[R] Adding Column to Data Frames Using a Loop

[William Dunlap]

> 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
> 
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