Delete rows at intervals

Hi, I have a very large matrix, and I would like to delete rows at specific intervals.

For example, I have the following matrix : [ 1 2 ; 3 4 ; 5 6 ; 7 8; 9 10; 11 12; 13 14; 15 16 ;17 18; 19 20; 21 22 ; 23 24]

I would like to delete every fourth row so the matrix looks like: [ 1 2; 3 4 ; 5 6; 9 10 ; 11 12; 13 14; 17 18; 19 20; 21 22 ]

Any help would be greatly appreciated.

Thanks,

Seamus

[Cristobal Silva]

You could select individually which rows you want, like

   A = [ 1 2 ; 3 4 ; 5 6 ; 7 8; 9 10; 11 12; 13 14; 15 16 ;17 18; 19 20; 21 22 ; 23 24]
   B = A([1:3,5:7,9:11],:)

Of course, for large data this would be impractical, so you can use a for loop for such task combined with mod operator and vertical concatenation

A = [ 1 2 ; 3 4 ; 5 6 ; 7 8; 9 10; 11 12; 13 14; 15 16 ;17 18; 19 20; 21 22 ; 23 24];
B = zeros(0,2);
for i=1:length(A)
  if (mod(i,4) ~= 0)
      B = vertcat(B,A(i,:));
  end
end

In other words, we create an empty 0-by-2 matrix (because A uses 2 cols per row) and then we just push to that matrix each row that we want to. In this case, we push every row that is not a multiple of 4 (using the mod function).

Answer 1

[Walter Roberson]

A(4:4:end,:) = [];

[Seamus]

Hi Walter, thanks very much for this. I may be incorrect but I think this extracts out every fourth row from the original matrix. In this instance I'm just looking to delete every fourth row, and leave the remainder.

[Walter Roberson]

No, extracting every 4th row would be

every4thRow = A(4:4:end,:)

on the right hand side. When you assign [] to an array element, that means to delete the element.

[Seamus]

My apologies, I read it wrong. That's perfect, thank you very much.