as a comma-separated list.

S.field1.field2.field3 = 42;
P = 'field1.field2.field3';
pathPieces = split(P, '.');
theAnswer = getfield(S, pathPieces{:})

Things get trickier if you have a more complicated indexing expression, but take a look at the help and documentation for getfield for how to handle such situations.

MATLAB: Initializing vs growing a structure

Like this:

Stucture_Size = 78965;  %number of elements
StructureName(Structure_Size) = struct();

Then StructureName will become a structure array of the proper size, starting with no fields.

As you add a previously-unused field to any element in the structure array, the field will be added to every element in the structure array, initialized to the empty double array in those other structure array elements. For example, if you set

StructureName(1).MyNewField = 'A' : 'Q';

then StructureName(483).MyNewField would pop into existence as the empty double array, not as an empty char array.

Each structure array member can have its field be a different type.

StructureName(1).MyNewField = 'A' : 'Q';
StructureName(2).MyNewField = serial('COM3');  %completely different type is OK!

When you make an assignment to all of an ordinary MATLAB variable, or you make an assignment to all of a particular structure array field name, then the memory used for the expression on the right hand side is shared with the left-hand side, not copied (but memory is copied if you assign to part of a variable, such as assigning to Q(8) = 17 ). Because of this, most often you do not need to preallocate the memory for a structure array member field name:

StructureName(278).MyNewField = zeros(1,500);  %legal but often useless

In particular the preallocation would be useless if you then assign to all of the member's field, such as

StructureName(278).MyNewField = rand(1,500);

It would not, however, be useless if you immediately start working with just part of the variable, such as

StructureName(278).MyNewField(43:59) = 0.5;

In a numeric array, if you had created the first 277 rows then assigning the 278'th row would be inefficient, but in structure arrays as long as you have pre-allocated the structure array itself, there is no inefficiency for having created values only for StructureName(1:277).MyNewField and not having preallocated for StructureName(278).MyNewField .

Best Answer

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MATLAB: Dot notation from string

MATLAB: Initializing vs growing a structure

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