Accessing data within a structure

I have this structure:

veh(x).meas(y).data.fileName

So i have a certain number x of vehicles (veh) and a certain number y of measurements (meas).

how can i get the fileName for every measurement as a list? (efficient)

Would like ot avoid a construct of numerous "numel" and "for" commands.

thanks!

Answer 1

[Jan]

The chosen data structure demands for loops. There is no "efficient" solution, which extracts the fields in a vectorized way. You can at least avoid some pitfalls:

fileList = {};
nVeh     = numel(veh);
for k = 1:nVeh
    aVeh  = veh(k);
    nMeas = numel(aVeh.meas);
    for kk = 1:nMeas
       fileList{end+1} = aVeh.meas(kk).data.fileName;
    end    
end

Here the output grows iteratively. If you know a maximum number of elements, you can and should pre-allocate the output:

nMax     = 5000;
fileList = cell(1, nMax);
iList    = 0;

nVeh     = numel(veh);
for k = 1:nVeh
    aVeh  = veh(k);
    nMeas = numel(aVeh.meas);
    for kk = 1:nMeas
       iList           = iList + 1;
       fileList{iList} = aVeh.meas(kk).data.fileName;
    end    
end
fileList = fileList(1:iList);   % Crop ununsed elements

Choosing nMax too large is very cheap, while a too small value is extremely expensive. Even 1e6 does not require a remarkable amount of time.

numel is safer than length. The temporary variable aVeh safes the time for locating veh(k) in the inner loop repeatedly.

If you do not have any information about the maximum number of output elements, collect the data in pieces at first:

nVeh  = numel(veh);
List1 = cell(1, nMax);
for k = 1:nVeh
    aVeh  = veh(k);
    nMeas = numel(aVeh.meas);
    List2 = cell(1, nMeas);
    for kk = 1:nMeas
       List2{kk} = aVeh.meas(kk).data.fileName;
    end
    List1{k} = List2;
end
fileList = cat(2, List1{:});

Answer 2

[Le Vu Bao]

I have just had the same kind of question. I think you can try a nested table, struct for your data. With table, it is simpler to query for data without using "for" or "numel"

[Christian Tieber]

Did it with loops at the end. Not very elegant. But it works.

fileList=[]
nVeh=length(veh)

for i=1:nVeh
    
    nMeas=length(veh(i).meas)
    
    for a=1:nMeas
       
        fileName = {veh(i).meas(a).data.fileName} 
        fileList=[List;fileName]
      
    end    
end