MATLAB: Help preallocating variables for a table

MATLABpreallocatetable

Hi everyone,
so i have written some code which goes like this:
function caTable = makeCaTable(caCM)
% creates a table with information about the frames(coordinates of the
% centroids and the total number of cells in the frame)
Frame = {};
PosX = {};
PosY = {};
NumberOfCells = {};
for i = 1:size(caCM,2)
      C = chop(caCM{1,i},0);
      for j = 1:size(C,1)
          Frame{end+1,1} = i;
          PosX{end+1,1} = C(j,1);
          PosY{end+1,1} = C(j,2);
          if(j == 1)
              NumberOfCells{end+1,1} = size(C,1);
          else
              NumberOfCells{end+1,1} = '-';
          end
      end
  end
  % write the table
  caTable = table(Frame, PosX, PosY, NumberOfCells);
it works and gives me the results i want. But now i want to preallocate the variables. I thought I do it like this:
[nrows,ncols] = cellfun(@size,caCM);
rows = sum(nrows);
Frames = cell(rows,1);
PosX = cell(rows,1);
PosY = cell(rows,1);
NumberOfObjects = cell(rows,1);
But if i do so i cant use my for loop the way i wrote it, since it would always put the values at the end of the preallocated matrices. But if i simply use:
Frames(j,1) = i
my table will not have all the fields. I really don't know how i can fix this. Maybe some of you could help me with that.
Thank you very much!

Best Answer

C is a matrix with doubles in it
In that case,
function caTable = makeCaTable(caCM)
   %doc goes here
   %  caCM: a 2D(?) cell array
   %  caTable:a table with 4 variables: Frame, PosX, PosY, NumberOfCells
 
   chopped = cellfun(@(x) chop(x, 0), caCM(1, :), 'UniformOutput', false)';  %transpose for easier repelem and cell2mat
   NumberOfCells = cellfun(@(m) size(m, 1), chopped);
   NumberOfCells = repelem(NumberOfCells , NumberOfCells );
   Frame = repelem((1:size(chopped, 1))', NumberOfCells);
   chopped = cell2mat(chopped);
   PosX = chopped(:, 1);
   PosY = chopped(:, 2);
   caTable = table(Frame, PosX, posY, NumberOfCells);
end
As a rule you shouldn't be using cell arrays unless you need to, they're slower to process and takes a lot more memory than a matrix (15 times more if each cell is just a scalar double).
Note that I'm replicating the number of cells for all rows of a frame instead of having '-' for all rows but the first of the frame. It's rarely a good idea to mix numbers and characters in a variable. If you want a different value for all but the first row, I'd recommend using NaN instead. It's trivially done:
     NumberOfCells = repelem(NumberOfCells, NumberOfCells);  %continuing on from there
     NumberOfCells([false; diff(NumberOfCells) == 0]) = NaN;
If you really want a '-', then it's slightly more complicated:
     ncells = repelem(NumberOfCells, NumberOfCells);  
     NumberOfCells = num2cell(ncells);
     NumberOfCells([false; diff(NumberOfCells) == 0]) = {'-'};
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