R - number of unique values in a column of data frame

Question

for a dataframe df, I need to find the unique values for some_col. Tried the following

length(unique(df["some_col"]))

but this is not giving the expected results. However length(unique(some_vector)) works on a vector and gives the expected results.

Some preceding steps while the df is created

df <- read.csv(file, header=T) typeof(df) #=> "list" typeof(unique(df["some_col"])) #=> "list" length(unique(df["some_col"])) #=> 1  

Answer 1

Try with [[ instead of [. [ returns a list (a data.frame in fact), [[ returns a vector.

df <- data.frame( some_col = c(1,2,3,4),                   another_col = c(4,5,6,7) )  length(unique(df[["some_col"]])) #[1] 4  class( df[["some_col"]] ) [1] "numeric"  class( df["some_col"] ) [1] "data.frame" 

You're getting a value of 1 because the list is of length 1 (1 column), even though that 1 element contains several values.

Answer 2

you need to use

length(unique(unlist(df[c("some_col")]))) 

When you call column by df[c("some_col")] or by df["some_col"] ; it pulls it as a list. Unlist will convert it into the vector and you can work easily with it. When you call column by df$some_col .. it pulls the data column as vector