[R] apply fn to many dataframes

[Wacek Kusnierczyk]

jim holtman wrote:
> 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
>   
>
>   

i think this is a typical case of where for-looping is the method of
choice, given the brief description below:

    dfs = list(d1, d2, d3, d4)
    for (i in 1:length(dfs))
       names(dfs[[i]])[2] = 'x2'

on the side, the solution above is in the good functional style, but
seems comparatively inefficient:

    dfs = replicate(1000, data.frame(a=1:1000, b=1000:1), simplify=FALSE)
    library(rbenchmark)
    benchmark(columns=c('test', 'elapsed'), replications=100,
       'for'=for (i in 1:length(dfs))
          names(dfs[[i]])[2] = 'foo',
       lapply={dfs = lapply(dfs, function(df) {
          names(df)[2] = 'foo'
          df })})

    #     test elapsed
    # 1    for   2.262
    # 2 lapply   3.211


vQ

> On Mon, May 25, 2009 at 6:19 PM, 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.
>>
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