Splitting arrays using loops

Question

Freja 2023-8-10

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2007102-splitting-arrays-using-loops

编辑： Bruno Luong 2023-8-11

would there be a way to create a loop for splitting an 8x8 array into seperate 4x4 arrays and then find the mean of each of the 4x4 arrays and place each of the mean values into a new array ?

5 个评论
显示 3更早的评论隐藏 3更早的评论

Stephen23 2023-8-11

在 MATLAB Online 中打开

"No need for permute, squeeze does the job just fine"

Not really, because SQUEEZE is fragile, unlike PERMUTE. It all looks "fine" ... until one day the user has data which consists of one row of blocks and are then astonished when the output has the wrong orientation:

A = reshape(1:32,4,8)
A = 4×8
     1     5     9    13    17    21    25    29
     2     6    10    14    18    22    26    30
     3     7    11    15    19    23    27    31
     4     8    12    16    20    24    28    32
M = squeeze(mean(reshape(A,[4 1 4 2]),[1 3])) % oops, wrong output
M = 2×1
    8.5000
   24.5000
M = mean(permute(reshape(A,4,1,4,2),[2,4,1,3]),3:4) % aaah, much better
M = 1×2
    8.5000   24.5000
M = permute(mean(reshape(A,4,1,4,2),[1,3]),[2,4,1,3]) % also this
M = 1×2
    8.5000   24.5000

SQUEEZE is just like LENGTH: used only by people who like hidden bugs in their code.

Bruno Luong 2023-8-11

编辑：Bruno Luong 2023-8-11

在 MATLAB Online 中打开

OP stated clearly he wants average on 8 x 8 matrix not 4 x 8.

But granted if one doesn't like squeeze, in this block average problem use reshape rather than permute for efficienty

A = reshape(1:32,4,8)
A = 4×8
     1     5     9    13    17    21    25    29
     2     6    10    14    18    22    26    30
     3     7    11    15    19    23    27    31
     4     8    12    16    20    24    28    32
M = reshape(mean(reshape(A,[4 1 4 2]),[1 3]), [1 2])
M = 1×2
    8.5000   24.5000

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

C B 2023-8-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2007102-splitting-arrays-using-loops#answer_1285902

在 MATLAB Online 中打开

I have already answered you same apart from making mean value as new array. here is how you can do it.

A = reshape(1:64, [8,8]);
mean_values = zeros(2,2);
% Loop to extract 4x4 matrices and compute mean
for row = 1:2
    for col = 1:2
        r_idx = (row-1)*4 + 1 : row*4;
        c_idx = (col-1)*4 + 1 : col*4;
        
        submatrix = A(r_idx, c_idx);
        mean_values(row, col) = mean(submatrix(:));
    end
end
disp(mean_values);
   14.5000   46.5000
   18.5000   50.5000