Add data of a matrix based on an other matrix

I would like to add a certain amount of data points of one matrix in a new matrix, based on a third matrix. It is because I need to add data points from one day (it is not predefined how many datapoints there are going to be) and then go to the next day and do the same. This is the code I tried for now.

if true
  % code
A = [1 1 1 2 2 2 3 4 5 6 6 6 7 8 9 10]; % Matrix that tells from which day data point of B is
B = rand(1,16).*100; % random data times 100 to get bigger numbers
i = 1; % variable used in the while loop
j = 1; % variable used in the while loop
C = []; % new matrix to store the summed data points for each day (it should become a 1x10 matrix in this specific case

while A(i) == i
   while A(i) == i
       C(1,i) = C(1,i) + B(1,j)
       j = j + 1;
   end
   i = i + 1;     
 end
end

Hoping to find an answer. - Philipp

Answer 1

[Stephen23]

Loops are not required. One easy way would be to use accumarray:

>> A = [1 1 1 2 2 2 3 4 5 6 6 6 7 8 9 10];
>> B = rand(1,16);
>> C = accumarray(A(:),B(:))
C =
   1.090191
   1.127991
   0.044079
   0.483190
   0.890956
   0.136439
   0.025175
   0.364094
   0.165231
   0.917592
>> size(C)
ans =
   10    1

For MATLAB releases >= R2015b you could use splitapply.

[Philipp Wirtz]

Thank you so much for the fast answer! That was just what I was looking for.

Answer 2

[Ameer Hamza]

splitapply(@sum, B, A)

Answer 3

[Cathal Cunningham]

Use logical indexing to do this

% Pre-build array
C = zeros(1,size(unique(A),2));
for i = 1:length(C)
    C(i) = sum(B(A==i));
end