delete some matrix elements

I want to delete the elements that are larger than a certain value, then the next element will replace the deleted one(shift forward). For example,

[1 2 9;
3 1 4;
2 9 7]

, delete those larger than 4, the output will be

[1 2 3;
1 4 2]

If the no. of elements is not the multiple of 3, the remaining ones will be deleted. Could anyone give me some ideas? Thanks everyone!

[Image Analyst]

Matrices have to remain rectangular. What would you want the answer to be if you deleted elements greater than 8 so just the 2 9's get removed? You can't have a matrix with 3 elements in the first row and second row and just one element in the last row. I.e. you can't have

[1 2 3
 1 4 2
 7    ]

[Patrick]

Sorry, the remaining ones will be deleted. How to do that? Thanks dude!

Answer 1

[Guillaume]

The way to delete elements of an array that fulfill a given condition such as greater than is with:

 A(A > value) = [];

So in your case:

 A(A > 4) = [];

Note that with a matrix, it will flatten it into a column vector, as there's no guarantee it deletes enough element to keep the same number of columns (or rows, or whatever dimension you want to stay the same).

For example, what matrix would you want if you started with

 A = [1 2 9; 3 1 4; 2 2 7];

If you know that the number of elements deleted is always going to produce a valid rectangular matrix with the same number of columns as you started with, the following would work:

 ncol = size(A, 2);
 A(A > 4) = [];
 A = reshape(A, [], ncol); %will error if the number of elements remaining in A is not a multiple of ncol

[Patrick]

Thanks @Guillaume for your detailed reply! If the no. of elements is not the multiple of 3, the remaining ones will be deleted. How can I do that?

[Guillaume]

It's the same syntax, but you give the index of the elements you want to delete directly. In that case it's the remainder (or modulo) of the number of elements after your deletion divided by the number of columns, counted from the end, thus:

 ncol = size(A, 2);
 A(A > 4) = []; %remove all elements greater than 4
 todelete = mod(numel(A), ncol); %surplus to delete
 A(end-todelete:end) = [];
 A = reshape(A, [], ncol); %now guaranteed to succeed

[Patrick]

Thank you so much!

Answer 2

[Jeremy Kemmerer]

Hi Patrick,

A straightforward way to find and alter matrix elements with a certain property is logical indexing. In your case, if we call your matrix A, you could try something like:

>>idx = A>4; //create logical array  

>>A(idx) = []; //remove array elements greater than 4

This will produce a row vector of the remaining elements, which you can reform into a matrix using the “reshape” command.

Please refer to the following documentation for more information on the “reshape” function: http://www.mathworks.com/help/matlab/ref/reshape.html

Also, there is a blog topic on MATLAB Central which provides more information on logical indexing: http://blogs.mathworks.com/loren/2013/02/20/logical-indexing-multiple-conditions/

[Patrick]

Thank you Jeremy for your answer and the relevant materials!