MATLAB: How do cumulatively add a matrix column based on a vector

cumulatively addmatrix manipulationpatternvector index

Hello all, I have the following matrix,
t = 
  1.0000    7.0000         0         0
  2.0000    9.0000    0.6000    3.0000
  3.0000   11.0000   -0.5000   -0.5000
  4.0000   13.0000    0.6000    2.4000
  5.0000   14.0000   -0.7000   -2.5000
  6.0000   18.0000    0.7000    2.4000
  7.0000   19.0000   -0.2000    1.0000
  8.0000   20.0000         0    1.0000
  9.0000   22.0000    0.7000    2.1000
 10.0000   23.0000   -0.1000    1.4000
 11.0000   24.0000         0    1.4000
 12.0000   25.0000    0.2000    0.4000
 13.0000   26.0000         0    0.4000
 14.0000   28.0000    0.5000    0.5000
 15.0000   29.0000   -0.2000   -0.9000
 16.0000   30.0000    0.2000    0.5000
 17.0000   31.0000   -0.1000   -0.2000
 18.0000   32.0000    0.3000         0
 19.0000   33.0000   -0.3000   -2.1000
 20.0000   34.0000    0.2000   -0.7000
 21.0000   35.0000   -0.1000   -1.4000
 22.0000   39.0000         0   -1.4000
 23.0000   42.0000   -0.3000   -3.5000
 24.0000   44.0000    0.3000   -1.4000
 25.0000   45.0000   -0.2000   -2.8000
& a vector m, corresponding to column 1 of t, as
   m = [1 2 4 9 12 14 18];
So, I need to cumulatively add the values in column 3 of matrix t from
   0 (1+1 : 2-1), that is m(1)+1 : m(2)-1
   3 (2+1 : 4-1)
   5, 6, 7, 8 (4+1 : 9-1)
   10, 11 (9+1 : 12-1)
   13 (12+1 : 14-1)
   15, 16, 17 (14+1 : 18-1)
   19 to 25 (18+1 : end)
& put these in column 4. Right now it has incorrect values. The starting points should all be 0. column 3 (m(n)) = 0, meaning m(1) = 0; m(2) = 0, m(4) = 0, etc.
My method with loops produces a lot of overlap & doesn't quite work. Hope this is not too confusing.
Any suggestions? Thanks Damo Nair

Best Answer

Try this:
t = [...
  1.0000    7.0000         0         0
  2.0000    9.0000    0.6000    3.0000
  3.0000   11.0000   -0.5000   -0.5000
  4.0000   13.0000    0.6000    2.4000
  5.0000   14.0000   -0.7000   -2.5000
  6.0000   18.0000    0.7000    2.4000
  7.0000   19.0000   -0.2000    1.0000
  8.0000   20.0000         0    1.0000
  9.0000   22.0000    0.7000    2.1000
  10.0000   23.0000   -0.1000    1.4000
  11.0000   24.0000         0    1.4000
  12.0000   25.0000    0.2000    0.4000
  13.0000   26.0000         0    0.4000
  14.0000   28.0000    0.5000    0.5000
  15.0000   29.0000   -0.2000   -0.9000
  16.0000   30.0000    0.2000    0.5000
  17.0000   31.0000   -0.1000   -0.2000
  18.0000   32.0000    0.3000         0
  19.0000   33.0000   -0.3000   -2.1000
  20.0000   34.0000    0.2000   -0.7000
  21.0000   35.0000   -0.1000   -1.4000
  22.0000   39.0000         0   -1.4000
  23.0000   42.0000   -0.3000   -3.5000
  24.0000   44.0000    0.3000   -1.4000
  25.0000   45.0000   -0.2000   -2.8000];
% Extract column 1 of t.
column1 = t(:, 1);
% & a vector m, corresponding to column 1 of t, defined as
m = [1 2 4 9 12 14 18];
for k = 1 : length(m) - 1
  index1 = m(k) + 1;
  index2 = m(k + 1) - 1;
  theSum(k) = sum(column1(index1:index2));
  fprintf('Summing column1 from index %d to %d and getting %.1f.\n', index1, index2, theSum(k));
end
You get:
Summing column1 from index 2 to 1 and getting 0.0.
Summing column1 from index 3 to 3 and getting 3.0.
Summing column1 from index 5 to 8 and getting 26.0.
Summing column1 from index 10 to 11 and getting 21.0.
Summing column1 from index 13 to 13 and getting 13.0.
Summing column1 from index 15 to 17 and getting 48.0.
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