Combining 2 vectors in alternating way by step of 4 in R

Question

I have two vector of same length:

a = c(723, 680, 2392, 2063, 721, 746, 2053, 2129)
b = c(1, 2, 3, 4, 5, 6, 7, 8)

Now I want to merge them but with a step of 4 element, to be more precise I want as output:

723 680 2392 2063 1 2 3 4 721 746 2053 2129 5 6 7 8

akrun · Accepted Answer · 2020-08-08T17:27:49.897

4

We can do this by splitting. Create a function to create a grouping variable with gl that increments at blocks of 'n' (here n is 4), then split both the vectors into a list, use Map to concatenate the corresponding list elements and unlist the list to create a vector

f1 <- function(x, n) split(x, as.integer(gl(length(x), n, length(x))))
unlist( Map(c, f1(a, 4), f1(b, 4)), use.names = FALSE)
#[1]  723  680 2392 2063    1    2    3    4
#[9]  721  746 2053 2129    5    6    7    8

Or if the lengths are the same, then we can rbind and concatenate after creating a matrix

c(rbind(matrix(a, nrow =4), matrix(b, nrow = 4)))
#[1]  723  680 2392 2063    1    2    3    4  721  746 2053 2129    5    6    7    8

edited Aug 08 '20 at 17:27

answered Aug 08 '20 at 17:20

akrun

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1

Thanks it works. I used the second one since I have 2 vectors of the same length. I thought that we should use rbind with c; it's a little tricky with n=4. – Will Aug 08 '20 at 17:42
1

@Will the `rbind` with `c` should have worked if we didn't have to split up at each 'n' which the `matrix` call will do with dim attribute – akrun Aug 08 '20 at 17:44

score 2 · Answer 2 · answered Aug 08 '20 at 18:17

Here are some other base R options

Using order

c(a,b)[order(c(2 * ceiling(seq_along(a) / 4) - 1, 2 * ceiling(seq_along(b) / 4)))]

Using split + unlist

unlist(
  split(
    c(a, b),
    c(2 * ceiling(seq_along(a) / 4) - 1, 2 * ceiling(seq_along(b) / 4))
  ),
  use.names = FALSE
)

Combining 2 vectors in alternating way by step of 4 in R

2 Answers2