MATLAB: Sum of rows with whose row numbers are specified by another array

adding rowssumsumming

I have an array of the form:
TTX =
    20     9     2     7
    40    10    10     1
    60     2    10     9
     0    10     5    10
    40     7     9     7
    40     1     2     8
     0     3     5     8
    60     6    10     4
    20    10     8     7
     0    10    10     2
I want to add the elements of rows 1:3, then add the elements of the rows 4:5, then 6:end.
How can I do that?

Best Answer

From R2015b you can use splitapply:
>> TTX = [...
    20     9     2     7
    40    10    10     1
    60     2    10     9
     0    10     5    10
    40     7     9     7
    40     1     2     8
     0     3     5     8
    60     6    10     4
    20    10     8     7
     0    10    10     2];
>> [U,~,X] = unique(TTX(:,1));
>> Y = splitapply(@(x)sum(x,1),TTX(:,2:4),X);
>> M = [U,Y]
M =
     0    23    20    20
    20    19    10    14
    40    18    21    16
    60     8    20    13
EDIT: this matches the original question's output explanation and example.
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