dataframe - Creating/Populating Empty Data Frames in R

Question

I am working with R. I found this link here on creating empty data frames in R: Create an empty data.frame .

I tried to do something similar:

df <- data.frame(Date=as.Date(character()),
                 country=factor(), 
                 total=numeric(), 
                 stringsAsFactors=FALSE) 

Yet, when I try to populate it:

df$total = 7

I get the following error:

Error in `$<-.data.frame`(`*tmp*`, total, value = 7) : 
  replacement has 1 row, data has 0

df[1, "total"] <- rnorm(100,100,100)

Error in `[<-.data.frame`(`*tmp*`, 1, "total", value = c(-79.4584309347689,  : 
  replacement has 100 rows, data has 1

Does anyone know how to fix this error?

Thanks

Answer 1

An option is to specify the row index

df[1, "total"] <- 7

-output

str(df)
#'data.frame':  1 obs. of  3 variables:
# $ Date   : Date, format: NA
# $ country: Factor w/ 0 levels: NA
# $ total  : num 7

---

The issue is that when we select a single column and assign on a 0 row dataset, it is not automatically expanding the row for other columns. By specifying the row index, other columns will automatically filled with default NA

Regarding the second question (updated), a standard data.frame column is a vector and the length of the vector should be the same as the index we are specifying. Suppose, we want to expand to 100 rows, change the index accordingly

df[1:100, "total"] <- rnorm(100, 100, 100) # length is 100 here
dim(df)
#[1] 100   3

Or if we need to cram everything in a single row, then wrap the rnorm in a list

df[1, "total"] <- list(rnorm(100, 100, 100))

In short, the lhs should be of the same length as the rhs. Another case is when we are assigning from a different dataset

df[seq_along(aa$bb), "total"] <- aa$bb 

---

This can also be done without initialization i.e.

df <- data.frame(total = aa$bb)