Blockwise sum of matrix elements

Question

I want to go from something like this:

1> a = matrix(c(1,4,2,5,2,5,2,1,4,4,3,2,1,6,7,4),4)
1> a
     [,1] [,2] [,3] [,4]
[1,]    1    2    4    1
[2,]          


        

Answer 1

I guess that might help you, but still it uses sapply which can be considered as loop-ish tool.

a <- matrix(c(1,4,2,5,2,5,2,1,4,4,3,2,1,6,7,4),4)
block.step <- 2
res <- sapply(seq(1, nrow(a), by=block.step), function(x) 
    sapply(seq(1, nrow(a), by=block.step), function(y) 
        sum(a[x:(x+block.step-1), y:(y+block.step-1)])
    )
)
res

Is it anyhow helpful ?