R help - Dec 2010 - How can I refer to actual (n) and previous (n-1) elements in a vector?

Hello,


How can I apply a function on a vector that refers to actual (n) and previous
elements in the vector (e.g. n-1)?


For example:
I would like to calculate the sum of (n-1) + n for each element of a vector and
get a vector as a result.


Besides others I tried this:


v<-c(3,6,8,1,1,3,9,5,6,3)
for (i in 1:NROW(v)){a[i]<-a[i-1]+a[i]}


I would like to get this result:
9,14,9,2,4,12,14,11,9


I would greatly appreciate your help!
Marianne 		 	   		  
	[[alternative HTML version deleted]]

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Marianne Stephan
> Sent: Monday, December 06, 2010 9:13 AM
> To: r-help at r-project.org
> Subject: [R] How can I refer to actual (n) and previous (n-1) elements
> in a vector?
> 
> 
> Hello,
> 
> 
> How can I apply a function on a vector that refers to actual (n) and
> previous elements in the vector (e.g. n-1)?
> 
> 
> For example:
> I would like to calculate the sum of (n-1) + n for each element of a
> vector and get a vector as a result.
> 
> 
> Besides others I tried this:
> 
> 
> v<-c(3,6,8,1,1,3,9,5,6,3)
> for (i in 1:NROW(v)){a[i]<-a[i-1]+a[i]}
> 
> 
> I would like to get this result:
> 9,14,9,2,4,12,14,11,9
> 
> 
> I would greatly appreciate your help!
> Marianne
> 	[[alternative HTML version deleted]]
How about something like

v[-n] +  v[-1]

Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Nordlund, Dan (DSHS/RDA)
> Sent: Monday, December 06, 2010 9:21 AM
> To: r-help at r-project.org
> Subject: Re: [R] How can I refer to actual (n) and previous (n-1)
> elements in a vector?
> 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> > project.org] On Behalf Of Marianne Stephan
> > Sent: Monday, December 06, 2010 9:13 AM
> > To: r-help at r-project.org
> > Subject: [R] How can I refer to actual (n) and previous (n-1)
> elements
> > in a vector?
> >
> >
> > Hello,
> >
> >
> > How can I apply a function on a vector that refers to actual (n) and
> > previous elements in the vector (e.g. n-1)?
> >
> >
> > For example:
> > I would like to calculate the sum of (n-1) + n for each element of a
> > vector and get a vector as a result.
> >
> >
> > Besides others I tried this:
> >
> >
> > v<-c(3,6,8,1,1,3,9,5,6,3)
> > for (i in 1:NROW(v)){a[i]<-a[i-1]+a[i]}
> >
> >
> > I would like to get this result:
> > 9,14,9,2,4,12,14,11,9
> >
> >
> > I would greatly appreciate your help!
> > Marianne
> > 	[[alternative HTML version deleted]]
> 
> How about something like
> 
> v[-n] +  v[-1]
> 
Sorry for the noise.  That should have been

v[-length(v)] + v[-1]


Hope this is more helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

Hi Marianne,

You have to be very careful with subsetting when doing something like that -
that's where you went wrong with your original construct. This version
works:
> v<-c(3,6,8,1,1,3,9,5,6,3)
> a <- numeric(length(v)-1)
> for (i in 2:length(v)) {a[i-1] <- v[i-1] + v[i]}
> a
[1]  9 14  9  2  4 12 14 11  9


But here's a more elegant way:
> v[1:(length(v)-1)] + v[2:length(v)]
[1]  9 14  9  2  4 12 14 11  9

and I'm sure there are even nicer solutions.

Sarah

On Mon, Dec 6, 2010 at 12:13 PM, Marianne Stephan
<mariannestephan at hotmail.com> wrote:
>
> Hello,
>
>
> How can I apply a function on a vector that refers to actual (n) and
previous elements in the vector (e.g. n-1)?
>
>
> For example:
> I would like to calculate the sum of (n-1) + n for each element of a vector
and get a vector as a result.
>
>
> Besides others I tried this:
>
>
> v<-c(3,6,8,1,1,3,9,5,6,3)
> for (i in 1:NROW(v)){a[i]<-a[i-1]+a[i]}
>
>
> I would like to get this result:
> 9,14,9,2,4,12,14,11,9
>
>
-- 
Sarah Goslee
http://www.functionaldiversity.org

Hi:

This is called a 'rolling sum', for which there is a very convenient
function called rollapply() in the zoo package. You first need to convert v
to a zoo object (in this case, an indexed vector):

library(zoo)
> rollapply(zoo(v), 2, FUN = sum)
 1  2  3  4  5  6  7  8  9
 9 14  9  2  4 12 14 11  9

The second argument is the width of the window to which the function is to
be applied - in this case, you want a window of width two and a sum
function.

HTH,
Dennis

On Mon, Dec 6, 2010 at 9:13 AM, Marianne Stephan <
mariannestephan@hotmail.com> wrote:
>
> Hello,
>
>
> How can I apply a function on a vector that refers to actual (n) and
> previous elements in the vector (e.g. n-1)?
>
>
> For example:
> I would like to calculate the sum of (n-1) + n for each element of a vector
> and get a vector as a result.
>
>
> Besides others I tried this:
>
>
> v<-c(3,6,8,1,1,3,9,5,6,3)
> for (i in 1:NROW(v)){a[i]<-a[i-1]+a[i]}
>
>
> I would like to get this result:
> 9,14,9,2,4,12,14,11,9
>
>
> I would greatly appreciate your help!
> Marianne
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Hello everybody,
I would like to calculate the median for each factor combination, with only one
value per factor combination as an output.
Could anybody help me?


For example:
# make table
g<-1:2
group<-rep(g, each=5)
session<-c(1,1,2,2,2,1,1,1,2,2)
rt<-seq(length=10,300, 800)
rt<-round(rt, digits=2)
table<-data.frame(group, session, rt)
table

   group session     rt
1      1       1 300.00
2      1       1 355.56
3      1       2 411.11
4      1       2 466.67
5      1       2 522.22
6      2       1 577.78
7      2       1 633.33
8      2       1 688.89
9      2       2 744.44
10     2       2 800.00




Besides others I tried the following:
median<-ave(rt, group, session, FUN=median)
median<-frameApply(table,by=c("group", "session"),
on="rt", fun=median)

I would appreciate your help a lot.
Marianne 		 	   		  
	[[alternative HTML version deleted]]