CODEWITHSUNDEEP
rmatrixvectorizationlinear-algebra
Tamara
Tamara

Reputation: 11

Cumulative sum of matrix elements

I have a matrix defined as the pair-wise differences between the elements of an array:

a <- as.matrix(dist(c(1,2,3,4,5)))

I need to compute without looping the sum of the pair-wise differences between the first two elements, the first three elements, etc. i.e., I need to arrive to the array:

v <- c(1,4,10,20)

Upvotes: 1

Views: 507

Answers (2)

daroczig
daroczig

Reputation: 28632

I don't know if you indeed want the to call the cumulative sum function twice, as I think "the sum of the pair-wise differences between the first two elements, the first three elements, etc." should result in:

c(1, 3, 6, 10)

Anyway, this should work with non-sequential x as well for your required output:

> cumsumdiff <- function (x) cumsum(cumsum(sapply(x[-1], `-`, x[1])))
> cumsumdiff(1:5)
[1]  1  4 10 20

Or based on @Jota's suggestion using the distance matrix:

> cumsumdiff <- function(x) cumsum(cumsum(unname(as.matrix(dist(x))[1, -1])))
> cumsumdiff(1:5)
[1]  1  4 10 20

Upvotes: 2

akrun
akrun

Reputation: 887213

Try

 head(cumsum(cumsum(1:5)),-1)
 #[1]  1  4 10 20

Upvotes: 2

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