Can I replace NAs when joining two data frames with dplyr?

by

coalesce might be something you need. It fills the NA from the first vector with values from the second vector at corresponding positions:

library(dplyr)
df1 %>% 
        left_join(df2, by = "fruit") %>% 
        mutate(var2 = coalesce(var2.x, var2.y)) %>% 
        select(-var2.x, -var2.y)

#     fruit var1 var3 var2
# 1  apples    1   NA    3
# 2 oranges    2    7    5
# 3 bananas    3   NA    6
# 4  grapes    4    8    6

Or use data.table, which does in-place replacing:

library(data.table)
setDT(df1)[setDT(df2), on = "fruit", `:=` (var2 = i.var2, var3 = i.var3)]
df1
#      fruit var1 var2 var3
# 1:  apples    1    3   NA
# 2: oranges    2    5    7
# 3: bananas    3    6   NA
# 4:  grapes    4    6    8

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