CODEWITHSUNDEEP
r
AnilGoyal
AnilGoyal

Reputation: 26218

How to do some operation in the elements of a vector with previous elements

This may have been a very basic question, but I am scratching my head ..

Suppose I have a vector v with 10 elements

v <- 1:10
> v
 [1]  1  2  3  4  5  6  7  8  9 10

Now I want to perform some operation (2 argument function) say + on its elements with previous elements, to get an output like.. 1+1 1+2 2+3 3+4.. and so on. For the first element where no previous value is there, I'll take first value only. I can perform this operation by manually creating another vector something like c(v[1], v[-length(v)]), but I think/presume there may be some direct method/in-built function to do so.

> v + c(v[1], v[-length(v)])
 [1]  2  3  5  7  9 11 13 15 17 19

#OR if product is the operation

> v * c(v[1], v[-length(v)])
 [1]  1  2  6 12 20 30 42 56 72 90

Please guide

Upvotes: 1

Views: 320

Answers (2)

jay.sf
jay.sf

Reputation: 72593

Perhaps you may want to try mapply.

v <- 1:10

init <- v[1]

mapply(`+`, c(init, v[-length(v)]), v)
#  [1]  2  3  5  7  9 11 13 15 17 19

mapply(`*`, c(init, v[-length(v)]), v)
#  [1]  1  2  6 12 20 30 42 56 72 90

mapply(`^`, c(init, v[-length(v)]), v)
# [1]  1  1  8  81  1024  15625  279936  5764801  134217728  3486784401

Upvotes: 1

Ronak Shah
Ronak Shah

Reputation: 388797

This can be achieved with dplyr::lag or data.table::shift, although I am not sure if this is what you were looking for.

v + dplyr::lag(v, default = v[1])
#[1]  2  3  5  7  9 11 13 15 17 19

Upvotes: 1

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