Asked by vikash kumar
on 21 Aug 2019

I want to manipulate n*n matrix such the last 10 non-zeros value in every column of matrix and change them into zero.Matrix is in the form of binary[0,1].

Answer by Andrei Bobrov
on 21 Aug 2019

Accepted Answer

Let A - your binary array (n x n)

m = 10;

B = cumsum(A);

out = A.*(B(end,:) - m >= B);

Answer by John D'Errico
on 21 Aug 2019

Edited by John D'Errico
on 21 Aug 2019

You want to reset the last 10 non-zeros to zero, in each column. So, let me build an example matrix.

A = rand(20,5) > 0.25

A =

20×5 logical array

1 0 0 1 1

1 1 1 1 1

1 1 0 1 1

1 1 1 0 1

0 1 1 1 1

1 1 1 1 1

1 1 0 1 0

1 1 0 1 1

1 1 1 1 1

0 0 1 1 0

0 0 1 0 1

0 0 1 1 1

1 1 0 0 0

0 1 0 1 1

1 1 1 1 1

1 0 0 1 0

1 1 1 0 1

1 0 1 1 1

1 1 1 0 0

1 1 1 1 1

In this kind of boring example, there should be roughly 15 non-zeros in each column. But you want to kill off only the last 10 in each column.

The trivial way to do this is to use a loop. It is always a good idea to think of a SIMPLE way to solve a problem, and only then, if it is a problem, then worry about finding a better solution.

B = A;

for i = 1:size(A,2)

B(find(A(:,i),10,'last'),i) = 0;

end

We can verify that it worked, simply enough.

sum(A,1)

ans =

15 14 13 15 15

sum(B,1)

ans =

5 4 3 5 5

B

B =

20×5 logical array

1 0 0 1 1

1 1 1 1 1

1 1 0 1 1

1 1 1 0 1

0 1 1 1 1

1 0 0 1 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

It is clear the loop worked as expected. It is simple, and sufficient to solve the problem, and not even that inefficient. The code is concise, and easy to understand. It will run in almost any version of MATLAB since it only needs the 'last' option for find, which was introduced into MATLAB, and that must have been well over 20 years since that happened.

If there were issues with time (and ONLY then) I would then consider more sophisticated ideas. Do you have many thousands, or even millions of columns to work on? Will this code run millions of times? If not, then don't bother looking for something more efficient.

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Answer by KSSV
on 21 Aug 2019

If A is your matrix. To get non-zero indices use:

idx = A>0 ;

To replace them to zero use:

A(idx) = 0 ;

John D'Errico
on 21 Aug 2019

KSSV
on 21 Aug 2019

This one?

idx = A>0 ;

A(idx) = 0 ;

madhan ravi
on 21 Aug 2019

KSSV no, if for instance , last two non zeros in each column to zero:

>> A

A =

4×2 logical array

0 0

1 1

0 1

1 1

>> Expected_result

Expected_result =

4×2 logical array

0 0

0 1

0 0

0 0

>>

Clear now?

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## madhan ravi (view profile)

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https://es.mathworks.com/matlabcentral/answers/476979-how-to-change-non-zero-value-into-0#comment_737430

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