Question

I have a data frame like this:

 GN  SN  
  a   b   
  a   b   
  a   c   
  d   e   
  d   f 
  d   e

I would like the following output:

GN: a SN: 2 b 1 c
GN d SN: 2 e 1 f

In other words I would like to have a sort of table() of the data.frame on the column S.N. First of all I split the data.frame according to $GN, so I have blocks. At this point I’ m not able to have the counting of the elements on column SN according to the split I’ve done. Is the "apply" function a way to do this? And how can I save a general output belonging from split function?

Answer 1

With your data:

df <- data.frame(GN = rep(c("a","b"), each = 3),
                 SN = c(rep("b", 2), "c", "e", "f", "e"))

We could do:

> lapply(with(df, split(SN, GN)), table)
$a

b c e f 
2 1 0 0 

$b

b c e f 
0 0 2 1

But if you don’t want all the levels (the 0 entries) then we need to drop the empty levels:

> lapply(with(df, split(SN, GN)), function(x) table(droplevels(x)))
$a

b c 
2 1 

$b

e f 
2 1

Writing out the individual tables to a file

This isn’t perfect but at least you can work with it

## save tables
tmp <- lapply(with(df, split(SN, GN)), function(x) table(droplevels(x)))

## function to write output to file `fname`
foo <- function(x, fname) {
    cat(paste(names(x), collapse = " "), "\n", file = fname, append = TRUE)
    cat(paste(x, collapse = " "), "\n", file = fname, append = TRUE)
    invisible()
}

fname <- "foo.txt"
file.create(fname)                # create file fname
lapply(tmp, foo, fname = fname)   # run our function to write to fname

That gives:

R> readLines(fname)
[1] "b c " "2 1 " "e f " "2 1 "

or from the OS:

$ cat foo.txt
b c 
2 1 
e f 
2 1