Matrix indexing, getting back my original matrix.

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Raldi
Raldi 2014-6-14
评论： Joseph Cheng 2014-6-14
Hi,
I have a matrix
mat= [ 1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9];
and I sub-sample it using the simple command
newMat= mat(1:4:end, 1:4:end);
Is there a way later on to get back the indexes I lost from the original matrix, if I want to reconstruct it in a new matrix likewise
newMat2= zeros(size(mat, 1), size(mat, 2));
newMat2(1:4:end, 1:4:end) = newMat;
So what I want is to replace the columns and rows I lost with lets say NaN.
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Jan
Jan 2014-6-14
I do not get the problem. In your example you fill the missing values with zeros. So all you want to do is using nan() instead of zeros()?

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采纳的回答

Joseph Cheng
Joseph Cheng 2014-6-14
Were you looking to get something like this?
mat= [ 1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9; ...
1 2 3 4 5 6 7 8 9];
selCol = 1:4:size(mat,2);
selRow = 1:2:size(mat,1);
newMat= mat(selRow, selCol);
newMat2= NaN*zeros(size(mat));
newMat2(selRow,selCol) = newMat;
newMat2 =
1 NaN NaN NaN 5 NaN NaN NaN 9
NaN NaN NaN NaN NaN NaN NaN NaN NaN
1 NaN NaN NaN 5 NaN NaN NaN 9
NaN NaN NaN NaN NaN NaN NaN NaN NaN
1 NaN NaN NaN 5 NaN NaN NaN 9
NaN NaN NaN NaN NaN NaN NaN NaN NaN
1 NaN NaN NaN 5 NaN NaN NaN 9
NaN NaN NaN NaN NaN NaN NaN NaN NaN
1 NaN NaN NaN 5 NaN NaN NaN 9
  1 个评论
Joseph Cheng
Joseph Cheng 2014-6-14
There may be a way to figure out the increments based on the new mat size and original mat but need to think of the conditions.
For instance if you go 1:4:100 you get a 1x25. which you can see 4. but you can just round or floor or ceil +1 for all cases.

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更多回答（1 个）

Azzi Abdelmalek
Azzi Abdelmalek 2014-6-14
You haven't lost anything
mat=repmat(1:9,6,1) % Example
new_mat=mat(1:4,1:4)
You have both the original matrix and the new one, maybe you can give more details about loosing indices
  1 个评论
Raldi
Raldi 2014-6-14
No this was just an example to illustrate my problem, assume the matrices are not on the same function and there is no way to access my original matrix. What I actually want is to find a way to select all the rows and columns that I left out in my original matrix and populate them with some new values (for simplicity here NaN)

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