R: how to check whether a vector is ascending/descending

Question

vector1 = c(2, 2, 2, 2, 2, 2)
vector2 = c(2, 2, 3, 3, 3, 3)
vector3 = c(2, 2, 1, 2, 2, 2)

I want to know if the numbers in the vector are ascending/sta

Answer 1

You can diff to compute the differences between elements and all to check if they are all non-negative:

all(diff(vector1) >= 0)
# [1] TRUE
all(diff(vector2) >= 0)
# [1] TRUE
all(diff(vector3) >= 0)
# [1] FALSE

The above code checks if all the vectors are non-decreasing, and you could replace >= 0 with <= 0 to check if they're non-increasing. If instead your goal is to identify vectors that are either non-decreasing or non-increasing (aka they don't have an increasing and a decreasing step in the same vector), there's a simple modification:

!all(c(-1, 1) %in% sign(diff(vector1)))
# [1] TRUE
!all(c(-1, 1) %in% sign(diff(vector2)))
# [1] TRUE
!all(c(-1, 1) %in% sign(diff(vector3)))
# [1] FALSE