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Problem :

I am very new to R. Now a days if I try to use the join function in a dplyr package then I often face following warning:

Warning message:
In left_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector
I am unable to find any relevant information about above warning. I need expert help from R expert to fix above warning.
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1 Answer

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Solution :

I have good exposure to R so I am trying to help you. The above warning is clearly informing you that one of the columns which are being used in your join is clearly a factor and also that factor had clearly the different levels in your different datasets. So if you do not want to lose any of your information then your factors should be converted to the character values. Please follow below example:

library(dplyr)
x<-data.frame(a=letters[1:7])
y<-data.frame(a=letters[4:10])

class(x$a) 
# [1] "factor"

# NOTE these are very different
levels(x$a)
# [1] "a" "b" "c" "d" "e" "f" "g"
levels(y$a)
# [1] "d" "e" "f" "g" "h" "i" "j"

m <- left_join(x,y)
# Joining by: "a"
# Warning message:
# joining factors with different levels, coercing to character vector 

class(m$a)
# [1] "character"

You must make sure that both of your factors have a same level before merging them

combined <- sort(union(levels(x$a), levels(y$a)))
n <- left_join(mutate(x, a=factor(a, levels=combined)),
    mutate(y, a=factor(a, levels=combined)))
# Joining by: "a"
class(n$a)
#[1] "factor"

 Above solution will surely fix your problem related to R.

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