Add column with counts of another [duplicate]

Answer 1

And a data.table version for good measure.

library(data.table)
df <- as.data.table(df)

Once you have the data.table, it's then a simple operation:

df[,count := .N,by="gender"]
df

#   gender age count
#1:      m  18     4
#2:      f  14     2
#3:      m  18     4
#4:      m  18     4
#5:      m  15     4
#6:      f  15     2