Finding the index inside a vector satisfying a condition

Question

I am looking for a condition which will return the index of a vector satisfying a condition.

For example- I have a vector b = c(0.1, 0.2, 0.7, 0.9) I wa

Answer 1

Use which and take the first element of the result:

which(b > 0.65)[1]
#[1] 3

Answer 2

You may use which.max:

which.max(b > 0.65)
# [1] 3

From ?which.max: "For a logical vector x, [...] which.max(x) return[s] the index of the first [...] TRUE

b > 0.65
# [1] FALSE FALSE  TRUE  TRUE

You should also have a look at the result of your code subset(b, b > 0.65) to see why it can't give you the desired result.

Answer 3

Be careful, which.max is wrong if the condition is never met, it does not return NA:

> a <- c(1, 2, 3, 2, 5)
> a >= 6
[1] FALSE FALSE FALSE FALSE FALSE
> which(a >= 6)[1]
[1] NA  # desirable
> which.max(a >= 6)
[1] 1   # not desirable

Why? When all elements are equal, which.max returns 1:

> b <- c(2, 2, 2, 2, 2)
> which.max(b)
[1] 1

Note: FALSE < TRUE