R help - Oct 2005 - Insert value from same column of another row (lag across observations)

I know I've done this before, but it's been a while and I can't find
quite
what I need in the help files or archives.

I have a text field in a very large data frame. I'd like to add a column
that represents the value from an existing field, from the next record (the
data are sorted). I'm trying to represent "what happens tomorrow",
so the
"today" row would have an "NA" value (unknown).

So if x is:
> x
[,1]
[1,] "today"
[2,] "yesterday"
[3,] "the day before"
[4,] "two days before"

any y is
> cbind(x,c(1,2,3,4))
[,1] [,2]
[1,] "today" "1"
[2,] "yesterday" "2"
[3,] "the day before" "3"
[4,] "two days before" "4"


z should become

[,1] [,2] [,3]
[1,] "today" "1" "NA"
[2,] "yesterday" "2" "1"
[3,] "the day before" "3" "2"
[4,] "two days before" "4" "3"

I've tried
z<-cbind(y,c("NA",y[-1,2]))

but I get
[1,] "today" "1" "NA"
[2,] "yesterday" "2" "2"
[3,] "the day before" "3" "3"
[4,] "two days before" "4" "4"

So the "lagging" doesn't work.

Any ideas?

--
---------------------------------------
David L. Van Brunt, Ph.D.
mailto:dlvanbrunt@gmail.com

	[[alternative HTML version deleted]]

On Sun, 2005-10-09 at 14:14 -0500, David L. Van Brunt, Ph.D.
wrote:
> I know I've done this before, but it's been a while and I can't
find quite
> what I need in the help files or archives.
> 
> I have a text field in a very large data frame. I'd like to add a
column
> that represents the value from an existing field, from the next record (the
> data are sorted). I'm trying to represent "what happens
tomorrow", so the
> "today" row would have an "NA" value (unknown).
> 
> So if x is:
> > x
> [,1]
> [1,] "today"
> [2,] "yesterday"
> [3,] "the day before"
> [4,] "two days before"
> 
> any y is
> > cbind(x,c(1,2,3,4))
> [,1] [,2]
> [1,] "today" "1"
> [2,] "yesterday" "2"
> [3,] "the day before" "3"
> [4,] "two days before" "4"
> 
> 
> z should become
> 
> [,1] [,2] [,3]
> [1,] "today" "1" "NA"
> [2,] "yesterday" "2" "1"
> [3,] "the day before" "3" "2"
> [4,] "two days before" "4" "3"
> 
> I've tried
> z<-cbind(y,c("NA",y[-1,2]))
> 
> but I get
> [1,] "today" "1" "NA"
> [2,] "yesterday" "2" "2"
> [3,] "the day before" "3" "3"
> [4,] "two days before" "4" "4"
> 
> So the "lagging" doesn't work.
> 
> Any ideas?

You are close, but you are eliminating the first value in y[, 2] by
using '-1' rather than the last:
> z <- cbind(y, c(NA, y[-nrow(y), 2]))
> z
     [,1]              [,2] [,3]
[1,] "today"           "1"  NA
[2,] "yesterday"       "2"  "1"
[3,] "the day before"  "3"  "2"
[4,] "two days before" "4"  "3"

Do not quote the NA, as "NA" is a character vector, as opposed to a
missing value indicator. See ?NA for more information.

HTH,

Marc Schwartz