def filter_by_popularity(self, center, attractions):
'''
Filters attractions using knapsack.
weights = a weighted average of estimated time spent at location and distance from center
values = a weighted average of num checkins and overall star rating
'''
places, weights, values = [], [], []
for attraction in attractions:
place = \
Place(attraction["name"], \
attraction["location"]["latitude"], \
attraction["location"]["longitude"], \
attraction)
val = place.calculate_value(attraction["checkins"],
attraction["overall_star_rating"])
weight = place.calculate_weight(center)
values.append(val)
weights.append(weight)
places.append(place)
return self.knapsack(5000000, weights, values, len(places), places)
