Create an empty data.frame

Question

I\'m trying to initialize a data.frame without any rows. Basically, I want to specify the data types for each column and name them, but not have any rows created as a result

Answer 1

If you want to declare such a data.frame with many columns, it'll probably be a pain to type all the column classes out by hand. Especially if you can make use of rep, this approach is easy and fast (about 15% faster than the other solution that can be generalized like this):

If your desired column classes are in a vector colClasses, you can do the following:

library(data.table)
setnames(setDF(lapply(colClasses, function(x) eval(call(x)))), col.names)

lapply will result in a list of desired length, each element of which is simply an empty typed vector like numeric() or integer().

setDF converts this list by reference to a data.frame.

setnames adds the desired names by reference.

Speed comparison:

classes <- c("character", "numeric", "factor",
             "integer", "logical","raw", "complex")

NN <- 300
colClasses <- sample(classes, NN, replace = TRUE)
col.names <- paste0("V", 1:NN)

setDF(lapply(colClasses, function(x) eval(call(x))))

library(microbenchmark)
microbenchmark(times = 1000,
               read = read.table(text = "", colClasses = colClasses,
                                 col.names = col.names),
               DT = setnames(setDF(lapply(colClasses, function(x)
                 eval(call(x)))), col.names))
# Unit: milliseconds
#  expr      min       lq     mean   median       uq      max neval cld
#  read 2.598226 2.707445 3.247340 2.747835 2.800134 22.46545  1000   b
#    DT 2.257448 2.357754 2.895453 2.401408 2.453778 17.20883  1000  a 

It's also faster than using structure in a similar way:

microbenchmark(times = 1000,
               DT = setnames(setDF(lapply(colClasses, function(x)
                 eval(call(x)))), col.names),
               struct = eval(parse(text=paste0(
                 "structure(list(", 
                 paste(paste0(col.names, "=", 
                              colClasses, "()"), collapse = ","),
                 "), class = \"data.frame\")"))))
#Unit: milliseconds
#   expr      min       lq     mean   median       uq       max neval cld
#     DT 2.068121 2.167180 2.821868 2.211214 2.268569 143.70901  1000  a 
# struct 2.613944 2.723053 3.177748 2.767746 2.831422  21.44862  1000   b