R help - May 2013 - Adding Column to Data Frames Using a Loop

Dear R Helpers,

I am trying to do calculations on multiple data frames and do not want to
create a list of them to go through each one.  I know that lists have many
wonderful advantages, but I believe the better thing is to work df by df
for my particular situation.  For background, I have already received some
wonderful help on how to handle some situations, such as removing columns:


x=as.data.frame(matrix(c(1,2,3,
        1,2,3,
        1,2,2,
        1,2,2,
       1,1,1),ncol=3,byrow=T))

y=as.data.frame(matrix(c(1,2,3,
        1,2,3,
        1,2,2,
        1,2,2,
       1,1,1),ncol=3,byrow=T))

z=as.data.frame(matrix(c(1,2,3,
        1,2,3,
        1,2,2,
        1,2,2,
       1,1,1),ncol=3,byrow=T))

for(i in letters[24:26] ) assign( i, subset(get(i), select=-c(V1))  )
x
y
z

And I figured how to do further processing using functions:

myfunc<-function(DF){
 DF$V4<-DF$V2+DF$V3
 return(DF)
}
for(i in letters[24:26] ) assign( i, myfunc(get(i)))

But if I want to do a rather simple calculation and store it as a new
column in each data frame such as

x$V4<-x$V2+x$V3
y$V4<-y$V2+y$V3
z$V4<-z$V2+z$V3

is there a simpler way to do this than building a function as shown above?
 I tried a few variations of

i<-24
assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))

but keep getting syntax errors.

If anyone could help with the syntax as to how to accomplish the
calculation above without building a function, I would really appreciate
it.

--John Sparks

> I am trying to do calculations on multiple data frames and do not want to
> create a list of them to go through each one.  I know that lists have many
> wonderful advantages, but I believe the better thing is to work df by df
> for my particular situation.  
Can you give some details about why you believe this is true in your particular
situation?
This is not a rhetorical question -- what are the problems you run into with
storing a related set of data.frames in a list?  Have you tried using an
environment
instead of a list (of course you have, since the global environment is an
environment,
but putting them into an environment created by new.env() helps organize
things).

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org] On Behalf
> Of Sparks, John James
> Sent: Wednesday, May 01, 2013 9:53 AM
> To: r-help at r-project.org
> Subject: [R] Adding Column to Data Frames Using a Loop
> 
> Dear R Helpers,
> 
> I am trying to do calculations on multiple data frames and do not want to
> create a list of them to go through each one.  I know that lists have many
> wonderful advantages, but I believe the better thing is to work df by df
> for my particular situation.  For background, I have already received some
> wonderful help on how to handle some situations, such as removing columns:
> 
> 
> x=as.data.frame(matrix(c(1,2,3,
>         1,2,3,
>         1,2,2,
>         1,2,2,
>        1,1,1),ncol=3,byrow=T))
> 
> y=as.data.frame(matrix(c(1,2,3,
>         1,2,3,
>         1,2,2,
>         1,2,2,
>        1,1,1),ncol=3,byrow=T))
> 
> z=as.data.frame(matrix(c(1,2,3,
>         1,2,3,
>         1,2,2,
>         1,2,2,
>        1,1,1),ncol=3,byrow=T))
> 
> for(i in letters[24:26] ) assign( i, subset(get(i), select=-c(V1))  )
> x
> y
> z
> 
> And I figured how to do further processing using functions:
> 
> myfunc<-function(DF){
>  DF$V4<-DF$V2+DF$V3
>  return(DF)
> }
> for(i in letters[24:26] ) assign( i, myfunc(get(i)))
> 
> But if I want to do a rather simple calculation and store it as a new
> column in each data frame such as
> 
> x$V4<-x$V2+x$V3
> y$V4<-y$V2+y$V3
> z$V4<-z$V2+z$V3
> 
> is there a simpler way to do this than building a function as shown above?
>  I tried a few variations of
> 
> i<-24
>
assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))
> 
> but keep getting syntax errors.
> 
> If anyone could help with the syntax as to how to accomplish the
> calculation above without building a function, I would really appreciate
> it.
> 
> --John Sparks
> 
> ______________________________________________
> R-help at 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.

Hi,
You could use:
library(plyr)
for(i in letters[24:26]) assign(i,mutate(get(i),V4=V2+V3))
?x
?# V1 V2 V3 V4
#1? 1? 2? 3? 5
#2? 1? 2? 3? 5
#3? 1? 2? 2? 4
#4? 1? 2? 2? 4
#5? 1? 1? 1? 2
?y
#? V1 V2 V3 V4
#1? 1? 2? 3? 5
#2? 1? 2? 3? 5
#3? 1? 2? 2? 4
#4? 1? 2? 2? 4
#5? 1? 1? 1? 2


A.K.

>Dear R Helpers, 
>
>I am trying to do calculations on multiple data frames and do not want to 
>create a list of them to go through each one. ?I know that lists have many 
>wonderful advantages, but I believe the better thing is to work df by df 
>for my particular situation. ?For background, I have already received some 
>wonderful help on how to handle some situations, such as removing columns: 
>
>
>x=as.data.frame(matrix(c(1,2,3, 
?> ? ? ? 1,2,3, 
? >? ? ? 1,2,2, 
?? > ? ? 1,2,2, 
?? > ?? 1,1,1),ncol=3,byrow=T)) 
>
>y=as.data.frame(matrix(c(1,2,3, 
?> ? ? ? 1,2,3, 
? >? ? ? 1,2,2, 
? >? ? ? 1,2,2, 
? >? ?? 1,1,1),ncol=3,byrow=T)) 
>
?>z=as.data.frame(matrix(c(1,2,3, 
?> ? ? ? 1,2,3, 
? >? ? ? 1,2,2, 
? >? ? ? 1,2,2, 
? >? ?? 1,1,1),ncol=3,byrow=T)) 
>
>for(i in letters[24:26] ) assign( i, subset(get(i), select=-c(V1)) ?) 
>x 
>y 
>z 
>
>And I figured how to do further processing using functions: 
>
>myfunc<-function(DF){ 
> DF$V4<-DF$V2+DF$V3 
?>return(DF) 
>} 
>for(i in letters[24:26] ) assign( i, myfunc(get(i))) 
>
>But if I want to do a rather simple calculation and store it as a new 
>column in each data frame such as 
>
>x$V4<-x$V2+x$V3 
>y$V4<-y$V2+y$V3 
>z$V4<-z$V2+z$V3 
>
>is there a simpler way to do this than building a function as shown above? 
?>I tried a few variations of 
>
>i<-24 
>assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))
>
>but keep getting syntax errors. 
>
>If anyone could help with the syntax as to how to accomplish the 
>calculation above without building a function, I would really appreciate 
>it. 
>
>--John Sparks

I'm a little puzzled, because you're asking for something simpler than
  for(i in letters[24:26] ) assign( i, myfunc(get(i)))
and suggesting things along the lines of
 
assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))
To me, the former is much simpler, much easier to read and understand.

There are several problems with

i<-24
assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))

First, I think you must have meant

  i <- letters[24]

You seem to expect that
  paste( get(i) , "$V2+" )
would result in
  "x$V2+"
but it doesn't.

The expression
  get(i)
returns the entire object, in this case a data frame, in particular,
the data frame named 'x'. Look at what happens when you try to
paste a data frame with something else
> paste(x,'$V2+')
[1] "c(1, 1, 1, 1, 1) $V2+" "c(2, 2, 2, 2, 1) $V2+"
"c(3, 3, 2, 2, 1) $V2+"

same as
> paste( get('x') , '$V2+')
[1] "c(1, 1, 1, 1, 1) $V2+" "c(2, 2, 2, 2, 1) $V2+"
"c(3, 3, 2, 2, 1) $V2+"

With
  paste(get(i),"$V2+",get(i),"$V3",sep="")

you appear to be trying to construct


   "x$V2+x$V3"
and then hoping that expression will be executed. And then hoping that the
results will get assigned within the data frame to x$V4.

But assign() writes into an environment, not into a data frame. For now,
think of environments as one of the elements that you see when you run
  search()

It is possible to construct the expression you want executed, and cause it
to be executed, but it's kind of an advanced approach and I think more
trouble than it's worth. In 15 or so years of using R, I've used it
maybe
once or twice. I really think you're better off with lists, and would
second Bill Dunlap's request for more explanation of why you don't want
to
go that route.

If you really want to construct R expressions and cause them to be
evaluated, I'd start by looking at
  ?eval
  ?parse

Try:
 eval(parse(text='3+4'))

or
> eval(parse(text= paste('3','+','4')))
[1] 7






-- 
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062





On 5/1/13 9:53 AM, "Sparks, John James" <jspark4 at uic.edu>
wrote:
>Dear R Helpers,
>
>I am trying to do calculations on multiple data frames and do not want to
>create a list of them to go through each one.  I know that lists have many
>wonderful advantages, but I believe the better thing is to work df by df
>for my particular situation.  For background, I have already received some
>wonderful help on how to handle some situations, such as removing columns:
>
>
>x=as.data.frame(matrix(c(1,2,3,
>        1,2,3,
>        1,2,2,
>        1,2,2,
>       1,1,1),ncol=3,byrow=T))
>
>y=as.data.frame(matrix(c(1,2,3,
>        1,2,3,
>        1,2,2,
>        1,2,2,
>       1,1,1),ncol=3,byrow=T))
>
>z=as.data.frame(matrix(c(1,2,3,
>        1,2,3,
>        1,2,2,
>        1,2,2,
>       1,1,1),ncol=3,byrow=T))
>
>for(i in letters[24:26] ) assign( i, subset(get(i), select=-c(V1))  )
>x
>y
>z
>
>And I figured how to do further processing using functions:
>
>myfunc<-function(DF){
> DF$V4<-DF$V2+DF$V3
> return(DF)
>}
>for(i in letters[24:26] ) assign( i, myfunc(get(i)))
>
>But if I want to do a rather simple calculation and store it as a new
>column in each data frame such as
>
>x$V4<-x$V2+x$V3
>y$V4<-y$V2+y$V3
>z$V4<-z$V2+z$V3
>
>is there a simpler way to do this than building a function as shown above?
> I tried a few variations of
>
>i<-24
>assign(paste(i,"$V4",sep=""),paste(get(i),"$V2+",get(i),"$V3",sep=""))
>
>but keep getting syntax errors.
>
>If anyone could help with the syntax as to how to accomplish the
>calculation above without building a function, I would really appreciate
>it.
>
>--John Sparks
>
>______________________________________________
>R-help at 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.