R Multiply second dimension of 3D Array by a Vector for each of the 3rd dimension

Question

When trying to multiply the first dimension of an array by each index of a vector by the second dimension, my array is converted to a matrix and things get squirrelly. I can only do the proper multiplication long-hand.

What a mouth full...

It's easier to explain with code...

Arr <- array(runif(10*5*3), dim = c(10,5,3))
dim(Arr)
Vect <- c(1:5)

Arr[,1,1] <- Arr[,1,1]*Vect[1]
Arr[,1,2] <- Arr[,1,2]*Vect[1]
Arr[,1,3] <- Arr[,1,3]*Vect[1]

Arr[,2,1] <- Arr[,2,1]*Vect[2]
Arr[,2,2] <- Arr[,2,2]*Vect[2]
Arr[,2,3] <- Arr[,2,3]*Vect[2]

Arr[,3,1] <- Arr[,3,1]*Vect[3]
Arr[,3,2] <- Arr[,3,2]*Vect[3]
Arr[,3,3] <- Arr[,3,3]*Vect[3]

Arr[,4,1] <- Arr[,4,1]*Vect[4]
Arr[,4,2] <- Arr[,4,2]*Vect[4]
Arr[,4,3] <- Arr[,4,3]*Vect[4]

Arr[,5,1] <- Arr[,5,1]*Vect[5]
Arr[,5,2] <- Arr[,5,2]*Vect[5]
Arr[,5,3] <- Arr[,5,3]*Vect[5]

How do I clean this up to be one command?

Answer 1

We can also replicate the 'Vect' and multiply with 'Arr'. The col is a convenient function that gives the numeric index of columns.

res1 <- Arr * Vect[col(Arr[,,1])]

Or we explicitly do the rep

res2 <- Arr* rep(Vect, each=dim(Arr)[1])
identical(res1, res2)
#[1] TRUE

Answer 2

Cast Vect into an array first, then element multiply:

varr <- aperm(array(Vect, dim = c(5L, 10L, 3L)), perm = c(2L, 1L, 3L))

Arr <- varr * Arr

(of course we don't need to store varr if you want this in one command)

(also, turns out this is basically what sweep does under the hood...)

Answer 3

The aaply() function from the plyr package does exactly what you're looking for. It can operate on arrays of any dimension and split them however you like. In this case you're splitting by rows so:

library(plyr)
Arr2 <- aaply(Arr, 1, function(x,y){x*y}, Vect)

Answer 4

Try:

sweep(Arr,2,Vect,FUN="*")