identifying unique elements in sub lists and altering

Question

I have the following \'list\' in R:

[[1]]
[1] 17336  5246  8597  5246 17878 19701

[[2]]
[1] 19701 37748 18155  5246  8597

[[3]]
[1] 12297 19701 17878  5246 173         


        

Answer 1

Using @SymbolixAU's data:

lapply(lst, function(x) tail(x, -Position(isTRUE, x==5, nomatch=-Inf)) )
#[[1]]
#[1] 1 2 3
# 
#[[2]]
#[1] 2 3 4
#
#[[3]]
#numeric(0)
# 
#[[4]]
#[1] 6

To explain how this works:
1) The middle Position part just returns the negative of the index of the first time a 5 is detected, i.e.:

sapply(lst, function(x) -Position(isTRUE, x==5, nomatch=-Inf) )
#[1] Inf Inf  -3  -2

2) tail(x, -n) just removes the first n values from a vector. When run with Inf instead, nothing is removed. Hence why Inf is used in the instance when no 5 is found.