Randomly deleting a 'one' in a column of a binary matrix

Hi,

I have a matrix that looks like this:

     1     1     1     1     1     1
     0     1     1     1     1     1
     0     0     1     1     1     1
     0     0     1     1     1     1
     0     0     0     1     1     1
     0     0     0     1     1     1
     0     0     0     0     1     1
     0     0     0     0     1     1
     0     0     0     0     0     1
     0     0     0     0     0     1

What should I do when I randomly want to delete a 'one' in (let's say) column 4? Thanks in advance,

Tim

Answer 1

[Wayne King]

If you want to just choose one of the 1's from the matrix to set equal to 0, then

    indices = find(A>0);
    chooseset = randperm(length(indices));
    index = chooseset(1);
    A(indices(index)) = 0;

If you really want to restrict it to just column 4

      indices = find(A(:,4)>0);
      chooseset = randperm(length(indices));
      index = indices(chooseset(1));
      A(index,4) = 0;

Recently randperm() has been updated so you can just get 1 value back.

      indices = find(A(:,4)>0);
      choice = randperm(length(indices),1);
      index = indices(choice);
      A(index,4) = 0;

[Tim]

Thanks!

Answer 2

[Azzi Abdelmalek]

 A=[1     1     1     1     1     1
     0     1     1     1     1     1
     0     0     1     1     1     1
     0     0     1     1     1     1
     0     0     0     1     1     1
     0     0     0     1     1     1
     0     0     0     0     1     1
     0     0     0     0     1     1
     0     0     0     0     0     1
     0     0     0     0     0     1]
m=size(A,2)
idx=randi(m,1)
A(:,idx)=[]  % this will remove column number idx

%I 'm not sur what you mean by delete a one, if you want raplace them by 0

 A(:,idx)=0

[Tim]

Thank you for your quick response! However, if I'm correct this will delete a whole column, instead of just a '1' in column 4. What I mean is that I want to randomly delete one of the six '1's in column 4.

Cheers, T.

[Wayne King]

My answer below will change just one of the 1's to a 0.

[Azzi Abdelmalek]

Tim you did'nt read all the answer, at the end I said use

A(:,idx)=0

istead of

A(:,idx)=[]

which means use the below code

m=size(A,2)
idx=randi(m,1)
A(:,idx)=0

[Tim]

Hi Azzi,

You're right, I did not read that part of the answer. This will however replace one of the columns with only zeros instead of take one of the '1's from a column. Wayne already helped me with that, but thanks again for your help!

Cheers, T.