How to sum up in matrix

Assume matrix S as follows:

S = [41  105
41  85
41  35
88  575
88  40
77  10
77  125
77  105
78  10
78  15
78  335
99  90
99  20
99  30
12  45
12  45
12  130
];

I want to build a new matrix N including 4 columns:

N = [41  105  240  345
41  85  345  430
41  35  430  465
88  575  240  815
88  40  815  855
77  10  240  250
77  125  250  375
77  105  375  480
78  10  240  250
78  15  250  265
78  335  265  600
99  90  240  330
99  20  330  350
99  30  350  380
12  45  240  285
12  45  285  330
12  130  330  460
];

% Third column: The first column in matrix S is unique ID. When ID is unique, then the corresponding array in third column of matrix N should be 240. Other arrays in column 3 is calculated based on the sum of above arrays in column 2 and 3---> 345 = 105+240 &&&&&& 430 = 85 + 345 Finally the last column (4th) is sum up between second and third columns

[Jan]

What have you tried so far? Can e.g. the 41 appear later in the array also? How should it be treated then?

[Zhan]

No, 41 can't appear later. I solved in Excel using a binary code, 1 if ID changed, 0 when ID is repeated. Then simple sum function.

Answer 1

[Roger Stafford]

   N = [S,zeros(size(S,1),2)];
   N(1,3) = 240;
   t = 240;
   for k = 2:size(S,1);
     if N(k,1)~=N(k-1,1)
       N(k,3) = 240;
       t = 240;
     else
       t = t + N(k-1,2);
       N(k,3) = t;
     end
   end
   N(:,4) = N(:,2) + N(:,3);