Given an element of a list, how do I recover its index inside the list?

Question

My problem is basically this:

I have a list, L, where each element is a matrix of the same dimension. I need to multiply each matrix inside this list by its corresponding element in an outside vector h, and then sum all the matrices.

set.seed(101)
L <- replicate(3, matrix(rnorm(4), 2), simplify = FALSE)
h <- 2:4

# I need this
L[[1]] * h[1] + L[[2]] * h[2] + L[[3]] * h[3]

Given that I need to experiment with a different number of matrices, and I have a bunch of them, I've got to do it in a smart way. My idea was

L1 <- lapply(L, function(x) x * h[x])
L2 <- Reduce('+', L1)

Where "h[x]" would be indexing the vector h by the index of the matrix x inside the list L, so I would get

 L1 = list(L[[1]] * h[1], L[[2]] * h[2], L[[3]] * h[3])

So, the question is, how to I get the index of an element in a list by using the element itself? Something like h[L[[m1]]] to get h[1].

Or, if you got any other way of solving my problem, how do I do it?

Answer 1

I think you're looking for mapply()/Map() (Map is slightly easier here because it doesn't try to simplify the results):

?Map:

‘Map’ applies a function to the corresponding elements of given vectors ... ‘Map’ is a simple wrapper to ‘mapply’ which does not attempt to simplify the result ...

?mapply:

‘mapply’ applies ‘FUN’ to the first elements of each ... argument, the second elements, the third elements, and so on

Set up example:

set.seed(101)
L <- replicate(3,matrix(rnorm(4),2),simplify=FALSE)
h <- 2:4

Do it:

Reduce("+",Map("*",L,h))