How to merge 2 vectors alternating indexes?

I would like to merge 2 vectors this way :

a = c(1,2,3)
b = c(11,12,13)
merged vector : c(1,11,2,12,3,13)

How could I do it ?

Solution 1:

This will work using rbind :

c(rbind(a, b))

For example:

a = c(1,2,3)
b = c(11,12,13)

c(rbind(a,b))

#[1]  1 11  2 12  3 13

The rbind() answer by @jalapic is excellent. Here's an alternative that creates a new vector then assigns the alternating values to it.

a <- c(1,2,3)
b <- c(11,12,13)

x <- vector(class(a), length(c(a, b)))
x[c(TRUE, FALSE)] <- a
x[c(FALSE, TRUE)] <- b
x
# [1]  1 11  2 12  3 13

And one more that shows append

c(sapply(seq_along(a), function(i) append(a[i], b[i], i)))
# [1]  1 11  2 12  3 13

Just wanted to add a simpler solution that works for when vectors are unequal length and you want to append the extra data to the end.

> a <- 1:3
> b <- 11:17
> c(a, b)[order(c(seq_along(a)*2 - 1, seq_along(b)*2))]
 [1]  1 11  2 12  3 13 14 15 16 17

Explanation:

c(a, b) creates a vector of the values in a and b.
seq_along(a)*2 - 1 creates a vector of the first length(a) odd numbers.
seq_along(b)*2 creates a vector of the first length(b) even numbers.
order(...) will return the indexes of the numbers in the two seq_along vectors such that x[order(x)] is an ordered list. Since the first seq_along contains the even numbers and the second seq_along has the odds, order will take the first element from the first seq_along, then the first elements of the second seq_along, then the second element from the first seq_along, etc. interspersing the two vector indexes and leaving the extra data at the tail.
By indexing c(a, b) using the order vector, we will intersperse a and b.

As a note, since seq_along returns numeric(0) when the input is NULL this solution works even if one of the vectors is length 0.