pandas: count things

Question

In the following, male_trips is a big pandas data frame and stations is a small pandas data frame. For each station id I\'d like to know how many male trips took place. The

Answer 1

My answer below works in Pandas 0.7.3. Not sure about the new releases.

This is what the pandas.Series.value_counts method is for:

count_series = male_trips.start_station_id.value_counts()

It should be straight-forward to then inspect count_series based on the values in stations['id']. However, if you insist on only considering those values, you could do the following:

count_series = (
                male_trips[male_trips.start_station_id.isin(stations.id.values)]
                    .start_station_id
                    .value_counts()
               )

and this will only give counts for station IDs actually found in stations.id.