How to concatenate factors, without them being converted to integer level?

Question

I was surprised to see that R will coerce factors into a number when concatenating vectors. This happens even when the levels are the same. For example:

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Answer 1

This is a really bad R gotcha. Along those lines, here's one that just swallowed several hours of my time.

x <- factor(c("Yes","Yes","No", "No", "Yes", "No"))
y <- c("Yes", x)

> y
[1] "Yes" "2"   "2"   "1"   "1"   "2"   "1"  
> is.factor(y)
[1] FALSE

It appears to me the better fix is Richie's, which coerces to character.

> y <- c("Yes", as.character(x))
> y
[1] "Yes" "Yes" "Yes" "No"  "No"  "Yes" "No" 
> y <- as.factor(y)
> y
[1] Yes Yes Yes No  No  Yes No 
Levels: No Yes

As long as you get the levels set properly, as Richie mentions.