Class Discovery

See http://www.cs.bham.ac.uk/~wbl/biblio/gecco1999/GP-417.pdf

CDGA uses a Genetic Algorithm to discover a classification rule for a given dataset.
A dataset can be seen as a table:

An attribute can be numerical, for example a "temperature" attribute, or categorical, for example a "color" attribute. For classification purposes, one of the categorical attributes is designated as a label, which means that its value defines the class of the rows.
A classification rule can be represented as follows:

For a given target class and a weight threshold, the classification rule can be read :

for each row of the dataset
  if (rule.w1 < threshold || (rule.w1 >= threshold && row.value1 rule.op1 rule.value1)) &&
     (rule.w2 < threshold || (rule.w2 >= threshold && row.value2 rule.op2 rule.value2)) &&
     ...
     (rule.wN < threshold || (rule.wN >= threshold && row.valueN rule.opN rule.valueN)) then
    row is part of the target class

Important: The label attribute is not evaluated by the rule.

The threshold parameter allows some conditions of the rule to be skipped if their weight is too small. The operators available depend on the attribute types:

• for a numerical attributes, the available operators are '<' and '>='
• for categorical attributes, the available operators are '!=' and '=='

The "threshold" and "target" are user defined parameters, and because the label is always a categorical attribute, the target is the (zero based) index of the class label value in all the possible values of the label. For example, if the label attribute can have the following values (blue, brown, green), then a target of 1 means the "blue" class.

For example, we have the following dataset (the label attribute is "Eyes Color"):

and a classification rule:

and the following parameters: threshold = 1 and target = 0 (brown).

This rule can be read as follows:

for each row of the dataset
  if (0 < 1 || (0 >= 1 && row.value1 < 20)) &&
     (1 < 1 || (1 >= 1 && row.value2 != light)) then
    row is part of the "brown Eye Color" class

Please note how the rule skipped the label attribute (Eye Color), and how the first condition is ignored because its weight is < threshold.

Running the example:

NOTE: Substitute in the appropriate version for the Mahout JOB jar

1. cd <MAHOUT_HOME>/examples
2. ant job

3. <HADOOP_HOME>/bin/hadoop dfs -put <MAHOUT_HOME>/examples/src/test/resources/wdbc wdbc

4. <HADOOP_HOME>/bin/hadoop dfs -put <MAHOUT_HOME>/examples/src/test/resources/wdbc.infos wdbc.infos

5. <HADOOP_HOME>/bin/hadoop jar <MAHOUT_HOME>/examples/build/apache-mahout-examples-0.1-dev.job org.apache.mahout.ga.watchmaker.cd.CDGA <MAHOUT_HOME>/examples/src/test/resources/wdbc 1 0.9 1 0.033 0.1 0 100 10
   

   CDGA needs 9 parameters:

• param 1 : path of the directory that contains the dataset and its infos file
• param 2 : target class
• param 3 : threshold
• param 4 : number of crossover points for the multi-point crossover
• param 5 : mutation rate
• param 6 : mutation range
• param 7 : mutation precision
• param 8 : population size
• param 9 : number of generations before the program stops

For more information about 4th parameter, please see Multi-point Crossover.
For a detailed explanation about the 5th, 6th and 7th parameters, please see Real Valued Mutation.

TODO: Fill in where to find the output and what it means.

The info file:

To run properly, CDGA needs some informations about the dataset. Each dataset should be accompanied by an .infos file that contains the needed informations. for each attribute a corresponding line in the info file describes it, it can be one of the following:

• IGNORED
  if the attribute is ignored
• LABEL, val1, val2,...
  if the attribute is the label (class), and its possible values
• CATEGORICAL, val1, val2,...
  if the attribute is categorial (nominal), and its possible values
• NUMERICAL, min, max
  if the attribute is numerical, and its min and max values

This file can be generated automaticaly using a special tool available with CDGA.

$ <HADOOP_HOME>/bin/hadoop jar <MAHOUT_HOME>/examples/build/apache-mahout-examples-0.1-dev.job org.apache.mahout.ga.watchmaker.cd.tool.CDInfosTool dataset_path

• the tool searches for an existing infos file (must be filled by the user), in the same directory of the dataset with the same name and with the ".infos" extension, that contain the type of the attributes:
  • 'N' numerical attribute
  • 'C' categorical attribute
  • 'L' label (this also a categorical attribute)
  • 'I' to ignore the attribute
    each attribute is in a separate
• A Hadoop job is used to parse the dataset and collect the informations. This means that the dataset can be distributed over HDFS.
• the results are written back in the same .info file, with the correct format needed by CDGA.