MATLAB: Keep m*n structure after indexing a matrix

indexingmatrices

I have a matrix (A) and a logical index matrix (i) of the same size. When I do A(i) the result is a Vektor. How can I keep the m*n structure of the Matrix?

Best Answer

How would you expect to keep the same "structure" (I guess you mean size) ? If the number of elements that the indexing operation returns is different to the number of elements in the matrix, then the output must have a different size to the input.
If you want to keep all of the elements, you could multiply the matrix by the logical matrix to make certain elements equal to zero:
>> A = randi(9,4,7)
A =
   6   9   4   5   8   7   3
   8   9   7   5   1   7   4
   2   3   2   8   6   8   8
   6   6   3   7   9   4   8
>> X = 1==mod(A,2); % detect all odd values
>> A.*X
ans =
   0   9   0   5   0   7   3
   0   9   7   5   1   7   0
   0   3   0   0   0   0   0
   0   0   3   7   9   0   0
Or you could turn them into some other value:
>> A(~X) = NaN
A =
   NaN     9   NaN     5   NaN     7     3
   NaN     9     7     5     1     7   NaN
   NaN     3   NaN   NaN   NaN   NaN   NaN
   NaN   NaN     3     7     9   NaN   NaN
But every element must have some value, and so you cannot have the output matrix having the same size unless you specify what those elements' values should be.
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