Append element to a cell

I am trying to loop over a cell array S and see if S{i,3} is equal to a j than add the elements of S{i,2} to X{j,1}

j = 1;
X = {};
while j < 29
    X{j,1} = j;
    for i = 1:length(S)
        if S{i,3} == j
            for k = 1:length(S{i,2})
                X{j,2} =[X{j,2}, S{i,2}(k,1)];
            end    
        end
    end
    j = j+1;
end

I get Index exceeds matrix dimensions for line 13.

Here is the cell array S:

And S{i,2} looks like this:

Answer 1

[Guillaume]

Index exceeds matrix dimensions

Well, then considering that the only indexing is of your cell array, either it doesn't have two columns or it doesn't have i rows. In any case, X{i, 2} is not valid. If X{i, 2} were valid your syntax is indeed correct.

[Afaf Arfaoui]

I edited my question. Actually the indexing of my cell array is not the only one

Answer 2

[Jan]

The message means, that X does not have either i rows or not 2 columns. You can check this easily using the debugger. Type this in the command window:

dbstop if error

Run the code again until stops at the error. Then check:

size(X)

[Afaf Arfaoui]

I get [1,1] for the size

[Afaf Arfaoui]

Actually if I run this

    j = 1;
    X = {}; 
    while j < 29
        X{j,1}=j;
        for i = 1:length(S)
            if S{i,3} == j
                for k = 1:length(S{i,2})
                    X{j,2} = S{i,2}(k,1);
                end    
            end
        end
        j = j+1;
    end

Without trying to save what's in X{j,2} I get a cell array with 28 rows (which is what I want) it's just not saving what's in X{j,2} and append the S{i,2}(k,1) to the end of it

[Jan]

This is not useful:

for k = 1:length(S{i,2})
   X{j,2} = S{i,2}(k,1);
end

It overwrite X{j,2} in each iteration. Maybe you want:

x{j,2} = S{i,2}

or

x{j,2} = S{i,2}(:, 1)

without a loop.

NOTE: Using length is prone to bugs, because it replies the longest dimension. If you want to get the length of the first dimension, use:

size(S, 1)

A cleaned version of your code:

X = cell(29, 2);  % Pre-allocate
For j = 1:29
  X{j,1} = j;
  for i = 1:size(S, 1)
      if S{i,3} == j
         X{j,2} = [X{i,2}, S{i,2}(:,1)];
      end
  end
end

The pre-allocation creates X{i, 2} implicitly as [].

Are you sure that [X{ i ,2}, S{i,2}(:,1)] is wanted, not j ?

[Afaf Arfaoui]

Thank you. You're right it's X{j,2}. But I want to append elementS in the same row in each iteration of j and not in different columns: