matlab - Sum of groups of four in a matrix -

i have following matrix: first column values of 1 5, second column 1 20, , third column random values.

1   1   2545 1   2   0 1   3   0 1   4   0 2   5   0 2   6   0 2   7   231 2   8   54587 3   9   41 3   10  1111 3   11  0 3   12  1213 4   13  0 4   14  0 4   15  0 4   16  0 5   17  898 5   18  6887 5   19  522 5   20  23     

what trying sum in groups of 4 when values different of zero. example, in matrix output want is:

1   nan 2   nan 3   nan 4   nan 5   8330 

assuming first column delineates values in third column belong group, easiest change values 0 nan, use accumarray sum of values belong each group. crucial because sum on matrix / array , any value nan, result nan. nice because if sum on each group, nan result if @ least 1 of values in group equal 0 before change.

i'm going assume matrix stored in x so:

x = [1   1   2545 1   2   0 1   3   0 1   4   0 2   5   0 2   6   0 2   7   231 2   8   54587 3   9   41 3   10  1111 3   11  0 3   12  1213 4   13  0 4   14  0 4   15  0 4   16  0 5   17  898 5   18  6887 5   19  522 5   20  23 ]; 

make copy of third column, , let's magic:

col = x(:,3); col(col == 0) = nan; out = accumarray(x(:,1), col); 

we get:

out =           nan          nan          nan          nan         8330 

the nice thing approach group id each value in matrix doesn't have in order have placed in post.

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if matrix guaranteed have order each group consists of consecutive 4-tuples of elements, can same thing nan assignment, can avoid using accumarray , reshape third column matrix of 4 rows sum on each row individually:

col = x(:,3); col(col == 0) = nan; out = sum(reshape(col, 4, []), 1);