Problem to delete rows in my matrix

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

[per isakson]

See http://uk.mathworks.com/matlabcentral/answers/312599-how-do-i-parse-this-complex-text-file-with-textscan#comment_407704

and try

for q = length(res)-1 : -1 : 1
    if res(q,3)==res(q+1,3)
        res(q,:) = [];
    end    
end

[Roger Stafford]

@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]

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

Answer 1

[Roger Stafford]

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) = [];

[Aude Rapet]

Ok thank you very much Roger!

The problem I have with my loop is also that it deletes the data I want at the end. I have this :

1592,61638027600  292,462164782916  1  2028
1591,36484205925  292,162090444115  2  2028
1593,62808256023  1584,64135103072  1  2029
1587,97242347533  1584,32241069947  2  2029
1594,59566486772  1371,42195936310  1  2030
1589,67204481414  1371,12117544568  2  2030
595,370627883948  566,978732597834  2  2031
964,731265222265  1166,89283712761  2  2032
1073,40350931937  850,071709928442  2  2033

and I want to keep only the one "paired" with 1-2 in column 3, but with my loop it deletes the last "pair" :

1592,61638027600  292,462164782916  1  2028
1591,36484205925  292,162090444115  2  2028
1593,62808256023  1584,64135103072  1  2029
1587,97242347533  1584,32241069947  2  2029
1594,59566486772  1371,42195936310  1  2030
1073,40350931937  850,071709928442  2  2033

Do you have an idea of how I could keep my "pair" 2030 (column 4), and delete 2031...2033?

Thank you for your reply!! Aude

[Aude Rapet]

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]

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]

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...