Using .SD (II)
You can also apply a single function to every column specified in .SDcols using lapply(). In this exercise, you will calculate the number of unique start stations and zip codes for each month using uniqueN(). uniqueN() returns the number of unique elements in a column/vector/data.table.
Questo esercizio fa parte del corso
Data Manipulation with data.table in R
Istruzioni dell'esercizio
Find the total number of unique start stations and zip codes for each month, using start_date to extract the month.
Esercizio pratico interattivo
Prova a risolvere questo esercizio completando il codice di esempio.
# Find the total number of unique start stations and zip codes per month
unique_station_month <- batrips[, ___(___, ___),
by = ___,
.SDcols = c("start_station", "zip_code")]
unique_station_month