Remove some elements from a matrix but conserve the same struct

Question

Rafael Freire 2011-3-29

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i have the matrix

   x=[0 -1 0  5  10  5  0  0;
0  5  10 20  10 5  0;
-1 0  5  10  5  2  0;
2  1  0  5   2  1  0;
0  0  -10 0  0  0  0;]

i would like to "disappear" the values between -1 and 5 and keep the matrix with same struct but only the wanted values. the new matrix going to be something like this:

x=

   [           10          ;
            10 20 10       ;  
               10          ;
                           ;
          -10              ;]

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Answer 1

Matt Fig 2011-3-29

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You will have to use cell arrays if you want to preserve the shape:

X = num2cell(x);  % A cell array with the same shape as x.
X(x<=5 & x>=-1) = {[]}

Is there a practical reason for doing this? It seems like a large waste of memory to me. If you need to keep the indices viewable, you also could do this:

X = x;
X(x<=5 & x>=-1) = 0;
X = sparse(X)

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Rafael Freire 2011-3-29

i have a surface [1023x1023 double] and i want to keep only the peaks that is above and under the values to reduce the number of points, after do all of thar i want to mesh the answer and thaks for you answer

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Answer 2

Sean de Wolski 2011-3-29

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Rafael, then set everything in the range you described to NaN, and mesh will pretend they're not there.

X(X>=-1&X<=5) = nan;
mesh(X);

You can reduce the size of the matrix first by zeroing out any rows/columns that contain no "peaks". Use the ANY function and the dimensional option to do this.