Find and delete a double value in matrix

I have several matrices with Double values. I want to delete(make null) n numbers. For example n=300 and I want to delete 300 values randomly. Here is a part of my code. I'm not sure if it's correct and how to continue it.

totalsize=size(matrixname,1)*size(matrixname,2);
n=totalsize*PER; %number of elements which values should be deleted
r = randi([1 totalsize],1,pen); %find n random elements to be deleted

Answer 1

[James Tursa]

Use randperm, not randi. The randperm function will ensure that you do not get duplicate indexes to delete, whereas using randi might produce duplicate indexes. Also, I am assuming PER is a percentage number, so you should use round or floor or ceil on the result of totalsize*PER to ensure it is an integer for downstream processing.

[Rihanna Waterson]

Thank you. And how to find and delete value of each nth element in a matrix? I know I cannot use find() because of decimal numbers.

Answer 2

[Jan]

totalsize = numel(matrixname);
n         = round(totalsize*PER);
index     = randperm(totalsize, n);  % n eandom distinct values from 1:totalsize
matrixname(index) = 0;

To delete each n.th element:

matrixname(1:n:totalsize) = 0;

[Rihanna Waterson]

Thanks. But they shouldn't be zero...because it will affect image reconstruction. I should delete them so they won't be used in image reconstruction. I've written this part but it seems the last line is not correct.

                  PER=0.0001;
                  totalsize=size(y_blk,1)*size(y_blk,2);
                  pen=floor(totalsize*PER);
                  r = randperm (totalsize,pen);
                  for l=0:pen
                     rem=rem(r,size(y_blk,1)); %row
                     div=floor(r/size(y_blk,1))+1; %column
                     y_blk(rem,div)=[];                   
                  end

[Jan]

@Rihanna: The term "delete(make null)" was not clear to me.

Please explain "it seems the last line is not correct" with details. I cannot run your code to check, what's going on. Note that deleting elements in a matrix might conflict with the definition of a matrix, that all columns have the same number of rows and vice versa.