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I need to remove every other 9 rows of a matrix

Asked by Joseph
on 25 Jun 2013
Latest activity Commented on by ADITHYAN
on 22 Apr 2015

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?


4 Answers

Answer by Matthew Eicholtz on 27 Jun 2013
 Accepted answer

How about this?

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,:) = [];


Great, I can easily modify this for any data size. Thank you. Thanks to everybody for their help.

Very elegant answer. Thanks a lot, helped me as well.

Answer by Andrei Bobrov
on 26 Jun 2013
Edited by 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),:) = [];


Thank you for your help. This works great for my data set of 57 rows but what if I had an increased data size.So for example say I had a data set of 190 rows or even more. How would I change this to accommodate for additional data? I attempted to run the code for a 190 row set and it didn't quite work.

Answer by Jan Simon
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)


Answer by Matt Kindig
on 26 Jun 2013
Edited by 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


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