Can the values of this structure be accessed/modified without using a for loop?

Question

Here’s my initial structure array:

A(1).B.C = 'a';
A(2).B.C = 'b';
A(3).B.C = 'a';
A(4).B.C = 'a';

I want to change the values of field C based on values of Values and indexes of IndexingArray:

Values = {'a', 'b'};
IndexingArray = [1 1 0 1];

So, my new structure array will be:

A(1).B.C = 'b';
A(2).B.C = 'b';
A(3).B.C = 'a';
A(4).B.C = 'b';

Is there a way to do it without using a for loop?

Answer 1

You can do this without loops using deal and comma separated list syntax, but it might look a little harder to read:

% Initialize A:
A(1).B.C = 'a';
A(2).B.C = 'b';
A(3).B.C = 'a';
A(4).B.C = 'a';
Values = {'a', 'b'};
IndexingArray = [1 1 0 1];

temp = [A.B];           % Structure array containing B substructures
[temp.C] = deal(Values{IndexingArray+1});  % Set field C of each structure element
temp = num2cell(temp);  % Convert to a cell array of 1-by-1 structures
[A.B] = deal(temp{:});  % Update B substructures

And in newer versions of MATLAB, you can omit deal altogether:

temp = [A.B];
[temp.C] = Values{IndexingArray+1};
temp = num2cell(temp);
[A.B] = temp{:};

This should allow you to update the B and C fields without affecting any other fields that might exist in your more complicated structure.

Answer 2

Instead of changing the value, it is simpler to rebuild the structure array.

• Get value by IndexingArray:

  val = Values(IndexingArray+1);
  

• Build array of structures using cell2struct, and convert to cell array using num2cell:

  T = num2cell(cell2struct(val, {'C'}, 1));
  

• Convert T result to array of structures using cell2struct:

  A = cell2struct(T', {'B'}, 1);
  

---

Here is the code sample to create A:

Values = {'a', 'b'};
IndexingArray = [1 1 0 1];

val = Values(IndexingArray+1);

T = num2cell(cell2struct(val, {'C'}, 1));

A = cell2struct(T', {'B'}, 1);

---

Building A in a single line of code:

A = cell2struct((num2cell(cell2struct(Values(IndexingArray+1), {'C'}, 1)))', {'B'}, 1);

---

Result (for testing):

>> A(1).B.C

ans =

b

>> A(2).B.C

ans =

b

>> A(3).B.C

ans =

a

>> A(4).B.C

ans =

b

---

Solution using arrayfun:

val = Values(IndexingArray+1);

A = arrayfun(@(x) struct('B', struct('C', val{x})), 1:4)

---

Update specific elements of A:

In case you need to update specific elements, instead overwriting A, you can apply arrayfun selectively, to the indexes, you know you need to update.

Example:

Assume A length is 6 elements, and you need to update the first 4, you can use the following code:

A(1:4) = arrayfun(@(x) struct('B', struct('C', val{x})), 1:4);

Assume you know you need to update only A(1) and A(4), you can use the following example:

A(1).B.C = 'a';
A(2).B.C = 'b';
A(3).B.C = 'a';
A(4).B.C = 'a';
A(5).B.C = 'c';
A(6).B.C = 'd';

Values = {'a', 'b'};
IndexingArray = [1 1 0 1];

val = Values(IndexingArray+1);

%List of indices of A to update
indices = [1, 4];

A(indices) = arrayfun(@(x) struct('B', struct('C', val{x})), indices);