Collaborative Filtering for fun Elissa Brown Erin Shellman Stella Rowlett

…and profit!💖 😹💻

How’s it done?‣ Collaborative filtering!

‣ Look for people who like the stuff you like, and recommend the things they’ve rated positively that you haven’t seen yet.

Leaf Shorts Floral ShortsElissa 5 5

Erin 2 5

Stella 4 1

Step 1: Collect ratings

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5

2 3 4 5Leaf Shorts

Flor

al S

horts Person

ElissaErinStella

Step 2: Find someone similar for Mr. New Customer‣ A new customer wants to

buy a pair of shorts for his new “girlfriend.”

‣Which of us is most similar to Justin?

💋

💍

💘

💌

Finding the nearest neighbor‣Remember the

Pythagorean Theorem?

a2 + b2 = c2

c =p

a2 + b2p(x1 � x2)2 + (y1 � y2)2

p(1� 5)2 + (4� 5)2 = 4.12

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1 2 3 4 5Leaf Shorts

Flor

al S

horts

PersonElissaErinJustinStella

What’s the distance between Justin and Stella?

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1 2 3 4 5Leaf Shorts

Flor

al S

horts

PersonElissaErinJustinStella?

What’s the distance between Justin and Stella?

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5

1 2 3 4 5Leaf Shorts

Flor

al S

horts

PersonElissaErinJustinStella

p(1� 4)2 + (1� 4)2 = 4.24

Meant to be!‣Justin and Erin are a

match made in heaven.

‣How can this help us shop for Justin’s girlfriend (not Erin)?p

(1� 2)2 + (4� 5)2 = 1.41

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1 2 3 4 5Leaf Shorts

Flor

al S

horts

PersonElissaErinJustinStella

💘

Leaf Shorts Floral Shorts Jungle ShortsElissa 5 5 1

Erin 2 5 5

Stella 4 1 2

Justin 1 4 ?

Step 3: Make a suggestion!

Let’s go shopping!💃

How should we store our ratings data?

How should we store our ratings data?

A dictionary!

# Store user ratings user_ratings = {"Elissa": {"Leaf Shorts": 5, "Floral Shorts": 5}, "Erin": {"Leaf Shorts": 2, "Floral Shorts": 5}, "Stella": {"Leaf Shorts": 4, "Floral Shorts": 1} }

Python Warm-up!# Store user ratings user_ratings = {"Elissa": {"Leaf Shorts": 5, "Floral Shorts": 5}, "Erin": {"Leaf Shorts": 2, "Floral Shorts": 5}, "Stella": {"Leaf Shorts": 4, "Floral Shorts": 1} } !# Print Stella's ratings. print user_ratings['Stella'] !# What did Elissa think of the floral ones? print user_ratings['Elissa']['Leaf Shorts'] !def what_did_they_think(rating): if rating > 3: opinion = "LOVED IT!" else: opinion = "HATED IT!" return opinion !my_humble_opinion = what_did_they_think(user_ratings['Erin']['Leaf Shorts']) !print "Erin's review of the Leaf Shorts: " + my_humble_opinion !!

What functions do we need to build a recommender?

What functions do we need to build a recommender?1. Function to compute distances

2. Function to find nearby people

3. Function to recommend items I haven’t rated yet

Pseudocode: Function to compute distances

compute_distancedef compute_distance(user1_ratings, user2_ratings): """ This function computes the distance between two user's ratings. Both arguments should be dictionaries keyed on users, and items. """ distances = [] for key in user1_ratings: if key in user2_ratings: distances.append((user1_ratings[key] - user2_ratings[key]) ** 2) total_distance = round(sum(distances) ** 0.5, 2) return total_distance

Pseudocode: Function to find closest match

find_nearest_neighborsdef find_nearest_neighbors(username, user_ratings): """ Returns the list of neighbors, ordered by distance. Call like this: find_nearest_neighbors('Erin', user_ratings) """ distances = [] for user in user_ratings: if user != username: distance = compute_distance(user_ratings[user], user_ratings[username]) distances.append((distance, user)) distances.sort() return distances

Pseudocode: Function to make recommendations

get_recommendationsdef get_recommendations(username, user_ratings): """ Return a list of recommendations. """ nearest_users = find_nearest_neighbors(username, user_ratings) recommendations = [] ! # Input user's ratings ratings = user_ratings[username] ! for neighbor in nearest_users: neighbor_name = neighbor[1] for item in user_ratings[neighbor_name]: if not item in ratings: recommendations.append((item, user_ratings[neighbor_name][item])) ! return sorted(recommendations, key = lambda personTuple: personTuple[1], reverse = True)

Limitations‣ What happens if everyone rated the

same group of items?

‣ What if there’s no overlap?

‣ What about new users with no ratings?

Now what?‣Ask your classmates to rate their electives,

and make a recommender to help students pick their classes.

‣Poll recent graduates from your school about their college, and make a recommender to help students pick colleges.

https://github.com/erinshellman/girls-who-code-recommender

🐱