R help - May 2009 - apply fn to many dataframes

Hi,

Say I have dataframes d1, d2, ... , dn, and I want to apply a 
function to all of them.  For example, say I want to change the name 
of the second variable in each dataframe to "x2".  The following
doesn't work:

a = list(d1,d2,d3,d4)
lapply(a,function(x) names(x)[2] = "x2")

What would work?

Thanks for any help.

You have to return a value from the function in the lapply and assign the
result to another object:
> df <- data.frame(a=1,b=2,c=3,d=4)
> a <- list(df,df,df,df)
> # to change the name of the second, you have to change the name and then
return
> # the dataframe as the return value and assign it back into another list
> a.1 <- lapply(a, function(x){
+     names(x)[2] <- 'newName'
+     x  # return
+ })
>
> a
[[1]]
  a b c d
1 1 2 3 4
[[2]]
  a b c d
1 1 2 3 4
[[3]]
  a b c d
1 1 2 3 4
[[4]]
  a b c d
1 1 2 3 4
> a.1
[[1]]
  a newName c d
1 1       2 3 4
[[2]]
  a newName c d
1 1       2 3 4
[[3]]
  a newName c d
1 1       2 3 4
[[4]]
  a newName c d
1 1       2 3 4
>

On Mon, May 25, 2009 at 6:19 PM, James Fearon <jfearon@stanford.edu>
wrote:
> Hi,
>
> Say I have dataframes d1, d2, ... , dn, and I want to apply a function to
> all of them.  For example, say I want to change the name of the second
> variable in each dataframe to "x2".  The following doesn't
work:
>
> a = list(d1,d2,d3,d4)
> lapply(a,function(x) names(x)[2] = "x2")
>
> What would work?
>
> Thanks for any help.
>
> ______________________________________________
> 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<http://www.r-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

	[[alternative HTML version deleted]]

Hi:?  you have to return the dataframe inside the lapply. I also changed    to
<- but I doubt that matters.
   d1 <- data.frame(x1=1,x3=4)
   d2 <- data.frame(x1=2,x3=5)
   d3 <- data.frame(x1=3,x3=6)
   d4 <- data.frame(x1=4,x3=7)
   a = list(d1,d2,d3,d4)
   print(a)
   lapply(a,function(.df) {
   ? ? ?  names(.df)[2] <- "x2"
   ? ? ?  .df
   })

   On May 25, 2009, James Fearon <jfearon at stanford.edu> wrote:

     Hi,
     Say I have dataframes d1, d2, ... , dn, and I want to apply a
     function to all of them. For example, say I want to change the name
     of the second variable in each dataframe to "x2". The following
doesn't
     work:
     a = list(d1,d2,d3,d4)
     lapply(a,function(x) names(x)[2] = "x2")
     What would work?
     Thanks for any help.
     ______________________________________________
     [1]R-help at r-project.org mailing list
     [2]https://stat.ethz.ch/mailman/listinfo/r-help
     PLEASE do read the posting guide
     [3]http://www.R-project.org/posting-guide.html
     and provide commented, minimal, self-contained, reproducible code.

References

   1. mailto:R-help at r-project.org
   2. https://stat.ethz.ch/mailman/listinfo/r-help
   3. http://www.R-project.org/posting-guide.html