Why do I get "invalid 'type' (character) of argument" using xtab() in R

The error…

Error in FUN(X[[i]], ...) : invalid 'type' (character) of argument

…is not new and I can find a lot of SO questions about it. But because of low quality MWE’s I’m not able to fit the answers to my own problem.

I’m interested not only in a quick code based solution but in a deeper understand why this error happens.

set.seed(0)

df <- data.frame(
    gender = sample(c("female", "male"), 100, replace = TRUE),
    foo = sample(c(TRUE, FALSE), 100, replace = TRUE),
    bar = sample(c(0, 1, 2, 3, 4), 100, replace = TRUE)
)

xtabs(gender ~ foo, data=df)

>Solution :

The left hand side of the formula is usually empty. If you have something there, it should be a vector or matrix of counts. You have gender there, which is a character variable and the error message comes when xtabs() tries to interpret it as counts.

So to fix this, just put the two factors on the right hand side:

xtabs(~ gender + foo, data=df)

BTW, even though neither one of those columns is actually a "factor", xtabs() will automatically convert them.