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basic indexing

3 messages · Warnes, Gregory R, Brian Ripley, Peter Dalgaard

Warnes, Gregory R
# 
>  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
[1] 8 9 2 8 8
[1] NA 8 9 2 8
[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,
-Greg


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Brian Ripley
#
On Wed, 10 Oct 2001, Warnes, Gregory R wrote:

            

      
      
      
    

        
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.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Peter Dalgaard
    

    

      
      #
    

  


  

      
Prof Brian D Ripley <ripley at stats.ox.ac.uk> writes:

            

      
      
      
    

        
...which is Splus compatible, although not desirable. From the Splus
docs:

NOTE:
    
   Logically k should have the opposite sign, but due to its pervasive
   use in other functions, lag remains unchanged. However, the new shift
   function (which calls lag by default) has the more logical
   convention.