MATLAB: Get new variable based on a condition

doubleMATLABsum

I have a double variable with 3 columns and 60000 rows, sorted by c2 and then by c1. For example:
     c1         c2      c3 
 M=[2008  1  1       
    2009  1  2
    2010  1  34
    1996  12  7
    1997  12  *11*
    1998  12  *5*
    1999  12  *5*  
    2000  12  4
    1998  13  14
    1999  13  1 ] %in this case I don't have the value for the two previous years
For each different c2, and if c1 is equal to 1999 I am trying to have a new variable P, with the year, with c2 and with the sum of the values in c3 from that year (1999 )and the two previous years/rows (1998 and 1997).
In this example my output would be:
P=[ 1999 12 21] %(5 + 5 + 11)
Thanks

Best Answer

Try this and see if it's what you want:
clc;
workspace;
M=[2008  1  1       
    2009  1  2
    2010  1  34
    1996  12  7
    1997  12  11
    1998  12  5
    1999  12  5  
    2000  12  4
    1998  13  14
    1999  13  1 ]
uniqueC2 = unique(M(:,2))
P = [0,0,0];
counter = 1;
for k = 1 : length(uniqueC2)
  % Find rows for this c2.
  thisC2 = M(:,2) == uniqueC2(k);
  % Find rows where c1 == 1999
  validRows = M(:,1) == 1999;
  % AND them
  validRows = validRows & thisC2;
  if any(validRows)
    % This c2 has at least one year with 1999
    % Find out what row it's in.
    the1999Row = find(validRows);
    % Sum this row only if there are at least two prior rows
    % with the same value of C2.
    if thisC2(the1999Row-1) && thisC2(the1999Row-2)
      % Prior 2 rows also belongs to this c2.
      theSum = M(the1999Row, 3) + M(the1999Row - 1, 3) + M(the1999Row - 2, 3);
      P(counter, :) = [1999, uniqueC2(k), theSum];
      counter = counter + 1;
    end
  end
end
% Print to command window:
P
It's easy to understand and fast. If you want something more compact, but probably harder to understand, someone will probably post a one-liner. But this gives exactly the output you asked for.
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