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.
Cet exercice fait partie du cours
Data Manipulation with data.table in R
Instructions
Find the total number of unique start stations and zip codes for each month, using start_date to extract the month.
Exercice interactif pratique
Essayez cet exercice en complétant cet exemple de 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