[R] index

[Richard.Cotton at hsl.gov.uk]

> I would like to know if it?s possible to create an index for a 
> variable/vector that specifies the value in the n position before 
> the actual value.
> 
> For example, if X <- c(letters[1:10]), x[10] is ?j?. I would like to
> create an index exp3 (the third position before) such as if z <- 
> x[exp3], then z[10] = ?g?.

x<- letters[1:10]

Then either
z<- c(rep(NA,3), x)
z[10] #"g"

or
exp3<- 1:length(x) - 3; exp3[exp3<=0] <- NA
z <- x[exp3]
z[10] #"g"

Regards,
Richie.

Mathematical Sciences Unit
HSL

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