How do I stop merge from converting characters into factors?

Question

E.g.

chr <- c("a", "b", "c")
intgr <- c(1, 2, 3)
str(chr)
str(base::merge(chr,intgr, stringsAsFactors = FALSE))

gives:

> str(base::merge(chr,intgr, stringsAsFactors = FALSE))
'data.frame':   9 obs. of  2 variables:
 $ x: Factor w/ 3 levels "a","b","c": 1 2 3 1 2 3 1 2 3
 $ y: num  1 1 1 2 2 2 3 3 3

I originally thought it has something to do with how merge coerces arguments into data frames. However, I thought that adding the argument stringsAsFactors = FALSE would override the default coercion behaviour of char -> factor, yet this is not working.

EDIT: Doing the following gives me expected behaviour:

options(stringsAsFactors = FALSE)
str(base::merge(chr,intgr))

that is:

> str(base::merge(chr,intgr))
'data.frame':   9 obs. of  2 variables:
 $ x: chr  "a" "b" "c" "a" ...
 $ y: num  1 1 1 2 2 2 3 3 3

but this is not ideal as it changes the global stringsAsFactors setting.

Answer 1

You can accomplish this particular "merge" using expand.grid(), since you're really just taking the cartesian product. This allows you to pass the stringsAsFactors argument:

sapply(expand.grid(x=chr,y=intgr,stringsAsFactors=F),class);
##           x           y
## "character"   "numeric"

---

Here's a way of working around this limitation of merge():

sapply(merge(data.frame(x=chr,stringsAsFactors=F),intgr),class);
##           x           y
## "character"   "numeric"

I would argue that it never makes sense to pass an atomic vector to merge(), since it is only really designed for merging data.frames.

Answer 2

We can use CJ from data.table as welll

library(data.table)
str(CJ(chr, intgr))
Classes ‘data.table’ and 'data.frame':  9 obs. of  2 variables:
#$ V1: chr  "a" "a" "a" "b" ...
#$ V2: num  1 2 3 1 2 3 1 2 3