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This Dojo is based on the concept of MapReduce, a programming model for processing large datasets. Google is often credited with having invented the method, and holds a patent. The distributed implementation was heavily used for many years inside the company. See for example:

The original paper introducing MapReduce is available online:

Warmup exercise

This exercise is an introduction to the general idea of decomposing a problem into map and reduce stages - if you are familiar with higher order functions and functional programming it will probably be very easy!

Implement a solution to the following problems using an iterative solution (e.g. for loops):

  • Given a list of integers, return the sum
  • Given a list of integers, return the sum of their squares
  • Given a list of integers, return the product of double their squares

Note: there are tests and stub implementations in the bbc.dojonorth.mapreduce.warmup.Exercise1 class (Scala)

These problems can be all be broken into 2 distinct parts: First, the input elements are transformed. Then, the transformed elements are combined into a result.

Try to reimplement your 3 solutions using the language's built-in map and reduce functions for these 2 stages.

Can you implement a higher order function mapReduce for all 3 problems? (N.B. a higher order function is one which takes functions as arguments).

Getting into the swing of it

In these exercises you will apply the same principles as before with a bigger set of data - in this case a sample of data from the million song database.

Getting the songs data

The song lyrics data is provided by Columbia University at:

There are 3 text files containing the data we need.

If you have WGET installed, you can easily download the zip files with the following commands:


Unzip these files somewhere in the project directory structure.

The first 2 files contain lists of (stemmed) words, followed by a list of track information. This consists of a Million Song Database Track ID, a musiXmatch ID and a list of word counts (words are indexed starting from 1). The 3rd file maps Million Song Database track IDs to metadata from the MusixMatch database. Specifically, we can get the artist and song titles - this will be useful for displaying results.

Code is provided to load these files. You can also read them into a SQLite database, which will fill up to around 2GB with indexes! The SQLite-generating code can be found here:

The MusixMatchSongMatcher class will load the "mxm_779k_matches.txt" file. It provides a dictionary mapping Track ID to "MSDTrack", a class with artist and title information. Using this is optional, but it will give more readable information for Tasks 3 and 4.

Task 1: Word statistics

In a similar way to the previous task, write map and reduce functions to find the average number of different words in a song, and the average total number of words in a song.

The expected results are as follows for the test data file:

  • Average different words: 81
  • Average total words: 212

For the training data file:

  • Average different words: 80
  • Average total words: 213


  • Think about averaging - can the mean be computed before all the values are available?
  • Can you combine

Task 2: Compute the word counts for all words across all songs

With the LyricDataLoader class, you have a method to get a list of tracks, each with a dictionary mapping words (strings) to their respective counts. Supply the load function with the path to the file you want to use (i.e. mxm_dataset_train.txt or mxm_dataset_test.txt).

  1. Write a map function which takes a track and produces the dictionary (i.e. Scala Map) of word counts.

  2. Write a reduce function which takes 2 dictionaries of word counts and merges them to a single dictionary.

  3. Combine your map and reduce functions to get the total word count for all the songs in the Test dataset. Try to find the top 10 words by count.

Hints (mainly Scala):

  • Use the HashMap[A,B] type rather than Map[A,B] if you want to benefit from the built-in merged function for combining two Maps.
  • In the Helpers object, there is an implicit conversion function for converting Map to HashMap
  • You could use any iterable data structure to store intermediate data instead of dictionaries.

Task 3: Grep

Your task now is to write a function which finds all songs containing a given word. Note that our dataset contains stemmed words!

  1. Write a map function which takes a song and returns a List with the Track ID, only if the track contains the specified word. If the song does not contain the word, it should return an empty List.

  2. Write a reduce function which takes 2 lists of Track IDs. It should combine the 2 lists.

  3. Use the map and reduce functions to define your grep function.


  • Try to use the MusixMatchSongMatcher class to give the results as a list of song titles instead of Track IDs

Task 4: Reverse index

Write map and reduce functions which enable you to create a dictionary of words to the Track IDs of songs containing them.

This reverse-index may be more useful than the Grep function you defined in the previous task. Can you think of any interesting queries using it?

Task 5: Obscenity

Try to build on your solutions to the previous tasks to find out which songs have the most profane lyrics.

You will need some way to measure how obscene a given song is.

Task 6: Real world MapReduce implementations

Have a look at some of the well known open source MapReduce implementations for your chosen programming language. Find out how you could port your code to use them. Most implementations use Hadoop under the hood.

Examples for Scala: Apache Spark, Scalding, Scoobi.

Amazon provide Elastic Map Reduce (EMR). This runs Hadoop jobs across EC2 instances. There is a QwikLab available here:


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