FW: [R] basic indexing

[Warnes, Gregory R]

 >  -----Original Message-----
 >  From: Duncan Murdoch [mailto:dmurdoch at pair.com]
 >  
 >  On Tue, 9 Oct 2001 21:00:32 -0400, Michaell Taylor
[michaell.taylor at reis.com]:
 >  
 >  >in stata this would be
 >  >gen z=x[N-1]
 >  
 >  In R use z <- c(NA, x[-length(x)])

This can be captured conveniently in the simple function:

lag <- function(x, n)
	c( rep(NA,n), x[1:(length(x)-n)] )

which is used as

> y
[1] 8 9 2 8 8
> lag(y,1)
[1] NA 8 9 2 8
> lag(y,2)
[1] NA NA 8 9 2



 >  >in stata this would be
 >  >gen z=x[N-1] if y==8
 >  
 >  In R you should probably use two statements for this.  The first
 >  constructs z as above, the second is
 >  
 >   z[y != 8] <- NA

and, of course,

> z <- ifelse(y==8, lag(y,1), NA )


-Greg


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