How to add count of unique values by group to R data.frame

Question

I wish to count the number of unique values by grouping of a second variable, and then add the count to the existing data.frame as a new column. For example, if the existing

Answer 1

This can be also achieved in a vectorized without by group operations by combining unique with table or tabulate

If df$color is factor, then

Either

table(unique(df)$color)[as.character(df$color)]
# black black black green green   red   red  blue  blue  blue 
#    2     2     2     1     1     2     2     3     3     3 

Or

tabulate(unique(df)$color)[as.integer(df$color)]
# [1] 2 2 2 1 1 2 2 3 3 3

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If df$color is character then just

table(unique(df)$color)[df$color]

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If df$color is an integer then just

tabulate(unique(df)$color)[df$color]