Problem with using `reshape` to reorder the matrix.

Question

wei zhang 2020-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/567270-problem-with-using-reshape-to-reorder-the-matrix

编辑： Image Analyst 2020-7-19

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I am trying to reorder my matrix "by block" in columns.

See the code below, it is a mini case of my problem. It only has 2 "blocks"(In the graph above, it has 3).

a = [1,7,4,10;
    2,8,5,11;
    3,9,6,12];
output = [a(:,[1,2]);a(:,[3,4])];

I think this way is not very delicate. I think I should use the combination of transpose and reshape. If it is impossible, I would like to know either.

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Answer 1

julian navas 2020-7-19

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编辑：julian navas 2020-7-19

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I think I don't get 100% your question but from the shape of 'output' see if this is what you need

a(:,[2,3])=a(:,[3,2])
reshape(a,[],2)
ans =
     1     7
     2     8
     3     9
     4    10
     5    11
     6    12

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Answer 2

Image Analyst 2020-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/567270-problem-with-using-reshape-to-reorder-the-matrix#answer_467760

编辑：Image Analyst 2020-7-19

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Did you just try a dumb for loop?

m = reshape(1:63, 3, 21)
[rowsm, columnsm] = size(m)
outputColumns = 7 % Columns to "lock"
numGroups = columnsm / outputColumns;
outputRows = rowsm * numGroups
m2 = zeros(outputRows, outputColumns);
row = 1;
for col = 1 : numGroups
	col1 = (col-1) * outputColumns + 1;
	col2 = col1 + outputColumns - 1;
 	m2(row:row+rowsm-1,:) = m(:, col1:col2);
	row = row + rowsm;
end
m2

In my example I started with a 3x21 matrix and used 7 columns for a groups, so the output has 7 columns and (21/7) * 3 = 9 rows.

m =
   4     7    10    13    16    19    22    25    28    31    34    37    40    43    46    49    52    55    58    61
   5     8    11    14    17    20    23    26    29    32    35    38    41    44    47    50    53    56    59    62
   6     9    12    15    18    21    24    27    30    33    36    39    42    45    48    51    54    57    60    63
m2 =
   4     7    10    13    16    19
   5     8    11    14    17    20
   6     9    12    15    18    21
  25    28    31    34    37    40
  26    29    32    35    38    41
  27    30    33    36    39    42
  46    49    52    55    58    61
  47    50    53    56    59    62
  48    51    54    57    60    63