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Hi guys,

how would you, if you have an arbitrary matrix, randomly delete one element in each row and column of said matrix.

So, for example, you start with a matrix A of size 5x5 and you wanna end up with B of size 4x4.

Thanks!

Edit: I tried it like this, but it results in an error:

a = magic(5);

inRow = randperm(size(a,1));

inCol = randperm(size(a,2));

a(inRow,inCol) = [];

Justace Clutter
on 12 Apr 2013

The problem with what you have tried is that you have to remove the entire column and the row and not just a single element. Try the following code instead

a = magic(5);

rowIndex = randi(size(a, 1), 1);

columnIndex = randi(size(a, 2), 1);

anew = a;

anew(rowIndex,:) = [];

anew(:,columnIndex) = [];

disp(a);

disp(anew);

The Result of this code is the following:

17 24 1 8 15

23 5 7 14 16

4 6 13 20 22

10 12 19 21 3

11 18 25 2 9

17 24 1 8

23 5 7 14

4 6 13 20

10 12 19 21

Justace Clutter
on 12 Apr 2013

Edited: Justace Clutter
on 12 Apr 2013

I have a few followup questions:

1: What do you do in the following case?

x o o x

x x x o

x x x o

Should it be:

x x x x

x x x

x

In the example you provided, it was clean, but I am not sure what to do in this case.

2: Does the order of the elements in the final matrix matter?

Cedric Wannaz
on 12 Apr 2013

Edited: Cedric Wannaz
on 12 Apr 2013

Look at the following and let me know if you have any question:

M = randi(10, 4, 5) ; % Random example.

[nr,nc] = size(M) ;

ind = sub2ind([nr,nc], randi(nr, 1, nc), 1:nc) ;

N = M(:) ;

N(ind) = [] ;

N = reshape(N, [], nc) ;

Running this gives e.g.:

>> M

M =

3 2 6 6 6

7 5 3 7 2

7 10 8 9 2

2 4 3 10 3

>> N

N =

3 2 6 7 6

7 10 3 9 2

7 4 8 10 2

PS: if you want to replace M with its reduced version, you don't need to build N; you can just have

[nr,nc] = size(M) ;

ind = sub2ind([nr,nc], randi(nr, 1, nc), 1:nc) ;

M = M(:) ;

M(ind) = [] ;

M = reshape(M, [], nc) ;

Justace Clutter
on 12 Apr 2013

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