读取csv文件时使用colClasses时R中的警告消息

Question

I am using lapply to read a list of files. The files have multiple rows and columns, and I interested in the first row in the first column. The code I am using is:

lapply(file_list, read.csv,sep=',', header = F, col.names=F, nrow=1, colClasses = c('character', 'NULL', 'NULL'))

The first row has three columns but I am only reading the first one. From other posts on stackoverflow I found that the way to do this would be to use colClasses = c('character', 'NULL', 'NULL') . While this approach is working, I would like to know the underlying issue that is causing the following error message to be generated and hopefully prevent it from popping up:

"In read.table(file = file, header = header, sep = sep, quote = quote, : cols = 1 != length(data) = 3"

Answer 1

It's to let you know that you're just keeping one column of the data out of three because it doesn't know how to handle colClasses of "NULL" . Note your NULL is in quotation marks.

An example:

write.csv(data.frame(fi=letters[1:3],
                            fy=rnorm(3,500,1),
                            fo=rnorm(3,50,2))
,file="a.csv",row.names = F)

write.csv(data.frame(fib=letters[2:4],
                     fyb=rnorm(3,5,1),
                     fob=rnorm(3,50,2))
          ,file="b.csv",row.names = F)

file_list=list("a.csv","b.csv")

lapply(file_list, read.csv,sep=',', header = F, col.names=F, nrow=1, colClasses = c('character', 'NULL', 'NULL'))

Which results in:

[[1]]
  FALSE.
1     fi

[[2]]
  FALSE.
1    fib

Warning messages:
1: In read.table(file = file, header = header, sep = sep, quote = quote,  :
  cols = 1 != length(data) = 3

Which is the same as if you used:

lapply(file_list, read.csv,sep=',', header = F, col.names=F,
 nrow=1, colClasses = c('character', 'asdasd', 'asdasd'))

But the warning goes away (and you get the rest of the row as a result) if you do:

lapply(file_list, read.csv,sep=',', header = F, col.names=F,
  nrow=1, colClasses = c( 'character',NULL, NULL))

You can see where errors and warnings come from in source code for a function by entering, for example, read.table directly without anything following it, then searching for your particular warning within it.