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

[Allaisone 1]

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.


Could you please let me know my mistake and how to correct my syntax ?


Kind Regards

Allaisone








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