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I have a vector in R:

> v <- c(5, 10, 15, 20, 25, 30, 35, 40, 45, 50)

I would like to apply a function to every nth element of the vector and have it return the new vector. For example, let's say I would like to multiply every third element of the vector by 2. I would then get

> v [1] 5 10 30 20 25 60 35 40 90 50

I have managed to extract elements using vector logic:

> v[c(rep(FALSE,2),TRUE)] [1] 15 30 45

meaning I have figured out how to access the elements and I am able to do things to them, but I don't know how to get them back into my original vector v.

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5 Answers 5

Reset to default
8

We need to assign 

v[c(FALSE, FALSE, TRUE)] <- v[c(FALSE, FALSE, TRUE)]*2
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6

Solution using ifelse:

ifelse(1:length(v) %% 3 == 0, v * 2, v) # [1] 5 10 30 20 25 60 35 40 90 50
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5

Another option would be to use seq. You specify n at will, and seq will start a sequence at n, by n until the end of your vector (length(v)).

n <- 3 v[seq(n, length(v), n)] <- v[seq(n, length(v), n)]*2 > v [1] 5 10 30 20 25 60 35 40 90 50
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3

Using tidyverse:

 library(tidyverse) p <- seq(v) %% 3 == 0 f <- function(x) x*2 map_if(v, p, f) %>% as_vector
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0

Using a while loop:

i=0 while (i < (length(v)-1) ){ i=i+3 v[i]=v[i]*2 } > v [1] 5 10 30 20 25 60 35 40 90 50
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1 Comment

A bit overkill to use loops on a vector, with this logic we can add forloop, apply family functions solutions, too.

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