How to access data in a nested structures

Hi,

how do I access parts of the following structure:

a(1).b.c = 1;
a(2).b.c = 2;

I want a vector containing the c's.

This seems to work:

vec = cat(1,cat(1,a(:).b).c);

Is this the correct way?

Why does this not work:

[a(:).b].c

Thanks!

Answer 1

[Stephen23]

"Is this the correct way?"

The usual MATLAB way is to use two lines, e.g.:

tmp = [a.b];
out = [tmp.c];

"Why does this not work: [a(:).b].c"

Because MATLAB does not allow indexing directly into the output of expressions (and accessing the field of a structure is a kind of indexing)

[Walter Roberson]

MATLAB does not allow indexing directly into the output of expressions

It does in some cases, since R2019b:

https://www.mathworks.com/help/releases/R2019b/matlab/matlab_prog/indexing-into-function-call-results.html

Their attempt of

cat(1,cat(1,a(:).b).c)

is legitimate in MATLAB since R2019b: the cat(1,a(:).b) is a function call, and the output of function calls can be dot indexed now.

[a(:).b].c

Nominally that is supposed to be equivalent to

horzcat(a(:).b).c

and if you express it in horzcat form then you get a result in R2019b and later (but you would want [horzcat(a(:).b).c] to bundle the results).

But the [] to horzcat equivalence is not complete, and the [] operation is not parsed as a function call for the purpose of dot indexing. It is an inconsistency in MATLAB.

Answer 2

[KSSV]

How about this?

[[a(:).b].c]

[Markus]

This does not work:

[[a(:).b].c]

↑

Error: Invalid use of operator.

[KSSV]

Okay...it worked in Octave...

Answer 3

[Walter Roberson]

a(1).b.c = 1;
a(2).b.c = 2;

arrayfun(@(S) S.b.c, a)

The above works only if the c is always a scalar.

a(1).b.c = [-1; 1];
a(2).b.c = [-2; 2];

cell2mat(arrayfun(@(S) S.b.c, a, 'UniformOutput', false))

This version works if the number of elements in c is the same each time, and the orientation of those elements is opposite the orientation of a. In the above example, the c are column vectors and a is a row vector and the result would have the c down columns.

a(1).b.c = [-1; 1];
a(2).b.c = [-2; 2];

cell2mat(arrayfun(@(S) S.b.c(:).', a(:), 'UniformOutput', false))

This version works if the number of elements in c is the same each time (but the shape does not need to be); the elements will become rows

temp = [a.b];
[temp.c]

This version works if the c are always scalar.

[Stephen23]

The last syntax "This version works if the c are always scalar" or any compatible sizes, e.g.:

>> a(1).b.c = [1;3;5];
>> a(2).b.c = [2;4;6];
>> temp = [a.b];
>> [temp.c]
ans =
          1.00          2.00
          3.00          4.00
          5.00          6.00

[Markus]

Thanks for the examples and detailed explanation. For my current use case I can get away with the simple scalar version.