CODEWITHSUNDEEP
r
BrTarolg
BrTarolg

Reputation: 35

Function which selects "closest elements" in vector/list

Lets say I have a vector of 10 numbers, i.e a<-(1:10)

I want a function where I can give an index, and then it returns the 4 "closest" elements. In this case: f(4) = c(2,3,5,6) f(2) = c(10,1,3,4)

Note that the original vector could be anything, so if a <- c(10,2,6,4,7,9,1) f(2) = c(1,10,6,4) f(5) = c(3,4,9,1)

Upvotes: 1

Views: 58

Answers (1)

John Coleman
John Coleman

Reputation: 51998

The natural way to do this is with modular arithmetic. This is a case in which R's 1-based indexing is mildly annoying. It is best to subtract 1 from the index, use modular arithmetic to get the 0-based indices that you would want, and then add 1 to get back to 1-based:

f <- function(v,i){
  m <- length(v)
  j <- i-1
  indices <- 1 + c(j-2,j-1,j+1,j+2) %% m
  v[indices]
}

#test:

a <- c(10,2,6,4,7,9,1)
print(f(a,2))
print(f(a,5))

Output:

[1]  1 10  6  4
[1] 6 4 9 1

Upvotes: 3

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