Hi there,
I've got this matrix D with, say 10 rows and 20 columns. For each row I want
to sum the first 3 non zero elements and put them in a vector z.

So if the first row D[1,] is
0 3 5 0 8 9 3 2 4 0

then I want z
z<-D[1,2]+D[1,3]+D[1,5]

But if there are less than 3 non zero elements, those should be summed. If
there are no non zero elements, the result must be zero.

So if the first row D[1,] is
0 0 3 0 1 0 0 0 0 0

then I want z
z<-D[1,3]+D[1,5]

Hope someone can help me out!

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