fwrite(, append = TRUE) appends wrong way

Question

I am facing a problem with the fwrite function from the DataTable package in R. In fact it appends the wrong way and I'd end up with something like:

**user  ref version status  Type    DataExtraction**                    
user1   2.02E+11    1   Pending 1   No                  
user2   2.02E+11    1   Saved   2   No"user3"   2.01806E+11 1   Saved   NB  No

I am using the function as follows :

library(data.table)
fwrite(Save, "~/Downloads/Register.csv", append = TRUE, sep = ",", quote = TRUE)

Reproducible example:

fwrite(data.table(user="user3", 
                  ref="204094093", 
                  version="2", 
                  status="Pending", 
                  Type="1",DataExtraction="No"),
       "~/Downloads/test.csv", sep = ",", append = FALSE)

fwrite(data.table(user="user3", 
                  ref="204094093", 
                  version="2", 
                  status="Pending", 
                  Type="1",DataExtraction="No"),
       "~/Downloads/test.csv", sep = ",", append = TRUE)

I'm not sure if it isolates the problem, but it seems that if I manually change something in the .csv file (for instance rename DataExtraction to Extraction), the problem of appending in the wrong way occurs. Does someone know what is going wrong?

Thanks!

Answer 1

When I run your example code I have no problems with the behavior - the file comes out as desired. Based on your comments about manually changing what is in the file, and what the undesired output looks like, here is what I believe is happening. When fwrite() (and many other similar IO functions) write to a file, each line has at the end of it a line break (in R, this is generally represented as \n). This is desired, so that subsequent lines of data indeed appear on subsequent lines of the file. Generally this will also mean that when you open a file in a text editor, there will be a blank line at the very end, since this reflects the line break in the last line that was written. (different editors handle this differently though). So, I suspect what is happening is that when you go in and manually edit the file in your editor, you are somehow losing that last line break. What this means is that when you go to write again using append, there is no line break at the end of the file, and therefore you get the undesired behavior of two lines of data on a single line of the file.

So, the solution would be to either find how to prevent your manual editing from deleting that last line break character. Barring that, there are ways to just write a single line break character to the file using R. E.g. with the cat() function.