R define dimensions of empty data frame

Question

I am trying to collect some data from multiple subsets of a data set and need to create a data frame to collect the results. My problem is don\'t know how to create an empt

Answer 1

It might help the solution given in another forum, Basically is: i.e.

Cols <- paste("A", 1:5, sep="")
DF <- read.table(textConnection(""), col.names = Cols,colClasses = "character")

> str(DF)
'data.frame':   0 obs. of  5 variables:
$ A1: chr
$ A2: chr
$ A3: chr
$ A4: chr
$ A5: chr

You can change the colClasses to fit your needs.

Original link is https://stat.ethz.ch/pipermail/r-help/2008-August/169966.html

Answer 2

seq_along may help to find out how many rows in your data file and create a data.frame with the desired number of rows

    listdf <- data.frame(ID=seq_along(df),
                              var1=seq_along(df), var2=seq_along(df))

Answer 3

You can do something like:

N <- 10
collect1 <- data.frame(id   = integer(N),
                       max1 = numeric(N),
                       min1 = numeric(N))

Now be careful that in the rest of your code, you forgot to use the row index for filling the data.frame row by row. It should be:

for(i in seq_len(N)){
   collect1$id[i] <- i
   ss1 <- subset(df1, df1$id == i)
   collect1$max1[i] <- max(ss1$value)
   collect1$min1[i] <- min(ss1$value)
}

Finally, I would say that there are many alternatives for doing what you are trying to accomplish, some would be much more efficient and use a lot less typing. You could for example look at the aggregate function, or ddply from the plyr package.

Answer 4

Would a dataframe of NAs work? something like:

data.frame(matrix(NA, nrow = 2, ncol = 3))

if you need to be more specific about the data type then may prefer: NA_integer_, NA_real_, NA_complex_, or NA_character_ instead of just NA which is logical

Something else that may be more specific that the NAs is:

data.frame(matrix(vector(mode = 'numeric',length = 6), nrow = 2, ncol = 3))

where the mode can be of any type. See ?vector

Answer 5

A more general method to create an arbitrary size data frame is to create a n-by-1 data-frame from a matrix of the same dimension. Then, you can immediately drop the first row:

> v <- data.frame(matrix(NA, nrow=1, ncol=10))
> v <- v[-1, , drop=FALSE]
> v
 [1] X1  X2  X3  X4  X5  X6  X7  X8  X9  X10
<0 rows> (or 0-length row.names)