How would I change every other element in a vector to the same value?

Question

If I have a vector such as:

vector <- c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1)

how would I change it for every other element to be one number? For example:

vector <- c(1, 2, 1, 2, 1, 2, 1, 2, 1, 2)

Would a for loop work best? I'm not sure how to make it every 2 elements.

Thanks!

A very similar question with answers: https://stackoverflow.com/questions/69125184/how-delete-every-3rd-element-of-a-vector/69125228#69125228 — GuedesBF, Mar 08 '23 at 03:20
[Another candidate ...](https://stackoverflow.com/questions/13461829) — Mikael Jagan, Mar 08 '23 at 03:32

GuedesBF · Answer 1 · 2023-03-08T03:15:19.190

2

We can modify inplace with a recycling logical index:

vector <- c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
vector[c(FALSE, TRUE)]<-2

Or use the modulo operator with replace

replace(vector, seq_along(vector)%%2==0, 2)

 [1] 1 2 1 2 1 2 1 2 1 2

edited Mar 08 '23 at 03:15

answered Mar 08 '23 at 03:11

GuedesBF

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benson23 · Answer 2 · 2023-03-08T03:17:27.020

2

This is not necessary to perform with a loop. We can use square brackets ([]) to index the vector, when the position of the element can be divided by 2 (%% 2 == 0), change the element to 2.

vector[seq_along(vector) %% 2 == 0] <- 2

[1] 1 2 1 2 1 2 1 2 1 2

edited Mar 08 '23 at 03:17

answered Mar 08 '23 at 03:13

benson23

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How would I change every other element in a vector to the same value?

2 Answers2