# I need to remove every other 9 rows of a matrix

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Joseph on 25 Jun 2013
This should be simple but its giving me some trouble. I have a matrix of 57 rows and 3 columns. This matrix is a condensed set of data from 3 data sets with 19 rows of data each. The first 9 rows of each individual data sets are unneeded and need to be removed. So in the full matrix I need to remove the first 9 rows of data, move down 10 rows, remove the next 9 rows of data, move down another 10 rows, and the remove the next 9 rows of data. Does anyone know how I can do this?

Matthew Eicholtz on 27 Jun 2013
r = 57; % total rows
c = 3; % total columns
d = 19; % rows in each data set
n = 9; % remove first n rows in each data set
A = cumsum(ones(r,c)); % example data
A(mod(1:r,d)<=n & mod(1:r,d)>0,:) = [];
Massimiliano Tondi on 12 Mar 2021
helped me as well, thanks a lot !

Andrei Bobrov on 26 Jun 2013
Edited: Andrei Bobrov on 27 Jun 2013
A = randi(100,57,5); % your data
a = 19*ones(size(A,1)/19,1);
A(bsxfun(@plus,cumsum(a) - a + 1,0:8),:) = [];
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Andrei Bobrov on 27 Jun 2013
corrected

Jan on 26 Jun 2013
A = rand(57,5);
s1 = size(A, 1);
index = zeros(1, s1);
index(1:19:s1) = 1;
index(10:19:s1) = -1;
R = A(logical(cumsum(index)), :);
(not tested)

Matt Kindig on 26 Jun 2013
Edited: Matt Kindig on 26 Jun 2013
So, to confirm, you need to do the following:
• remove rows 1-9 (9 rows)
• retain rows 10-19 (10 rows)
• remove rows 20-28 (9 rows)
• retain rows 29-38 (10 rows)
• remove rows 39-47 (9 rows)
• retain rows 48-57 (10 rows)
I think this should do it:
M = [(1:57)', rand(57,2)]; %random data for illustration
npts = size(M,1);
nremove = 9; %number of rows to remove at once
nretain = 10; %number of rows to retain at once
%which removes to remove
removeRows = cell2mat(arrayfun(@(k) k:(k+nremove-1), 1:(nremove+nretain):npts, 'UniformOutput', false));
removeRows(removeRows>npts)=[]; %ignore rows beyond size of M
M(removeRows,:) = []; %remove these rows