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日期:2021-01-13 08:25

Main Part 2 (Scala, 7 Marks)
You are asked to implement a Scala program for recommending movies accord?
ing to a ratings list. This part is due on 15 January at 5pm.
? Important
? Make sure the files you submit can be processed by just calling
scala <> on the commandline.1 Use the template files
provided and do not make any changes to arguments of functions or to
any types. You are free to implement any auxiliary function you might
need.
? Do not leave any test cases running in your code because this might
slow down your program! Comment out test cases before submission,
otherwise you might hit a time?out.
? Do not use any mutable data structures in your submissions! They are
not needed. This means you cannot create new Arrays or ListBuffers,
for example.
? Do not use return in your code! It has a different meaning in Scala than
in Java. It changes the meaning of your program, and you should never
use it.
? Do not use var! This declares a mutable variable. Only use val!
? Do not use any parallel collections! No .par therefore! Our testing and
marking infrastructure is not set up for it.
Also note that the running time of each part will be restricted to a maximum of
30 seconds on my laptop.
? Disclaimer
It should be understood that the work you submit represents your own effort!
You have not copied from anyone else. An exception is the Scala code I showed
during the lectures or uploaded to KEATS, which you can freely use.
1All major OSes, including Windows, have a commandline. So there is no good reason to not
download Scala, install it and run it on your own computer. Just do it!
1
Reference Implementation
Like the C++ part, the Scala part works like this: you push your files to GitHub
and receive (after sometimes a long delay) some automated feedback. In the
end we will take a snapshot of the submitted files and apply an automated
marking script to them.
In addition, the Scala part comes with reference implementations in form of
jar?files. This allows you to run any test cases on your own computer. For ex?
ample you can call Scala on the command line with the option ‐cp danube.jar
and then query any function from the template file. Say you want to find out
what the function produces: for this you just need to prefix it with the object
name CW7b. If you want to find out what these functions produce for the list
List("a", "b", "b"), you would type something like:
$ scala ‐cp danube.jar
scala> val ratings_url =
| """https://nms.kcl.ac.uk/christian.urban/ratings.csv"""
scala> CW7b.get_csv_url(ratings_url)
val res0: List[String] = List(1,1,4 ...)
Hints
Use .split(",").toList for splitting strings according to commas (similarly
for the newline character \n), .getOrElse(..,..) allows to query a Map, but
also gives a default value if the Map is not defined, a Map can be ‘updated’ by
using +, .contains and .filter can test whether an element is included in a
list, and respectively filter out elements in a list, .sortBy(_._2) sorts a list of
pairs according to the second elements in the pairs—the sorting is done from
smallest to highest, .take(n) for taking some elements in a list (takes fewer if
the list contains less than n elements).
2
Main Part 2 (7 Marks, file danube.scala)
You are creating Danube.co.uk which you hope will be the next big thing in
online movie renting. You know that you can save money by anticipating what
movies people will rent; you will pass these savings on to your users by offering
a discount if they rent movies that Danube.co.uk recommends.
Your task is to generate two movie recommendations for every movie a user
rents. To do this, you calculate what other renters, who also watched this
movie, suggest by giving positive ratings. Of course, some suggestions are
more popular than others. You need to find the two most?frequently suggested
movies. Return fewer recommendations, if there are fewer movies suggested.
The calculations will be based on the small datasets which the research lab
GroupLens provides for education and development purposes.
https://grouplens.org/datasets/movielens/
The slightly adapted CSV?files should be downloaded in your Scala file from
the URLs:
https://nms.kcl.ac.uk/christian.urban/ratings.csv (940 KByte)
https://nms.kcl.ac.uk/christian.urban/movies.csv (280 KByte)
The ratings.csv file is organised as userID, movieID, and rating (which is be?
tween 0 and 5, with positive ratings being 4 and 5). The file movie.csv is organ?
ised as movieID and full movie name. Both files still contain the usual CSV?file
header (first line). In this part you are asked to implement functions that pro?
cess these files. If bandwidth is an issue for you, download the files locally, but
in the submitted version use Source.fromURL instead of Source.fromFile.
Tasks
(1) Implement the function get_csv_url which takes an URL?string as argu?
ment and requests the corresponding file. The two URLs of interest are
ratings_url and movies_url, which correspond to CSV?files mentioned
above. The function should return the CSV?file appropriately broken up
into lines, and the first line should be dropped (that is omit the header of
the CSV?file). The result is a list of strings (the lines in the file). In case
the url does not produce a file, return the empty list.
[1 Mark]
(2) Implement two functions that process the (broken up) CSV?files from (1).
The process_ratings function filters out all ratings below 4 and returns
a list of (userID, movieID) pairs. The process_movies function returns a
list of (movieID, title) pairs. Note the input to these functions will be the
output of the function get_csv_url.
[1 Mark]
3
(3) Implement a kind of grouping function that calculates a Map containing
the userIDs and all the corresponding recommendations for this user (list
of movieIDs). This should be implemented in a tail?recursive fashion us?
ing a Map as accumulator. This Map is set to Map() at the beginning of
the calculation. For example
val lst = List(("1", "a"), ("1", "b"),
("2", "x"), ("3", "a"),
("2", "y"), ("3", "c"))
groupById(lst, Map())
returns the ratings map
Map(1 ‐> List(b, a), 2 ‐> List(y, x), 3 ‐> List(c, a)).
In which order the elements of the list are given is unimportant.
[1 Mark]
(4) Implement a function that takes a ratings map and a movieID as argu?
ments. The function calculates all suggestions containing the given movie
in its recommendations. It returns a list of all these recommendations
(each of them is a list and needs to have the given movie deleted, other?
wise it might happen we recommend the same movie “back”). For exam?
ple for the Map from above and the movie "y" we obtain List(List("x")),
and for the movie "a" we get List(List("b"), List("c")).
[1 Mark]
(5) Implement a suggestions function which takes a ratings map and a movieID
as arguments. It calculates all the recommended movies sorted accord?
ing to the most frequently suggested movie(s) sorted first. This function
returns all suggested movieIDs as a list of strings.
[1 Mark]
(6) Implement then a recommendation function which generates a maximum
of two most?suggested movies (as calculated above). But it returns the
actual movie name, not the movieID. If fewer movies are recommended,
then return fewer than two movie names.
[1 Mark]
(7) Calculate the recommendations for all movies according to what the rec?
ommendations function in (6) produces (this can take a few seconds).
Put all recommendations into a list (of strings) and count how often the
strings occur in this list. This produces a list of string?int pairs, where the
first component is the movie name and the second is the number of how
many times the movie was recommended. Sort all the pairs according to
the number of times they were recommended (most recommended movie
name first).
[1 Mark]
4

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