Help preallocating variables for a table

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Julian 2018-9-13

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https://ww2.mathworks.cn/matlabcentral/answers/418834-help-preallocating-variables-for-a-table

评论： Julian 2018-9-13

采纳的回答： Guillaume

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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!

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Guillaume 2018-9-13

编辑：Guillaume 2018-9-13

Please get rid of all the extra blank lines you've added to the code and in the future use the {}Code button to format code as such, as I've done for you now.

What is C (returned by chop)? A cell array? A matrix?

Why are PosX, PosY and particularly Frame cell arrays. Frame only stores scalar numerics. Not sure about PosX and PosY (depends on what C is).

Julian 2018-9-13

Thank you Guillaume. C is a matrix with doubles in it. They don't necessarly have to be cell arrays. I just built it that way because it was the first thing that came to my mind. They actually all hold scalar numerics.

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Answer 1

Guillaume 2018-9-13

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编辑：Guillaume 2018-9-13

在 MATLAB Online 中打开

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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Guillaume 2018-9-13

No, I understood the intent the first time round. It's my mistake, I meant to use the original NumberOfCell before it got repelem'd on that Frame line. Swapping the order of the two lines will fix the error:

   NumberOfCells = cellfun(@(m) size(m, 1), chopped);
   Frame = repelem((1:size(chopped, 1))', NumberOfCells);    %to be done first
   NumberOfCells = repelem(NumberOfCells , NumberOfCells);

Better would be not to reuse NumberOfCells for different purpose so replace the above three by:

ncells = cellfun(@(m) size(m, 1), chopped);
NumberOfCells = repelem(ncells, ncells);
Frame = repelem((1:size(chopped, 1))', ncells);

and then the order doesn't matter.

Julian 2018-9-13

Thank you so much Guillaume. I really learnt a lot! It works just fine now

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Answer 2

Steven Lord 2018-9-13

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I think you want to use the T = table('Size',sz,'VariableTypes',varTypes) syntax shown on the documentation page for the table function.

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Julian 2018-9-13

编辑：Julian 2018-9-13

Yes i did that:

    [nrows,ncols] = cellfun(@size,caCM);
rows = sum(nrows);
varTypes = {'double', 'double', 'double', 'double'};
varNames = {'Frame', 'PosX', 'PosY', 'NumberOfCells'};
sz = [rows 4];
caTable = table('Size', sz, 'VariableTypes', varTypes, 'VariableNames',varNames);

but I still don't know how to use the for loop to make it fill out the empty fields the right way.

Peter Perkins 2018-9-13

The preallocation syntax that Steve is referring to was introduced in R2018a.

But Guillaume's answer avoids needing to preallocate the table, because nothing is growing dynamically, and everything is a vectorized assignment. +1.

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