Concatenating data from structure array for plotting

I have a structure array with several levels of depth that I am trying to extract a specific set of data from in order to plot it. The plan is to draw individual elements from several different fields, and then combine them into a single array for plotting. I know that normally structures can be concatenated using [], but it doesn't seem to be working for this instance.

plot(x_values,[files([files.Param] == condition).Zone(1).Values(1,4)]);

Expected one output from a curly brace or dot indexing expression, but there were multiple results.

I know that the inner conditional statement is working, as I can extract it and get the appropriate logic matrix, but that means that I am looking at multiple elements of 'files' (which is exactly what I want to do), but they do not seem to be concatenating properly. Any suggestions would be appreciated.

Answer 1

[Stephen23]

Nested structures do not allow that syntax, so the simplest solution is to use temporary variables to unnest those structures (temporary varaibles will not copy data in memory, so don't worry about using them).

https://www.mathworks.com/matlabcentral/answers/421867-accessing-all-lower-elements-of-a-structure

https://www.mathworks.com/matlabcentral/answers/329538-why-do-i-get-this-error-for-this-non-scalar-structure#comment_436903

https://www.mathworks.com/matlabcentral/answers/408691-is-there-a-way-to-extract-a-set-of-values-from-a-structured-array#comment_586986

[Bob Thompson]

Ok, so, now that I've had a chance to review those pages, basically gathering the data the way I would like is not possible without using loops to call the individual pieces, because arrays work off comma separated lists, and those don't combine well with structures. Typically, your suggestion, therefore, of a more efficient way to accomplish this type of data management is to use multidimensional arrays with only a single level of structure. Does that sound about correct?

You refer to non-scalar structures in one of the posts. What are those, and how do they differ from regular structures?

In your post here you mention creating a temporary variable. I don't know that I've ever done that intentionally before, so would you be willing to show me an example?

[Stephen23]

"your suggestion, therefore, of a more efficient way to accomplish this type of data management is to use multidimensional arrays with only a single level of structure. Does that sound about correct?"

Yes. MATLAB code is usually simplest and most efficient when using arrays. Nested structures are usually not worth the bother. Structures (scalar or non) are quite handy though.

"You refer to non-scalar structures in one of the posts. What are those, and how do they differ from regular structures?"

All structures are exactly the same type of object, the only difference is the size: a scalar structure is scalar, a non-scalar structure is not scalar. I use this term because it is used in the documentation:

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

MATLAB does not actually have special "scalar" data classes:

https://www.mathworks.com/help/matlab/matlab_prog/fundamental-matlab-classes.html

For example, this is a scalar double:

1

but because all scalars are arrays that scalar double is actually just a 1x1x1x1x... double array, and it has exactly the same class as any other AxBxCx... double array. In exactly the same way this scalar structure

S.blah = 1

is exactly the same class as any non-scalar structure with size AxBxCx... the only difference is the size. When you are using a comma separated list this makes a difference because some syntaxes are easiest to use with scalar structures. Really many operations on scalar structures are just degenerate cases of comma separated lists... but I digress.

"In your post here you mention creating a temporary variable. I don't know that I've ever done that intentionally before, so would you be willing to show me an example?"

>> S(1).A.B = 1;
>> S(2).A.B = 2;
>> tmp = [S.A];
>> tmp.B
ans =  1
ans =  2

Where S.A.B would throw an error. Read more here:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

[Bob Thompson]

Ok, yeah. I am already largely using non-scalar structures, since I have multiple structures under 'files', 'zones', and some other stuff.

Note this will not work for a non-scalar S. Read more here:

If I understand this correctly, then because I already have non-scalar structures then the temporary variable won't work. Any other suggestions before I restructure my variable to remove the nested structure?

[Stephen23]

" I am already largely using non-scalar structures, since I have multiple structures under 'files', 'zones', and some other stuff."

What you descibe "I have multiple structures under 'files'" are nested structures (which may or may not be scalar). You seem to be confusing nested structures (structures inside structures, like you described) and non-scalar structures (structures with non-scalar size).

"If I understand this correctly, then because I already have non-scalar structures then the temporary variable won't work. Any other suggestions before I restructure my variable to remove the nested structure?"

I have no idea what would work because nowhere have you given any information on the sizes of your structures. For a start, what size is the files structure? What sizes are the nested Zone structures?

[Bob Thompson]

The size is not specific, but should follow something like this:

files (1x5) struct

- name (1x1) char

- param (1x1) double

- zone (1x4) struct

-- name (1x1) char

-- number (1x1) double

-- Max (5x4) double

-- Average (5x1) double

[Stephen23]

What is the size of the Values field?

Please describe which data combination you want to extract from this structure. In your question you seem to want to extract this:

1. N elements of files (determined by a logical array), followed by
2. one element of the structure Zone, followed by
3. one element of the array Values.

That would mean getting one value for each element of files, so it appears that you were hoping to concatenate those N values into one numeric vector. Is that correct?

[Bob Thompson]

Sorry, 'values' can be either 'Max' or 'Average'. I didn't refer back to my original post when I listed the sizes of the structure elements.

That would mean getting one value for each element of files, so it appears that you were hoping to concatenate those N values into one numeric vector. Is that correct?

Yes, that is correct.

[Stephen23]

As I mentioned earlier, working with nested functions is not much fun.

You could try something like this:

fun = @(s) s.Zone(1).Values(1,4);
V = arrayfun(fun,files([files.Param]==condition))

[Bob Thompson]

That should be sufficient for this, thanks.

And in the future I know better how to avoid this whole situation, so double thanks.

Answer 2

[Mehmet Burak Ekinci]

You may use custom matlab functions from file exchange to get values from nested struct arrays

https://www.mathworks.com/matlabcentral/fileexchange/116130-matlab-struct-operations

See Answer in link for example.

https://www.mathworks.com/matlabcentral/answers/331374-how-can-i-plot-nested-structure-fields

[Stephen23]

Or use the inbuilt GETFIELD and SETFIELD :

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

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

or use comma-separated lists:

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

https://www.mathworks.com/matlabcentral/answers/1656435-tutorial-comma-separated-lists-and-how-to-use-them

and then you can avoid needing to use any third-party functions.