[R] How to count the number of different elements in a column

[Hard Core]

Hello,
Suppose that i have a dataframe
a <- read.dta("banca_impresa.dta")

i have a column with 17900 obs like 

1
2
3
1
6
7
8
3
4
4

and i want to know the number of the different values so in this case it
would be 7 
How can i do? 
Thank you



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