One of the basic data types in R is factors. In my experience factors are basically a pain and I never use them. I always convert to characters. I feel oddly like I'm missing something.
Are there some important examples of functions that use factors as grouping variables where the factor data type becomes necessary? Are there specific circumstances when I should be using factors?
Only with factors we can handle NAs by setting them as factor level. This is handy because many functions leave out NA values. Let's generate some toy data:
df <- data.frame(x= rnorm(10), g= c(sample(1:2, 9, replace= TRUE), NA))
If we want means of x grouped by g we can use
aggregate(x ~ g, df, mean)
g x
1 1 1.0415156
2 2 -0.3071171
As you can see we do not get the mean of x for the case where g is an NA. Same problem is true if we use by instead (see by(df$x, list(df$g), mean)). There are many other similiar examples where functions (by default or in general) do not consider NAs.
But we can add NA as a factor level. See here:
aggregate(x ~ addNA(g), df, mean)
addNA(g) x
1 1 -0.2907772
2 2 -0.2647040
3 <NA> 1.1647002
Yeah, we see the mean of x where g has NAs. One could argue that same output is possible with paste0 which is true (try aggregate(x ~ paste0(g), df, mean)). But only with addNA we can backtransform the NAs to actual missings. So let's firstly transform g with addNA and then backtransform it:
df$g_addNA <- addNA(df$g)
df$g_back <- factor(as.character(df$g_addNA))
[1] 2 2 1 1 1 2 2 1 1 <NA>
Levels: 1 2
Now the NAs in g_back are actual missings. See any(is.na(df$g_back)) which returns a TRUE.
This even works in strange situations where "NA" was a value in the original vector! For example, the vector vec <- c("a", "NA", NA) can be transformed using vec_addNA <- addNA(vec) and we can actually backtransform this with
as.character(vec_addNA)
[1] "a" "NA" NA
On the other hand, to my knowledge we can not backtransform vec_paste0 <- paste0(vec) because in vec_paste0 the "NA" and the NA are the same! See
vec_paste0
[1] "a" "NA" "NA"
I started the answer with "Only with factors we can handle NAs by setting them as factor level.". In fact I would be careful using addNA but regardless of the risk associated with addNA the fact stands that there is no similiar option for characters.
