# Removing rows in a matrix

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hamzah khan on 19 Oct 2020
Commented: Ameer Hamza on 20 Oct 2020
Hello,
I have this matrix, called A matrix. I am attaching the file with the question. Also the picture of some of its data values
I need to remove some of its rows in a loop. I want to keep rows 1,2 then remove 3,4,5,6, keep 7,8,then remove 9,10,11,12, keep 13,14.. like this it goes on.keep 2 remove 4 rows.
Does anyone know?

Ameer Hamza on 19 Oct 2020
Try this
n = size(Identify_3);
idx = (mod((1:n)-1, 6)+1) <= 2;
Identify_3 = Identify_3(idx, :);

hamzah khan on 19 Oct 2020
Thanks :)
hamzah khan on 19 Oct 2020
Hi,
How can i edit your code to keep the rows 1.48,96,144,192 and so on, basically after the first row, we take 48th row and then double 49+48=96 and so on...
Also if I want to change your code for the original matrix, what should I change?
Ameer Hamza on 20 Oct 2020
You can do it like this
D = C([1 48:48:end], :)

Bjorn Gustavsson on 19 Oct 2020
Edited: Bjorn Gustavsson on 19 Oct 2020
If you can determine/calculate all rows you want to remove at once it is best to do the re-sizing of the array at once, instead of row-by-row (if I've understood this right, the reallocation-operation is the main argument to not automatically and incrementally grow arrays, ought to be similar for shrinking.):
idx2remove = [];
for i1 = 1:size(A,1)
if some_conds(A(i1,:))
idx2remove = [idx2remove,i1];
end
end
A(idx2remove,:) = [];
Ah, if you have to remove rows [3,4,5,6]+6*n then something like this should work:
idx2remove = 1:size(A,1);
idx2remove = reshape(idx2remove,6,[]);
idx2remove = idx2remove(3:end,:);
A(idx2remove(:),:) = [];
If this doesn't work out because your A doesn't have a multiple of 6 rows, then you'll have to modify the creation of the remove-array a bit. You could also go for creating an array with row-indices to keep and do:
A = A(idx2keep,:);
HTH

#### 1 Comment

hamzah khan on 19 Oct 2020
Thanks :)

KSSV on 19 Oct 2020
Let A be your data. It seems every four rows have real value of complex number very less. We can use keep these rows.
idx = real(abs(A(:,1)))<=10^-10 ; % this will give indices of what you want to keep
iwant = A(idx,:)

#### 1 Comment

hamzah khan on 19 Oct 2020
Thanks :)

R2019a

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