Blog posts on Data Science, Machine Learning, Data Mining, Artificial Intelligence, Spark Machine Learning

Sunday, May 25, 2014

Basic recommendation engine using R

In our day to day life, we come across a large number of Recommendation engines like Facebook Recommendation Engine for Friends’ suggestions, and suggestions of similar Like Pages, Youtube recommendation engine suggesting videos similar to our previous searches/preferences. In today’s blog post I will explain how to build a basic recommender System.

Types of Collaborative Filtering:

  1. User based Collaborative Filtering
  2. Item based Collaborative filtering
 In this post will explain about User based Collaborative Filtering. This algorithm usually works by searching a large group of people and finding a smaller set with tastes similar to yours. It looks at other things they like and combines them to create a ranked list of suggestions.

Implementing User Based Collaborative Filtering:
This involves two steps:
  1. Calculating Similarity Function 
  2. Recommend items to users based on user Similarity Score
Consider the below data sample of Movie critics and their movie rankings, the objective is to recommend the unrated movies based on similar users:

Step1- Calculate Similarity Score for CHAN:

Creating Similarity score for people helps us to identify similar people. We use Cosine based Similarity function to calculate the similarity between the users. Know more about cosine similarity here. In R we have a cosine function readily available:
user_sim = cosine(as.matrix(t(x)))

Step2- recommending Movies for CHAN:

For recommending movies for Chan using the above similarity matrix, we need to first fill the N/A where he has not rated. As first step, separate the non-rated movies by Chan and a weighted matrix is created by multiplying user similarity score (user_sim[,7]) with ratings given by other users.
Next step is to sum up all the columns of the weight matrix, then divide by the sum of all the similarities for critics that reviewed that movie. The result calculation gives what the user might rate this movie, the results as below:
The above explanation is written in the below R function:
rec_itm_for_user = function(userNo) 
{ #calcualte column wise sum 
col_sums= list()
 rat_user = critics[userNo,2:7]
tot = list()
 for(i in 1:ncol(rat_user)){ 
 col_sums[x] = sum(weight_mat[,i],na.rm=TRUE)
 temp =[,i])
 for(j in 1:nrow(temp))
{ if(![j,1]))
 sum_temp = sum_temp+user_sim[j,7]
 tot[z] = sum_temp z=z+1 
 for(i in 1:ncol(rat_user)){ 
 rat_user[1,i] = col_sums[[z]]/tot[[z]] z=z+1 
Calling the above function gives the below results:

Titanic Batman Inception Superman.Returns spiderMan Matrix

2.811   4.5     2.355783           4            1    3.481427
Recommending movies for Chan will be in the order: Matrix (3.48), Titanic(2.81), Inception(2.35).
complete sourceCode is available on github


  1. If you are facing any issue in your Epson Printer like Epson Error Code 0xF3 then you can resolve this issue by the help of expert’s technicians of Epson Printer or you may dial their toll-free i.e 1-888-500-9609.

  2. Attend The Data Science Training in Bangalore From ExcelR. Practical Data Science Training in Bangalore Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Data Science Courses in Bangalore.
    ExcelR business analytics courses

  3. Thanks for sharing this useful information! Hope that you will continue with the kind of stuff you are doing. If you want you can also read this blog which talk about how to Fix Windows 7 Update Error 8007000e in a very easy way.

  4. Great article like this require readers to think as they read. I took my time when going through the points made in this article. I agree with much this information.
    Best Data Science training in Mumbai

    Data Science training in Mumbai

  5. I feel very grateful that I read this. It is very helpful and very informative and I really learned a lot from it.
    360digitmg data science institutes

  6. The content is well recognized, so no one could claim that it is just one person's opinion yet it covers and justifies all the valid points. Hope to read some more work from you.
    SAP training in Kolkata
    SAP training Kolkata
    Best SAP training in Kolkata
    SAP course in Kolkata


  7. What a useful information provided by this blog! It’s remarkable. Thanks for helping me out… I will definitely get in touch with your blogs,
    Epson printer in error state

  8. Attend The Business Analytics Courses From ExcelR. Practical Business Analytics Courses Sessions With Assured Placement Support From Experienced Faculty. ExcelR Offers The Data Analytics Courses.
    Business Analytics Courses