Why does dplyr/tibble not merge factors with different levels?

[Benoit Daloze]

Hello,

This code:

a = data.frame(f=factor(c("a", "b")), g=c("a", "a"))
b = data.frame(f=factor(c("a", "c")), g=c("a", "a"))
a = a %>% group_by(g) %>% mutate(n=1)
b = b %>% group_by(g) %>% mutate(n=2)
rbind(a,b)

produces:

# A tibble: 4 x 3
# Groups:   g [1]
      f      g     n
  <chr> <fctr> <dbl>
1     a      a     1
2     b      a     1
3     a      a     2
4     c      a     2
Warning messages:
1: In bind_rows_(x, .id) : Unequal factor levels: coercing to character
2: In bind_rows_(x, .id) :
  binding character and factor vector, coercing into character vector
3: In bind_rows_(x, .id) :
  binding character and factor vector, coercing into character vector

What is the problem here? Could the factors not be trivially merged without any loss?

Using plain data.frame would merge the factor levels and produce no warnings.

Is there a reason for this rather strict behavior?

See https://stackoverflow.com/questions/46876312/how-to-merge-factors-when-binding-two-dataframes-together

for more context about this problem.

Best regards,

Benoit Daloze

[Patrick Kilduff]

Hi Benoit,

This happens because R ‘assumes’ that a column of class factor in a data.frame has all possible levels. In your example, the “a” and “c” levels of “f” are in separate data.frames. Because R doesn’t understand all the possible levels in each data.frame (i.e., “a”, “b”, and “c”), but only the ones that are present, it converts the “f” column in each data.frame to class character before combining the two. Character vectors don’t have that restriction.

Converting to factors to character before “rbind”-ing or merging, will get around this warning message. If you need to recode the column to factor for your analysis, after combining the data you can do:

out <- rbind(a, b)

out$f <- as.factor(out$f)

More details can be found here: http://adv-r.had.co.nz/Data-structures.html. 

Good luck,

Patrick

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