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Decision Tree

A Ruby library which implements ID3 (information gain) algorithm for decision tree learning. Currently, continuous and discrete datasets can be learned.

  • Discrete model assumes unique labels & can be graphed and converted into a png for visual analysis
  • Continuous looks at all possible values for a variable and iteratively chooses the best threshold between all possible assignments. This results in a binary tree which is partitioned by the threshold at every step. (e.g. temperate > 20C)


  • ID3 algorithms for continuous and discrete cases, with support for inconsistent datasets.
  • Graphviz component to visualize the learned tree
  • Support for multiple, and symbolic outputs and graphing of continuous trees.
  • Returns default value when no branches are suitable for input


  • Ruleset is a class that trains an ID3Tree with 2/3 of the training data, converts it into set of rules and prunes the rules with the remaining 1/3 of the training data (in a C4.5 way).
  • Bagging is a bagging-based trainer (quite obvious), which trains 10 Ruleset trainers and when predicting chooses the best output based on voting.

Blog post with explanation & examples


require 'decisiontree'

attributes = ['Temperature']
training = [
  [36.6, 'healthy'],
  [37, 'sick'],
  [38, 'sick'],
  [36.7, 'healthy'],
  [40, 'sick'],
  [50, 'really sick'],

# Instantiate the tree, and train it based on the data (set default to '1')
dec_tree =, training, 'sick', :continuous)

test = [37, 'sick']
decision = dec_tree.predict(test)
puts "Predicted: #{decision} ... True decision: #{test.last}"

# => Predicted: sick ... True decision: sick

# Specify type ("discrete" or "continuous") in the training data
labels = ["hunger", "color"]
training = [
        [8, "red", "angry"],
        [6, "red", "angry"],
        [7, "red", "angry"],
        [7, "blue", "not angry"],
        [2, "red", "not angry"],
        [3, "blue", "not angry"],
        [2, "blue", "not angry"],
        [1, "red", "not angry"]

dec_tree =, training, "not angry", color: :discrete, hunger: :continuous)

test = [7, "red", "angry"]
decision = dec_tree.predict(test)
puts "Predicted: #{decision} ... True decision: #{test.last}"

# => Predicted: angry ... True decision: angry


The MIT License - Copyright (c) 2006 Ilya Grigorik

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