Finding smallest value in nested structure

Dear all,

I have a structure as shown here:

subject(i).cineLoop.frame(j).data(k).area

Where 'area' is an amount (e.g. 2328).

How can I find the smallest area in the entire structure? Do I have to resort to for loops? Or am I better off using a different kind of container?

I found another thread concerning this problem: http://www.mathworks.nl/matlabcentral/answers/40479-multi-level-indexing , however I didn't manage to get the solution working for structures with more than 2 levels.

Any help is much appreciated!

Answer 1

[Robert Cumming]

One way is to use a self calling function (this will find "area" at any level):

Here is the bones of one - you would call it by:

( yourStruct, 'area' );

function SelfCallingFunction ( str, var )
 fnames = fieldnames ( str );
 for i=1:length(fnames)
   if strcmp ( fnames{i}, var )
     disp ( str.(fnames{i} ) );
   elseif isstruct ( str.(fnames{i}) )
     SelfCallingFunction ( str.(fnames{i}), var )
   end      
 end
end

Note: This will only display the variable in question - but you can expand on this to make it store the "area" and returned it.

There is a bit more work to do - and you really need to do it yourself to learn how it works but also to be able to maintain the code in the future.

[B]

Dear Robert,

Thanks for your reply.

Throughout my code I make ample use of for loops to read dat from and write data to the structure. However, I was wondering whether I HAVE to use for loops, or if there is a more elegant way.

[Robert Cumming]

Maybe I misunderstood - I thought you wanted to find all "area" which could be anywhere...

Also - In my opinion there is nothing wrong with for loops if they are used correctly...

More compact code can also mean harder to understand and thus maintain (by yourself or colleagues)

[B]

Thanks for your reply. I'm glad to hear that for loops are the way to go. I've been using MATLAB for a while now, making the transitition from C++. Since a lot of containers from STL have their own syntax to handle data, I was wondering whether I was overlooking some basic functionality of MATLAB.

[Jeremy S]

Looping through the fieldnames() output is unfortunately the way to do it. Its a little easier if you put a number in the fieldname, i.e. area1, area2, etc... and then use a strcat(['area',str2num(i)]) in a for loop instead of using fieldnames().

Answer 2

[Image Analyst]

Just a guess. Try this (untested):

allAreas = [subject.cineLoop.frame.data.area];
[minArea, indexOfMin] = min(allAreas);

I know it works for cell arrays, such as you get from regionprops(). Add indexes at places if you want to narrow it down. However the index is the index into allAreas, not the i, j, and k separately. You might have to keep track of the min as you traverse your triple for loop if you want to know the i, j, and k individually.

[B]

Thanks for your reply. Unfortunately when trying

allAreas = [subject.cineLoop.frame.data.area];

the following error occurs:

Dot name reference on non-scalar structure.

Any other thoughts?

[Darik]

I think you might have to concatenate one level at a time

allCineLoops = [subject.cineLoop];
allFrames = [allCineLoops.frame];
allData = [allFrames.data];
allArea = [allData.area];
minArea = min(Area);

[Image Analyst]

Try this:

clc;
% Generate some sample data
for i = 1 : 3
  for j = 1 : 4
    for k = 1 : 5
      subject(i).cineLoop.frame(j).data(k).area = rand(1);
    end
  end
end

tic; % Start timer.
% Now find the smallest area.
% First initialize the min values we want to keep track of.
iAtMinArea = 1;
jAtMinArea = 1;
kAtMinArea = 1;
minArea = inf;
% Now scan the structure looking for the global min.
for i = 1 : 3
  for j = 1 : 4
    for k = 1 : 5
      if subject(i).cineLoop.frame(j).data(k).area < minArea
        iAtMinArea = i;
        jAtMinArea = j;
        kAtMinArea = k;
        minArea = subject(i).cineLoop.frame(j).data(k).area;        
      end
    end
  end
end
% Found it. Now print them to the command window
toc; % Stop timer and print out elasped time.

fprintf('The min area = %f and occurs at i = %d, j = %d, k = %d.\n',...
  minArea, iAtMinArea, jAtMinArea, kAtMinArea)
% Typical printout:
% Elapsed time is 0.001091 seconds.
% The min area = 0.010271 and occurs at i = 3, j = 3, k = 5.

Answer 3

[Jos (10584)]

I think the for-loop approach is the best option. You can use that to store each value in a matrix first:

    Ni = numel(subject)
    Nj = numel(subject(1).cineLoop.frame)
    Nk = numel(subject(1).cineLoop.frame(1).data)

    Area = zeros(Nk,Nj,Nk) ;

    for i=1:Ni
      for j=1:Nj
        for k=1:Nk
          Area(i,j,k) = subject(i).cineLoop.frame(j).data(k).area ;
        end
      end
    end

[MinArea, idx) = min(Area(:))
[mi,mj,mk] = ind2sub([Ni,Nj,Nk],idx)