MATLAB: Extracting values data from variable field to a matrix

dataextractfield

I want to extract in a matrix all the datas of the field Latency. I passed to extract the first value with the code

 trials_struct{1,1}.Latency

But I don't find the solution to get all value in one matrix in one loop.

Best Answer

Your misleadingly named trials_struct is actually a 1x186 cell array, each cell contains one scalar structure with multiple fields. Unfortunately the fieldnames are not all the same: if they were then your data would be better stored in a non-scalar structure, then the solution to your task is also trivial:

[real_struct.Latency]

Unfortunately because the fieldnames differ you will have to stick with a cell array and use a loop or cellfun:

cellfun(@(s)s.Latency,trials_struct)

In this case it worked, but note that if any of the scalar structures does not have that field Latency, then the code will throw an error.

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MATLAB: How to add element to a cell array struct without loop

This would be trivially easy if you had stored your data in a non-scalar structure (instead of inefficiently in lots of separate structures in a cell array):

>> [S(1:10).data] = deal(1)
S = 
1x10 struct array with fields:
    data
>> S(1).data
ans =
     1
>> S(6).data
ans =
     1

MATLAB: Looping over inputs provided by structure

The basic problem is that you are confusing the number of fields in a structure with the size of the structure. However these are two totally independent things: a structure is just like any other array: it can be empty, scalar, vector, etc. The number of fields it has is independent to its size. All of your structures are scalar, with various number of fields. Because your structures are scalar, it is not so interesting to check their size. What you actually want is to access the fields of the structure, something like this:

fn1 = fieldnames(apples.AppleType);
for k = 1:numel(fn1)
    tmp = apples.AppleType.(fn1{k})
    ...
end

Better Alternative: storing the data in nested structures is confusing and complicated to access. I would recommend that you store your data in one simple non-scalar structure, without any nested structures at all:

S(1).name  = 'Green';
S(1).taste = 'Sour';
S(1).color = 'Bright';
S(2).name  = 'Red';
S(2).taste = 'Sweet';
S(2).color = 'Bright';

or equivalently by calling struct just once:

S = struct('name',{'Greeen','Red'}, 'taste',{'Sour','Sweet'}, 'color',{'Bright','Bright'})

Then it is trivial to access any apple/s using indexing, e.g. in a loop:

for k = 1:numel(S)
    S(k)
end

Using a non-scalar structure really makes accessing your data easier, for example, to get all of the apples names in a cell array all you need is this:

>> {S.name}
Green
Red

Best Answer

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MATLAB: How to add element to a cell array struct without loop

MATLAB: Looping over inputs provided by structure

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