Find and delete a double value in matrix

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Rihanna Waterson
Rihanna Waterson 2017-2-24
评论： Jan 2017-2-27
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

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回答（2 个）

James Tursa
James Tursa 2017-2-24
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.
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Rihanna Waterson
Rihanna Waterson 2017-2-24
编辑：Rihanna Waterson 2017-2-24
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.

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Jan
Jan 2017-2-24
编辑：Jan 2017-2-24
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;
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Rihanna Waterson
Rihanna Waterson 2017-2-24
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
Jan 2017-2-27
@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.

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