Problem to delete rows in my matrix

Question

Aude Rapet 2016-11-21

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评论： Aude Rapet 2016-11-21

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.
Error in trackthebeads (line 8)
    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

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Roger Stafford 2016-11-21

编辑：Roger Stafford 2016-11-21

@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 2016-11-21

Thank you for your reply Per! It helps me a lot!

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Answer 1

Roger Stafford 2016-11-21

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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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Roger Stafford 2016-11-21

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 2016-11-21

Yes, what I want to do is to select "paired" numbers by my function. So I could either

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
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...

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