Question

Just had a conversation with coworkers about this, and we thought it’d be worth seeing what people out in SO land had to say. Suppose I had a list with N elements, where each element was a vector of length X. Now suppose I wanted to transform that into a data.frame. As with most things in R, there are multiple ways of skinning the proverbial cat, such as as.dataframe, using the plyr package, comboing do.call with cbind, pre-allocating the DF and filling it in, and others.

The problem that was presented was what happens when either N or X (in our case it is X) becomes extremely large. Is there one cat skinning method that’s notably superior when efficiency (particularly in terms of memory) is of the essence?

Answer 1

Since a data.frame is already a list and you know that each list element is the same length (X), the fastest thing would probably be to just update the class and row.names attributes:

set.seed(21)
n <- 1e6
x <- list(x=rnorm(n), y=rnorm(n), z=rnorm(n))
x <- c(x,x,x,x,x,x)

system.time(a <- as.data.frame(x))
system.time(b <- do.call(data.frame,x))
system.time({
  d <- x  # Skip 'c' so Joris doesn't down-vote me! ;-)
  class(d) <- "data.frame"
  rownames(d) <- 1:n
  names(d) <- make.unique(names(d))
})

identical(a, b)  # TRUE
identical(b, d)  # TRUE

Update – this is ~2x faster than creating d:

system.time({
  e <- x
  attr(e, "row.names") <- c(NA_integer_,n)
  attr(e, "class") <- "data.frame"
  attr(e, "names") <- make.names(names(e), unique=TRUE)
})

identical(d, e)  # TRUE

Update 2 – I forgot about memory consumption. The last update makes two copies of e. Using the attributes function reduces that to only one copy.

set.seed(21)
f <- list(x=rnorm(n), y=rnorm(n), z=rnorm(n))
f <- c(f,f,f,f,f,f)
tracemem(f)
system.time({  # makes 2 copies
  attr(f, "row.names") <- c(NA_integer_,n)
  attr(f, "class") <- "data.frame"
  attr(f, "names") <- make.names(names(f), unique=TRUE)
})

set.seed(21)
g <- list(x=rnorm(n), y=rnorm(n), z=rnorm(n))
g <- c(g,g,g,g,g,g)
tracemem(g)
system.time({  # only makes 1 copy
  attributes(g) <- list(row.names=c(NA_integer_,n),
    class="data.frame", names=make.names(names(g), unique=TRUE))
})

identical(f,g)  # TRUE