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.
This exercise is part of the course
Data Manipulation with data.table in R
Exercise instructions
Find the total number of unique start stations and zip codes for each month, using start_date to extract the month.
Hands-on interactive exercise
Have a go at this exercise by completing this sample code.
# 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