[R] how to loop through dataframe objects in environment

[Jeff Newmiller]

This advice works best when you use the lapply function to load your data frames to begin with. That way your like-structured data frames are grouped into one list that you can loop through without complicated use of get and assign.

dtadir <- "mydatadir"
fnames <- list.files( dtadir )
dtalist <- lapply( fnames, function( fn ) { read.csv( file.path( dtadir, fn ) ) }
for ( idx in seq_along( dtalist ) ) {
  dtalist[[ idx]]$result <- dtalist[[ idx ]][ , 3 ] + dtalist[[ idx ]][ , 4 ]
}

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Sent from my phone. Please excuse my brevity.

On October 15, 2014 8:59:01 PM PDT, Vikash Kumar <vikash.kr.117 at gmail.com> wrote:
>Hi Stephen,
>
>Try out lapply(). It would help you loop through all data frames and
>sum.
>
>Regards,
>Vikash
>
>On Thu, Oct 16, 2014 at 1:58 AM, Stephen HK Wong <honkit at stanford.edu>
>wrote:
>
>> Dear All,
>>
>> I  have many 50 objects they are all dataframes. Each dataframe has
>many
>> rows and four column. I simply want to do an addition of 3rd and 4th
>column
>> and save the result into 5th column. Since there are many dataframes,
>I
>> don't want to do it one by one,  is there a way to loop through all
>objects
>> and perform the similar action ?
>>
>> One way I can think of is like this:
>>
>> for (i in 1:50){
>> get(ls()[i])[,3]+get(ls()[i][,4]
>> }
>>
>> But I don't know how to save the addition result back to 5th column
>of
>> each dataframe.
>>
>>
>> Many Thanks!
>>
>>
>>
>> Stephen HK Wong
>>
>> ______________________________________________
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>> 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.
>>
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>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.