R: Only keep the 3 (x) first characters in a all rows in a column?

Question

I have imported a few thousand xls files into a data.frame and I added a column with the filename.

Thus I have the data

data1  data2  data3  filname
A      A2     A3     301fg.xls
B      B2     B3     302gfg.xls
C      C2     C3     303gfsddf.xls
.,.,.,.

I want to renamne the names in the filename column to only contain the 3 first characters/digits thus giving:

data1  data2  data3  filname
A      A2     A3     301
B      B2     B3     302
C      C2     C3     303
.,.,.,.

Answer 1

df$filname <- sub("^(\\d{3}).*$", "\\1", df$filname)

要么

df$filname <- substr(df$filname, 0, 3)

Answer 2

@lukeA has posted the most logical answer to this, but you could also use read.fwf :

> read.fwf(textConnection(mydf$filname), 3)
   V1
1 301
2 302
3 303

Depending on the str of your data, it might need to be read.fwf(textConnection(as.character(mydf$filname)), 3) if mydf$filename is a factor variable.