This example explains how to run the K-Means algorithm using the SPMF open-source data mining library.
How to run this example?
What is K-Means?
K-Means is one of the most famous clustering algorithm. It is used to automatically separate a set of instances (vectors of double values) into groups of instances (clusters) according to their similarity. Thus, K-Means is used to automatically group similar instances together into clusters.
In this implementation the user can choose between various distance functions to assess the similarity between vectors. SPMF offers the Euclidian distance, correlation distance, cosine distance, Manathan distance and Jaccard distance.
What is the input?
K-Means takes as input a set of instances having a name and containing one or more double values, a parameter K (a positive integer >=1) indicating the number of clusters to be created, and a distance function.
The input file format of K-Means is a text file containing several instances.
The first lines (optional) specify the name of the attributes used for describing the instances. In this example, two attributes will be used, named X and Y. But note that more than two attributes could be used. Each attribute is specified on a separated line by the keyword "@ATTRIBUTEDEF=", followed by the attribute name
Then, each instance is described by two lines. The first line (which is optional) contains the string "@NAME=" followed by the name of the instance. Then, the second line provides a list of double values separated by single spaces.
An example of input is provided in the file "inputDBScan2.txt" of the SPMF distribution. It contains 31 instances, each described by two attribute called X and Y.
For example, the first instance is named "Instance1", and it is a vector with two values: 1 and 1 for the attributes X and Y, respectively.
This input file represents a set of 2D points. But note that, it is possible to use more than two attributes to describe instances. To give a better idea of what the input file looks like, here is a visual representation:
The K-Means algorithm will group the instances according to their similarity. To do this, it is also necessary to specify the distance function to be used for comparing the instances. The distance function can be the Euclidian distance, correlation distance, cosine distance, Manathan distance and Jaccard distance. In the command line or GUI of SPMF, the distance function is specified by using one of these keywords: "euclidian", "correlation", "cosine", "manathan" and "jaccard" as parameter. In this example, the euclidian distance is used.
What is the output?
K-Means groups instances in clusters according to their similarity. In SPMF, the similarity is defined according to the distance function chosen by the user such as the Euclidian distance. K-Means returns K clusters or less.
Note that running K-Means with the same data does not always generate the same result because K-Means initializes clusters randomly.
By running K-Means on the previous input file and K=3, we can obtain the following output file containing 3 clusters:
[Instance12 8.0 2.0][Instance13 9.0 2.0][Instance14 10.0 1.0][Instance28 9.0 3.0][Instance29 9.0 4.0][Instance30 9.0 5.0]
[Instance4 11.0 12.0][Instance5 11.0 13.0][Instance6 13.0 13.0][Instance7 12.0 8.5][Instance8 13.0 8.0][Instance9 13.0 9.0][Instance11 11.0 7.0][Instance15 7.0 13.0][Instance16 5.0 9.0][Instance17 16.0 16.0][Instance18 11.5 8.0][Instance20 13.0 10.0][Instance21 12.0 13.0][Instance21 14.0 12.5][Instance22 14.5 11.5][Instance23 15.0 10.5][Instance24 15.0 9.5][Instance25 12.0 9.5][Instance26 10.5 11.0][Instance27 10.0 10.5][Instance10 13.0 7.0]
[Instance1 1.0 1.0][Instance2 0.0 1.0][Instance3 1.0 0.0]
The output file format is defined as follows. The first few lines indicate the attribute names. Each attribute is specified on a separated line with the keyword "ATTRIBUTEDEF=" followed by the attribute name (a string). Then, the list of clusters is indicated. Each cluster is specified on a separated line, listing the instances contained in the cluster. An instance is a name followed by a list of double values separated by " " and between the "[" and "]" characters.
The clusters found by the algorithm can be viewed visually using the "Cluster Viewer" provided in SPMF. If you are using the graphical interface of SPMF, click the checkbox "Cluster Viewer" before pressing the "Run Algorithm" button. The result will be displayed in the Cluster Viewer.
As it can be seen in this example, the result somewhat make sense, as points close to each other are in the same cluster.
Applying K-Means to time series
Note that the K-Means algorithm implementation in SPMF can also be applied to time series database such as the file contextSAX.txt in the SPMF distribution. To apply K-Means to time series, it is necessary to set the "separator" parameter of the K-Means algorithm to "," since time series files separate values by "," instead of spaces.
Where can I get more information about K-Means?
K-Means was proposed by MacQueen in 1967. K-means is one of the most famous data mining algorithm. It is described in almost all data mining books that focus on algorithms, and on many websites. By searching on the web, you will find plenty of resources explaining K-Means.
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