Modify struct array and return struct array

Question

I have a struct array: a 1x10 struct array with fields: N, t, q, r, T, each of which is a vector of type double.

The 10 array entries each represent the outcome of a testing condition in an experiment. I would like to be able to make a function that takes two indices, index1 and index2, and modifies the constituent N, t, q, r vectors (T is a single number) so that they become length index1:index2. Something like

function sa = modifier(struct_array, index1, index2)
    sa = structfun(@(x) x(index1:index2), struct_array, 'UniformOutput', false)
    stuff
end

Now, where stuff is, I've tried using structfun and cellfun, see here except that those return a struct and a cell array, respectively, whereas I need to return a struct array.

The purpose of this is to be able to get certain sections of the experimental results, e.g. maybe the first five entries in each vector inside each cell correspond to the initial cycles of the experiment.

Please let me know if this is possible, and how I might go about it!

Answer 1

You can try this:

From this question's answer, I figured out how to loop through struct fields. I modified the code to address your question by extracting a subsample from each field that goes through the for loop and then copy the desired subset of that data into a new struct array with identically named fields.

% Define indexes for extraction
fieldsToTrim = {'a' 'b'};
idx = 2:3; % Create index vector for extracting selected data range

% Define test struct to be read
teststruct.a = [1 2 3];
teststruct.b = [4 5 6];
teststruct.c = [7 8 9];

% Get names of struct fields
fields = fieldnames(teststruct);

% Loop through each field and extract the subset
for i = 1:numel(fields)
    if max(strcmp(fields{i},fieldsToTrim)) > 0
        % If current matches one of the fields selected for extraction
        % extract subset
        teststructResults.(fields{i}) = teststruct.(fields{i})(idx);
    else
        % Else, copy all contents on field to resulting struct
        teststructResults.(fields{i}) = teststruct.(fields{i});
    end
end

Finally, to turn this into a function, you can modify the above code to this:

function teststructResults =  extractSubsetFromStruct(teststruct,fieldsToTrim,idx1, idx2)
    % idx1 and idx2 are the start and end indicies of the desired range

    % fieldsToTrim is a string array of the field names you want
    % included in the trimming, all other fields will be fully copied

    % teststruct is your input structure which you are extracting the
    % subset from

    % teststructResults is the output containing identically named
    % struct fields to the input, but only containing data from the selected range

    idx = idx1:idx2; % Create index vector for extracting selected data range

    % Get names of struct fields
    fields = fieldnames(teststruct);

    % Loop through each field and extract the subset
    for i = 1:numel(fields) 
        if max(strcmp(fields{i},fieldsToTrim)) > 0
            % If current matches one of the fields selected for extraction
            % extract subset
            temp = teststruct.(fields{i});
            teststructResults.(fields{i}) = temp(idx);
        else
            % Else, copy all contents on field to resulting struct
            teststructResults.(fields{i}) = teststruct.(fields{i});
        end
    end
end

I successfully ran the function like this:

teststruct = 

    a: [1 2 3]
    b: [4 5 6]
    c: [7 8 9]

>> extractSubsetFromStruct(teststruct,{'a' 'b'},2,3)

ans = 

    a: [2 3]
    b: [5 6]
    c: [7 8 9]