cell and matrix and for roof function

clear; clc;
N=6;
Ncell_1=11; Ncell_2=21; Ncell_3=31; Ncell_4=41; Ncell_5=51; Ncell_6=61;
Ncell=[Ncell_1,Ncell_2,Ncell_3,Ncell_4,Ncell_5,Ncell_6];

for i=1:N
    for j=1:N
        for m=1:Ncell(i) 
            for n=1:Ncell(j)
                
                A(m,n)=zeros;
            end
        end

        B{i,j}=[A];
    end              
end

my desired answer is that in the each element in the B cell is symetric like the following.

B

[11*11 doubles, 11*21 doules, 11*31 doubles, 11*41 doubles, 11*51 doubles, 11*61 doubles]

[21*11 doubles, 21*21 doubles, 21*31 doubles, ...............................................21*61 doubles]

[31.............................................,31*31doubles,......................................................

[41...

[51...

[61*11doubles,....................................................................................................61*61 doubles]

But there is the problem in my code to get the desired answer. but i can't find it.

Answer 1

[Guillaume]

First, what is the point of:

Ncell_1=11; Ncell_2=21; Ncell_3=31; Ncell_4=41; Ncell_5=51; Ncell_6=61;
Ncell=[Ncell_1,Ncell_2,Ncell_3,Ncell_4,Ncell_5,Ncell_6];

when you could have just written:

Ncell = [11, 21, 31, 41, 51, 61];

A lot less typing, a lot easier to read and doesn't use numbered variables (If you start numbering variables, you're doing it wrong!).

You could also have written:

Ncell = 11:10:61;  

Also, we're not going to use N. Don't hardcode the size of arrays. If tomorrow you decide you want 7 elements, you have to make sure you change the Ncell array and the N scalar. If one doesn't match the other, your program won't behave correctly. So, instead simply tell matlab to use the number of elements in Ncell whatever that is.

Finally, you don't seem to understand how to construct a matrix of zeros. To construct a matrix of size m x n you don't use loops, you simply use:

A = zeros(m, n);

So, to summarise, the correct code if you were to use loops would be:

Ncell = [11, 21, 31, 41, 51, 61];
B = cell(numel(Ncell), numel(Ncell));  %always preallocate arrays before loops
for row = 1:numel(Ncell)
    for col = 1:numel(Ncell)
        B{row, col} = zeros(Ncell(row), Ncell(col));
    end
end

However, a much simpler way to get what you want would be:

Ncell = [11, 21, 31, 41, 51, 61];
B = mat2cell(zeros(sum(Ncell)), Ncell, Ncell);   %create a square matrix of size sum(Ncell) with zeros, then split into cell array according to Ncell with mat2cell

No loop required.

[Jeil Park]

Thank you so much.

But the above code is that I simplified my real code.

I really want to use four [for roof function] so that my real code works.

Could you let me know the problem in my code?

[Guillaume]

The main issue with your code is the lack of preallocation. At i=1, j=6, you've grown your A to a 11x61 matrix. At i=2, j=6, since you've not changed A, it's still a 11x61 matrix. You're first filling the first 11x11 elements, then growing it to 21 rows, so end up with a 21x61 matrix.

With the 4 loops, the correct code would be:

B = cells(numel(Ncell), numel(Ncell));  %preallocation of cell array. Not so critical but will speed up the code:
for i = 1:numel(Ncell)
    for j = 1:nueml(Ncell)
        A = zeros(Ncell(i), Ncell(j));  %preallocation of A, of the correct size. Critical!
        for m = 1:Ncell(i) 
            for n = 1:Ncell(j)
                A(m, n) = something;
            end
        end
        B{i, j} = A;
    end
end

However, if you explain what you truly want to do, we can probably find a way to do it without loops (which would be more efficient).

Answer 2

[Jeil Park]

Honestly, It is first time for me to use Matlab Answers.

It really works and is helpful. Thank you so much.

You save my day !

the response is so fast. It is really good.

Thanks a lot again.

[Star Strider]

If Guillaume’s Answer helped you solve your problem, please Accept it.

[Jeil Park]

I don't know how to accept it since it is first time.

Could you tell me how to accept? the answer helped me solve my problem a lot! Thanks

[Guillaume]

Click on the green accept button next to the answer you want to accept.