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.
Este exercício faz parte do curso
Data Manipulation with data.table in R
Instruções do exercício
Find the total number of unique start stations and zip codes for each month, using start_date to extract the month.
Exercício interativo prático
Experimente este exercício completando este código de exemplo.
# 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