R: Re: Refer to previous row

It works!

The rationale, if I have understood well, is to take my vector of N elements, ask to remove the first/last element, and replace the blank space with a given value.

Thank you all for the support!
-----Original Message-----
From: Duncan Murdoch <murdoch.duncan at gmail.com>
Date: Mon, 07 Jan 2013 13:16:50 
To: <donatellipaolo at gmail.com>
Cc: <r-help at r-project.org>
Subject: Re: [R] Refer to previous row

Hi all,

I have a very basic doubt -- but still, I am a newby!

My question is about referring to the previous row: in a sample as the
following...

ID  X1  X2
1   A   12
2   A   6
3   A   10
1   B   17
2   B   19
1   C   22
1   D   13
2   D   19
3   D   21

... I would like to create a dummy variable equal to 1 whenever the
value of ID of the current row is lower or equal than the value of ID
of the previous row -- check the new vector X3 I'd like to obtain:

ID  X1  X2  X3
1   A   12  0
2   A   6    0
3   A   10  0
1   B   17  1
2   B   19  0
1   C   22  1
1   D   13  1
2   D   19  0
3   D   21  0

I have searched a lot without finding a decent and working solution. I
suppose it is just some basic matter of indexing language, something
like

X3<- as.numeric ( ID[n] <= ID[n-1])

but it is not so simple!

Negative indexing lets you leave out an entry, so x[-1] leaves out the 
first entry, and x[-length(x)] leaves out the last one.  To talk about 
previous entries, you need to do something about the fact that the first 
row has no previous entry.  You gave X3[1] the value 0, suggesting that 
you want to implicitly have the "zeroth" row to have the smallest 
possible value.  So

prevID <- c( -Inf, ID[-length(ID)] )
X3 <- as.numeric( ID <= prevID )

should do what you want.

Duncan Murdoch