r - Shift values in single column of dataframe up

Question

Using example data like this:

example=data.frame(x=c(1,2,3,4,5,6,7,8), y=c(1,2,3,4,5,6,7,8), z=c(1,2,3,4,5,6,7,8))

which looks like this:

    x   y   z
1   1   1   1
2   2   2   2
3   3   3   3
4   4   4   4
5   5   5   5
6   6   6   6
7   7   7   7
8   8   8   8

I would like to shift all values in the z column upwards by two rows while the rest of the dataframe remains unchanged. The result should look like this:

    x   y   z
1   1   1   3
2   2   2   4
3   3   3   5
4   4   4   6
5   5   5   7
6   6   6   8
7   7   7   NA
8   8   8   NA

I only found ways to move the values of a column down, or a shifting of the whole dataframe.

Any ideas? Thanks!

Answer 1

Your problem simplifies to:

• Drop the first n elements in a vector
• Pad n values of NA at the end

You can do this with a simple function:

shift <- function(x, n){
  c(x[-(seq(n))], rep(NA, n))
}

example$z <- shift(example$z, 2)

The result:

example
  x y  z
1 1 1  3
2 2 2  4
3 3 3  5
4 4 4  6
5 5 5  7
6 6 6  8
7 7 7 NA
8 8 8 NA