Creating a Sequence with positive and negative numbers in R

Question

I have been trying to create a function for the sequence of following series:

1,-2,-3, 4, 5, 6, -7 ,-8, -9, -10........n (1 positive, 2 negatives, 3 positives, 4 negatives … and goes on up to n).

Creating a non-negative sequence is quite easy, but these negative terms are testing me.

If anyone can help me over this

Answer 1

Here's a way to do this.

myfun <- function(n) {
  myvec <- integer(n)
  for (i in seq_len(n)) {
    curtri <- ceiling(sqrt(i*2 + 0.25) - 0.5)
    myvec[i] <- i * (-1)^(curtri + 1)
  }
  return(myvec)
}

myfun(10)
 [1]   1  -2  -3   4   5   6  -7  -8  -9 -10

It takes advantage of the fact that you can find which triangular number you are at with sqrt(i*2 + 0.25) - 0.5. By applying even to non triangular numbers, we can determine the index of the next triangular number, and use that as the exponent for -1.

There's probably a better way, though.