Create dataframe from objects with some NULL values in R

Question

I have n objects that I want to combine together as a 1-row by n-column data frame. However, some of the objects may be NULL or empty strings and I would like the names of the objects to become the names of the dataframe.

So if I have

a <- 1
b <- 2
c <- 3
d <- NULL
e <- 'text'
f <- character(0)

I would like to do something like:

mydf <- data.frame(a, b, c, d, e)

and have the NULL values dropped when creating the data frame:

> mydf
  a b c    e
1 1 2 3 text

However, if I try it with the NULL objects and empty strings I get the following error:

Error in data.frame(a, b, c, d, e) : 
  arguments imply differing number of rows: 1, 0

And if I write some sort of function to filter out NULL objects, I lose the names of my objects and get bizarre column names.

Thanks for the help!

You can place it in a list i.e. list(a, b, c, d, e, f) or create a list column in data.frame ie. data.frame(a, b, c, d= I(list(d)), e) — akrun
– akrun, Commented Jul 11, 2017 at 1:14
As alistaire said, use NA instead. The reasoning is that assigning NULL to a variable or data frame column tells R to remove that variable or column from your workspace. It is not the same value as null in other languages. — sconfluentus
– sconfluentus, Commented Jul 11, 2017 at 1:24

G. Grothendieck · Accepted Answer · 2017-07-11 03:13:30Z

3

Try this:

L <- list(a = a, b = b, c = c, d = d, e = e, f = f)
DF <- as.data.frame(matrix(L, 1, dimnames = list(NULL, names(L))))
DF
##   a b c    d    e f
## 1 1 2 3 NULL text  

str(DF)
## 'data.frame':   1 obs. of  6 variables:
##  $ a:List of 1
##   ..$ a: num 1
##  $ b:List of 1
##   ..$ b: num 2
##  $ c:List of 1
##   ..$ c: num 3
##  $ d:List of 1
##   ..$ d: NULL
##  $ e:List of 1
##   ..$ e: chr "text"
##  $ f:List of 1
##   ..$ f: chr

Also note that the matrix m looks like this:

str(m)
## List of 6
##  $ : num 1
##  $ : num 2
##  $ : num 3
##  $ : NULL
##  $ : chr "text"
##  $ : chr(0) 
##  - attr(*, "dim")= int [1:2] 1 6
##  - attr(*, "dimnames")=List of 2
##   ..$ : NULL
##   ..$ : chr [1:6] "a" "b" "c" "d" ...

edited Jul 11, 2017 at 3:13

answered Jul 11, 2017 at 1:23

G. Grothendieck

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