How to produce a binary variable that takes value of 0 before the last non-zero value of another variable and 1 after it

Question

Given the variable

a <- c(1, 1, 1, 0, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0) 

I would like to mark the last non-zero value to produce a variable that takes value of 0 before the benchmark and 1 after it. In this case

b <- c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1)

A simple command that allows to mark the last non-zero value of a a column is

tail(which(a!=0),1) 

But how to create the variable I need conditional on such benchmark? Any help would be much appreciated!

Answer 1

One option could be:

+(cumsum(a) == max(cumsum(a)))

 [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1

Or:

+(cumsum(a) == sum(a))

Answer 2

We can use which to get the position, index where a is 1, wrap with max to return the last position, create a logical vector with sequence of 'a' and coerce to binary (+)

+(seq_along(a) >= max(which(a == 1)))
#[1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1

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Or use Position

+(seq_along(a) >= Position(function(x) x == 1, a, right = TRUE))