# Problem to delete rows in my matrix

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Aude Rapet on 21 Nov 2016
Commented: Aude Rapet on 21 Nov 2016
Hi, I am a beginner in matlab and I try to delete rows from my matrix when two following values in my 3rd column are the same. I tried this :
for q=1:length(res)-1
if res(q,3)==res(q+1,3)
res(q,:) = [];
end
end
But I have the error message :
Index exceeds matrix dimensions.
if res(q,3)==res(q+1,3)
I had 4020 values at the beginning, it managed to delete 36 but then it blocked I don't know why. Can you help me? Thanks, Aude

per isakson on 21 Nov 2016
and try
for q = length(res)-1 : -1 : 1
if res(q,3)==res(q+1,3)
res(q,:) = [];
end
end
Roger Stafford on 21 Nov 2016
@Per. I think this method might have the difficulty that a row might be deleted which might have provided a match in column 3 on the next step down, and that would abort another valid deletion. That is to say, there might be three successive equal elements in column 3 but only one row deletion as a result instead of two.
@Per Correction: On second thought I think your revised method actually does work properly. Forget the above paragraph.
Aude Rapet on 21 Nov 2016

Roger Stafford on 21 Nov 2016
The problem here is that you are reducing the row count of ‘res’ whenever you get a successive pair in column 3 that match. Hence when q gets near its upper end ‘res’ is no longer as large as it was initially.
One way of proceeding would be to simply collect all the row indices that need to be deleted, and then afterwards delete them all at once:
d = [];
for q=1:length(res)-1
if res(q,3)==res(q+1,3)
d = [d,q];
end
end
res(d) = [];

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Aude Rapet on 21 Nov 2016
I wrote this and I think it works but maybe there is an easier way :
for ind = length(res)-1 : -1 : 1
if res(ind,4)== res(ind+1,4)
res_select(ind:ind+1,:) = res(ind:ind+1,:) ;
end
end
for ind = length(res_select)-1 : -1 : 1 %to delete the zeros at the beginning of my new matrix res_select
if res_select(ind,:)==0
res_select(ind,:) = [] ;
end
end
what do you think about it?
Roger Stafford on 21 Nov 2016
That would appear to violate the rule that you originally stated: “delete rows from my matrix when two following values in my 3rd column are the same”. Can you carefully restate your criterion for deleting rows so as to include the case you mentioned here?
Aude Rapet on 21 Nov 2016
Yes, what I want to do is to select "paired" numbers by my function. So I could either
1. delete rows from my matrix when two following values in my 3rd column are the same : that means having 1 2 1 2 etc. alternatively in my 3rd column
2. either keep rows only when two following values in my 4rd (4!!) column are the same, and delete the others rows. With both ways I have the same selected rows at the end.And I think the second option is better, because with the first one I lose the end of my datas...