Retrieve data from a nested structure

Hi,

I am looking for some help with extracting data from a nested structure. I have a structure like that shown below.

I can call a single value from the structure easily e.g data.prop(1).Area will give me the value of Area from the first prop structure.

I wish to extract easily all of a certain field, the only way I can currently do this is with a loop. For example I might wish to make an array with all of the Areas.

 myData = data.prop(1:3).Area

However this doesn't work.

My current solution is to loop through the structure as follows:

for i = 1:length(data)
myData(i,1) = data.prop(i).Area
end

If there is a better way to achieve this I would be very interested. Thank you for the help

Answer 1

[Martin Dale]

% use a 2 stage process ( or more stages for a deeper nested structure)
tmp = [data.prop];
areas = [tmp.area];

Answer 2

[Walter Roberson]

myData = [data.prop.Area];

[matheu Broom]

Thanks, I have tried this but I get this answer,

Expected one output from a curly brace or dot indexing expression, but there were 1003 results.

Not sure what the problem is?

[Walter Roberson]

You could get that error if you omitted the [] that I showed, or if data is not a scalar structure (but if data were not a scalar structure you would have received an error in your for loop version.)

Example:

N = 10;
data.prop = struct('Area', num2cell(rand(1,N)), 'Perimeter', num2cell(randi(50,1,N)), 'Centre', num2cell(rand(N,2),2).');
[data.prop.Area]

[Guillaume]

the substructure prop is a non-scalar structure. The [] extraction cannot work.

Unfortunately, the loop is the only option. That's one of the reason I dislike multilevel structures.

[matheu Broom]

AHH, I think this is the issue, each of the prop is a structure holding a field, Area, Perimeter etc which each has one value. The loops are working but I just read about avoiding loops for efficiency and that its a bad coding practice.

I started using structures as its the only way I found of naming columns so that I know what that column contains.

[Walter Roberson]

N = 10;
data.prop = struct('Area', num2cell(rand(1,N)), 'Perimeter', num2cell(randi(50,1,N)), 'Centre', num2cell(rand(N,2),2).');
data.prop

ans = 1×10 struct array with fields:
    Area
    Perimeter
    Centre

[data.prop.Area]

ans = 1×10

0.9392 0.1636 0.5417 0.4702 0.3263 0.5196 0.4143 0.4565 0.1550 0.5646

You can see that my data.prop is a non-scalar structure and that it still works.