CODEWITHSUNDEEP
r
Yvonne Dong
Yvonne Dong

Reputation: 31

How to make rows in a data frame accumulative from right to left

I have a matrix like 

> A= [ 1 2 4
>      2 3 1
>      3 1 2 ]

and I would like to have result accumulative by each row from right to left like:

> A= [ 7 6 4
>      6 4 1
>      6 3 2 ]

I tried to use the solution in Cumulative sum in a matrix but it gives a different way of adding the matrix.

Upvotes: 2

Views: 116

Answers (2)

James
James

Reputation: 66844

You mentioned data.frame in the title, and there is a way of using Reduce and rev with a data.frame to get the result you want.

A <- data.frame(c(1,2,3),c(2,3,1),c(4,1,2))
data.frame(rev(Reduce("+",rev(A),accumulate=TRUE)))
  c.7..6..6. c.6..4..3. c.4..1..2.
1          7          6          4
2          6          4          1
3          6          3          2

Upvotes: 0

Ben Bolker
Ben Bolker

Reputation: 226192

A little bit magical, but:

t(apply(A,1,function(x) rev(cumsum(rev(x)))))

does it.

  • apply(A, 1, FUN) applies a function to each row of the matrix.
  • rev() reverses the vector and cumsum() computes the cumulative sum: we need rev(cumsum(rev(x))) to get the ordering you want
  • the final t() is necessary to transpose the results because apply always returns its results column-wise, as (very obscurely) noted here (from the "Value" section of ?apply):

If each call to ‘FUN’ returns a vector of length ‘n’, then ‘apply’ returns an array of dimension ‘c(n, dim(X)[MARGIN])’ if ‘n > 1’

Upvotes: 5

Related Questions