Shifting rows of matrix in matlab - matlab

Question

I have to shift certain rows in matlab. Like let say I have a matrix of size 50x50. And I have to shift certain rows lets say 15,18,45.. to the top and the remaining rows at the bottom. How can I accomplish this in matlab?

Answer 1

Have you tried the circshift function? Something like this could help:
A = [1:8; 11:18; 21:28; 31:38; 41:48]
A =
1 2 3 4 5 6 7 8
11 12 13 14 15 16 17 18
21 22 23 24 25 26 27 28
31 32 33 34 35 36 37 38
41 42 43 44 45 46 47 48
B = circshift(A, [3, 0])
B =
21 22 23 24 25 26 27 28
31 32 33 34 35 36 37 38
41 42 43 44 45 46 47 48
1 2 3 4 5 6 7 8
11 12 13 14 15 16 17 18

Answer 2

This is a problem that can be quite easily solved with the help of some simple indexing:
Matrix = [ 1 101 201 301
2 102 202 302
3 103 203 303
4 104 204 304
5 105 205 305
6 106 206 306
7 107 207 307
8 108 208 308
9 109 209 309
10 110 210 310];
rowsOnTop = [1 8 4];
rowsBelow = true(size(Matrix,1),1);
rowsBelow(rowsOnTop) = false;
Modified = [Matrix(rowsOnTop,:); Matrix(rowsBelow,:)]
Modified =
1 101 201 301
8 108 208 308
4 104 204 304
2 102 202 302
3 103 203 303
5 105 205 305
6 106 206 306
7 107 207 307
9 109 209 309
10 110 210 310

Answer 3

I understood that you want to move certain rows of matrix to the top and keep the rest on its place. For that you can use this:
Example matrix:
Matrix = [ 1:10; 101:110; 201:210; 301:310 ]';
Matrix =
1 101 201 301
2 102 202 302
3 103 203 303
4 104 204 304
5 105 205 305
6 106 206 306
7 107 207 307
8 108 208 308
9 109 209 309
10 110 210 310
Here's the code:
RowsVector = [ 3, 5, 8 ];
Edit: new better solution (presented here first because it's better).
NewMatrix = Matrix(cell2mat(arrayfun(#(x) x:size(Matrix,1):prod(size(Matrix)), [ RowsVector, setdiff(1:size(Matrix, 1), RowsVector) ]', 'UniformOutput', false)));
NewMatrix =
3 103 203 303
5 105 205 305
8 108 208 308
1 101 201 301
2 102 202 302
4 104 204 304
6 106 206 306
7 107 207 307
9 109 209 309
10 110 210 310
Edit: the rest of the answer is related to a [limited] older solution.
% RowsVector must be sorted, otherwise the reordering will fail.
Edit: fixed a bug with unordered RowsVector input.
RowsVector = sort(RowsVector);
for RowIndex = 1:size(RowsVector, 2)
row = RowsVector(RowIndex);
Matrix = vertcat(Matrix(row,:), Matrix);
Matrix(row+1,:) = [];
end
This is the result:
Matrix =
8 108 208 308
5 105 205 305
3 103 203 303
1 101 201 301
2 102 202 302
4 104 204 304
6 106 206 306
7 107 207 307
9 109 209 309
10 110 210 310

Answer 4

I'd solve this by defining a row permutation matrix to produce the desired result. If Matlab has a built-in function for this it escapes me, so I wrote one:
function P = rowpermat(vec)
P = zeros(length(vec));
for i = 1:length(vec)
P(i,vec(i)) = 1;
end
If vec is a permutation of 1:n this function will return a matrix which permutes the rows of an nxn matrix 1->vec(1), 2->vec(2), ... Note the absence of error checking and the like so use this in production code at your own risk.
In this case, if A is the matrix to permute, you might write:
rowpermat([15, 18, 45, 1:14,16:17,19:44,46:50])*A