Extracting elements from cell array based on data type

Question

I'm loading qxf data into MATLAB using the function qxf2mat. This creates the following 1x1 struct:

struct with fields:

                 qxfName: 'BSR1786214'
        numberOfChannels: 14
          numberOfCycles: 24477
          parameterCodes: {1×14 cell}
          parameterTitle: {1×14 cell}
     parameterShortTitle: {1×14 cell}
     parameterDefinition: {1×14 cell}
    minimumObservedValue: [1×14 double]
    maximumObservedValue: [1×14 double]
             absentValue: [1×14 double]
                datetime: [24477×20 char]
                BTVOLTCM: [1×24477 single]
          BTVOLTCM_flags: {1×24477 cell}
                HEADCM01: [1×24477 single]
          HEADCM01_flags: {1×24477 cell}
                ISCMBMA1: [1×24477 single]

What I want to do now is extract all the [1x24477 single] elements and put them into their own matrix. So the matrix would have 24477 rows, and in this example, 3 columns.

I've used struct2cell to convert the struct into a cell array, and was planning to use cell2mat after that, but I can't because of all the different data types.

Wouldn't be easier just to access them by name? Or do the names change? — Ander Biguri
The names change from dataset to dataset, so ideally I just want to access them with indexing. — Brad Reed
Do you know the size beforehand, or do you get it from numberofCycles? — Ander Biguri

Ander Biguri Ander Biguri · Accepted Answer · 2017-08-22T17:53:53

Here it is a way of doing it:

% Get the names of the fields
fnames=fieldnames(mystruct);

% Get the fields that are both not a cell, and the correct size
thisones=~structfun(@iscell,mystruct)&structfun(@(x)(size(x,2)==mystruct.numberofCycles),mystruct);

% to make sure they are always 3 (else the struct is different of what you showed)
assert(sum(thisones)==3);

%Get the index of the fields
indices=find(thisones);

% make a matrix by appending the columns
result=zeros(3,mystruct.numberofCycles);
for ii=1:3
   result(ii,:)=mystruct.(fnames{indices(ii)});
end

Note that this will not work if you have 2 structures with size 1xN that are different types, but are not a cell. You would need to make the thisones condition more complex.

Extracting elements from cell array based on data type

1 Answers