R help - Oct 2001 - FW: basic indexing

>  -----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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On Wed, 10 Oct 2001, Warnes, Gregory R wrote:
>  >  -----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
There is of course already a generic function called lag in R (in package
ts), so overwriting it would not be a good idea.

Especially since this one has argument n of the opposite sign to the
generic definition.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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