rowsum

Consider the array a : > a <- array(c(1:9, 1:9), c(3,3,2)) > a , , 1 [,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9 , , 2 [,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9 How do we efficiently compute the row sums of the matrices indexed by the third dimension, such that the result is: [,1] [,2] [1,] 12 12 [2,] 15 15 [3,] 18 18 ?? The column sums are easy via the 'dims' argument of colSums() : > colSums(a, dims = 1) but I cannot find a way to use rowSums() on the array to achieve the desired result, as it has a different interpretation of 'dims' to that of colSums() . It is simple to compute

So I am trying to sum the rows of a matrix, and there are inf's within it. How do I sum the row, omitting the inf's? Multiply your matrix by the result of is.finite(m) and call rowSums on the product with na.rm=TRUE . This works because Inf*0 is NaN . m <- matrix(c(1:3,Inf,4,Inf,5:6),4,2) rowSums(m*is.finite(m),na.rm=TRUE) A[is.infinite(A)]<-NA rowSums(A,na.rm=TRUE) Some benchmarking for comparison: library(microbenchmark) rowSumsMethod<-function(A){ A[is.infinite(A)]<-NA rowSums(A,na.rm=TRUE) } applyMethod<-function(A){ apply( A , 1 , function(x){ sum(x[!is.infinite(x)])}) } rowSumsMethod2<