Find index of last element in vector below threshold

Get the indexes of the satisfying elements and take the last one:

# 1
tail(which(v < 8), 1)
[1] 7

The above gives a zero length result if there are no satisfying elements. To specify a different result use this (where we have specified NA but that can be replaced with any other desired value such as 0, say):

# 2
(v < 0) |>
  which() |>
  tail(1) |>
  c(NA) |>
  head(1)
[1] NA

or

# 3
(v < 0) |>
  which() |>
  (\(x) if (length(x)) tail(x, 1) else NA)()
[1] NA

This one gives NA if there is no match:

# 4
rev(which(v < 0))[1]
[1] NA

This one gives 0 if there is no match. It is less efficient than the ones above but this may not matter if v is not large.

# 5
Reduce(max, init = 0, which(v < 0))
[1] 0

You can supply a condition to which(), which returns index values that match the condition. So:

which(v < 8)
#[1] 1 2 3 6 7

To get the index of the last value < 8, wrap it in max() e.g.:

max(which(v < 8))
#[1] 7

We might want to use Position(), one of those Common Higher-Order Functions in Functional Programming Languages which we tend to forget.

Position(\(x) x