R help - Sep 2004 - loops: pasting indexes in variables names

I cannot figure out how, using R, I can paste indexes or characters to the 
variable
names which are used within loops. I will explain this with a simple 
example:
Immagine I have a huge series of variables, each one taken two times, say
x1 x2 y1 y2 z1 z2.....
Now, immagine that I want to compute a variable from the difference of
each couple, say dx=x1-x2, dy=y1-y2, dz=z1-z2...
In Stata, for example,  this wold be straightforward:
foreach i in x y z   {
		gen  d`i'= `i'1-`i'2
		}
With R I tried to use paste( ) but I found that it applies to objects,
not to variable names.
best regards,
Umberto

Hi Umberto,

look at ?get, ?assign, ?paste and try the following:

x1 <- rnorm(10)
x2 <- rnorm(10)
y1 <- rnorm(10)
y2 <- rnorm(10)
z1 <- rnorm(10)
z2 <- rnorm(10)
######
names. <- c("x", "y", "z")
for(i in names.){
    res1 <- get(paste(i,"1",sep=""))
    res2 <- get(paste(i,"2",sep=""))
    assign(paste("d",i,sep=""), res1-res2)
}
#######
all.equal(dx, x1-x2)
all.equal(dy, y1-y2)
all.equal(dz, z1-z2)


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Umberto Maggiore" <umberto_maggiore at hotmail.com>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, September 22, 2004 2:12 PM
Subject: [R] loops: pasting indexes in variables names

>I cannot figure out how, using R, I can paste indexes or characters 
>to the variable
> names which are used within loops. I will explain this with a simple 
> example:
> Immagine I have a huge series of variables, each one taken two 
> times, say
> x1 x2 y1 y2 z1 z2.....
> Now, immagine that I want to compute a variable from the difference 
> of
> each couple, say dx=x1-x2, dy=y1-y2, dz=z1-z2...
> In Stata, for example,  this wold be straightforward:
> foreach i in x y z   {
> gen  d`i'= `i'1-`i'2
> }
> With R I tried to use paste( ) but I found that it applies to 
> objects,
> not to variable names.
> best regards,
> Umberto
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
>

Umberto Maggiore <umberto_maggiore <at> hotmail.com> writes:
> 
> I cannot figure out how, using R, I can paste indexes or characters to the 
> variable
> names which are used within loops. I will explain this with a simple 
> example:
> Immagine I have a huge series of variables, each one taken two times, say
> x1 x2 y1 y2 z1 z2.....
> Now, immagine that I want to compute a variable from the difference of
> each couple, say dx=x1-x2, dy=y1-y2, dz=z1-z2...
> In Stata, for example,  this wold be straightforward:
> foreach i in x y z   {
> 		gen  d`i'= `i'1-`i'2
> 		}
> With R I tried to use paste( ) but I found that it applies to objects,
> not to variable names.
> best regards,
> Umberto
You can try this:

paste. <- function(...) paste(..., sep = "")
for(i in c("x", "y", "z"))
   assign(paste.("d", i), get(paste.(i, 1) - paste.(i, 2)))

You also might review why you need this since you may prefer
to use vectors, matrices, arrays, lists or data frames for
this sort of processing.  For example, if they were in a data
frame, DF, then you could write:

   DF[,1] - DF[,2]

and perform all the subtractions at once.

Umberto Maggiore wrote:
> I cannot figure out how, using R, I can paste indexes or characters to 
> the variable
> names which are used within loops. I will explain this with a simple 
> example:
> Immagine I have a huge series of variables, each one taken two times, say
> x1 x2 y1 y2 z1 z2.....
> Now, immagine that I want to compute a variable from the difference of
> each couple, say dx=x1-x2, dy=y1-y2, dz=z1-z2...
> In Stata, for example,  this wold be straightforward:
> foreach i in x y z   {
>         gen  d`i'= `i'1-`i'2
>         }
> With R I tried to use paste( ) but I found that it applies to objects,
> not to variable names.
> best regards,
> Umberto
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
Try:
x1<-10
x2<-20
y1<-5
y2<-22
z1<-4
z2<-7
for(i in c("x","y","z")){
  eval(parse(text=paste("d",i,"<-",i,"1 -
",i,"2",sep="")))
}
ls(pattern="d")

output-start
Wed Sep 22 14:38:28 2004
[1] "dx" "dy" "dz"
output-end

but why don't  you  store x1,y1,z1  and x2,y2,z2  in a list:

a<-list(x=1:4, y=1:7, z=1:5)
b<-list(x=(1:4)*10, y=1:7, z=(1:5)-20)
d<-sapply(1:3, function(i) a[[i]]-b[[i]] )

@
output-start
Wed Sep 22 14:43:09 2004
[[1]]
[1]  -9 -18 -27 -36
[[2]]
[1] 0 0 0 0 0 0 0
[[3]]
[1] 20 20 20 20 20
output-end

Peter Wolf