[R] Adding a new conditional column to a list of dataframes

[David Winsemius]

> On Apr 15, 2018, at 4:08 AM, Allaisone 1 <Allaisone1 using hotmail.com> wrote:
> 
> 
> Hi all ..,
> 
> 
> I have a list of 7000 dataframes with similar column headers and I wanted to add a new column to each dataframe based on a certain condition which is the same for all dataframes.
> 
> 
> When I extract one dataframe and apply my code it works very well as follows :-
> 
> 
> First suppose this is my first dataframe in the list
> 
>> OneDF <- Mylist[[1]]
> 
>> OneDF
> 
> 
> ID       Pdate                  Tdate
> 
> 1         2010-09-30       2011-05-10
> 
> 2         2011-11-07       2009-09-31
> 
> 3         2012-01-05        2008-06-23
> 
> 
> To add a new column where "C" has to be written in that column only if the date in
> 
> "Tdate" column is less than the first date(row) in "Pdate" column.Otherwise "NA" is written.
> 
> I have written this code to do so :-
> 
> 
> OneDF$NewCol [ OneDF[ ,3] <  OneDF[ 1,2] ] <- "C"
> 
> 
> This gave me what I want as follows :-
> 
> 
> ID       Pdate                  Tdate                      NewCol
> 
> 1         2010-09-30       2011-05-10                NA
> 
> 2         2011-11-07       2009-09-31                  C
> 
> 3         2012-01-05        2008-06-23                 C
> 
> 
> However, when I tried to apply this code in a function and then apply this function
> 
> to all dataframes using lapply() function , I do not get what I want.
> 
> 
> I wrote this function first :-
> 
> 
> MyFunction <- function(x) x$NewCol [ x[ ,3] <  x[ 1,2] ] <- "C"
> 
> 
> Then I wrote this code to apply my function to all dataframes in "Mylist" :
> 
> 
> NewList <- lapply(names(Mylist), function(x) MyFunction(Mylist[[x]]))
> 
> 
> This returned a list of 7000 elements and each of which contain "C'' letter. Each
> 
> dataframe has become a vector of "C'' letter which is totally away from what I need.
> 
> I expected to see a list of my 7000 dataframes and each of which looks like the output
> 
> I have shown above with the new column.
> 
> 
> I spent a lot of time trying to know what  is the mistake I have made in these last two codes
> 
> but was not able to know the issue.

A function returns the result of the last function call. In your case the last function called was `[<-` and if you look at that function's help page you will find only the value of its RHS (in your case "C") is returned. That assignment function has is predominat action via side-effect rather than by a truly functional operation.. The function might have been written:

 MyFunction <- function(x) { x$NewCol [ x[ ,3] <  x[ 1,2] ] <- "C"; x } # so that x gets returned

I say "might" since you have not included a reproducible example.

Another point: The `$` operator is not ideal for work within functions.

And. Noting this:

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David Winsemius
Alameda, CA, USA

'Any technology distinguishable from magic is insufficiently advanced.'   -Gehm's Corollary to Clarke's Third Law