FR for `fread`: if `select` is used, `colClasses` need only correspond to the columns in `select`

[MichaelChirico]

I never filed a FR for a question I raised a year ago on SO.

The current canon for using select and colClasses simultaneously is (IMO) unwieldy.

Consider:

#file to read
ffile <- paste0(paste(paste0("V", 1:20), collapse = ","),
              "\na,b,c,d,e,1,2,3,4,5,1.1,1.2,1.3,1.4,1.5,",
              "TRUE,FALSE,TRUE,FALSE,TRUE")

#columns to take
sel <- c(2, 10, 13:15, 20)
#types of all columns
tps <- rep(c("character", "integer",
           "numeric", "logical"),
         rep(5, 4))

Here's the best I could come up with as a programmatic way to use fread:

DT <- fread(ffile, select = paste0("V", sel),
            colClasses =
              sapply(unique(tps[sel]),
                     function(x) paste0("V", sel[which(tps[sel] == x)])))

(gross; could be spelled out explicitly as the following, but this is generally unsatisfying:)

DT <- fread(ffile, select = paste0("V", sel),
            colClasses =
              list(character = "V1", integer = "V10",
                   numeric = c("V13","V14","V15"),
                   logical = c("V20")))

To me it would make much more sense to be able to simply write:

DT <- fread(ffile, select = paste0("V", sel), colClasses = tps[sel])

But this currently produces the error:

Error in fread(ffile, select = paste0("V", sel), colClasses = tps[sel]) :
colClasses is unnamed and length 6 but there are 20 columns. See ?data.table for colClasses usage.

Parsimonious, and as far as I can tell unambiguous. Any reason why this wouldn't work?

[MichaelChirico]

What about just reversing the order of dealing with select vs. colClasses -- switch Lines #1141 - 1157 with Lines #1059 - 1112, and reset ncol to length(select) within the select condition?

[dselivanov]

+1 for this, also annoying me. Will try to check.

[arunsrinivasan]

Quite a few issues marked for v1.9.8 already. Can't take a look anytime soon. Glad if you could look into this. Thanks.

[renkun-ken]

Really need this.