MATLAB – TreeBagger example

Did you know that Decision Forests (or Random Forests, I think they are pretty much the same thing) are implemented in MATLAB? In MATLAB, Decision Forests go under the rather deceiving name of TreeBagger.

Here’s a quick tutorial on how to do classification with the TreeBagger class in MATLAB.

% Since TreeBagger uses randomness we will get different results each 
% time we run this.
% This makes sure we get the same results every time we run the code.
rng default
% Here we create some training data.
% The rows< represent the samples or individuals.
% The first two columns represent the individual's features.
% The last column represents the class label (what we want to predict)
trainData = [ ...
    [6,  300,  1];
    [3,  300,  0];
    [8,  300,  1];
    [11, 2000, 0];
    [3,  100,  0];
    [6,  1000, 0];
features = trainData(:,(1:2))
classLabels = trainData(:,3)
% How many trees do you want in the forest? 
nTrees = 20;
% Train the TreeBagger (Decision Forest).
B = TreeBagger(nTrees,features,classLabels, 'Method', 'classification');
% Given a new individual WITH the features and WITHOUT the class label,
% what should the class label be?
newData1 = [7, 300];
% Use the trained Decision Forest.
predChar1 = B.predict(newData1);
% Predictions is a char though. We want it to be a number.
predictedClass = str2double(predChar1)
% predictedClass =
%      1
% So we predict that for our new piece of data, we will have a class label of 1 
% Okay let's try another piece of data.
newData2 = [7, 1500];
predChar2 = B.predict(newData2);
predictedClass2 = str2double(predChar2)
% predictedClass2 =
%      0
% It predicts that the new class label is a 0.

There’s an excellent tutorial in the MATLAB documentation here that covers a lot more.

8 thoughts on “MATLAB – TreeBagger example”

  1. Hi Jer,

    Thanks for the help. Actually I am working on the segmentation of organ from ct scan. But I am not able to understand what will be the features like shape or size.. Can you guide me for this?

    Thanks and Regards

  2. Hi Jer,

    Thanks for sharing such a nice tutorial. It helps me a lot to understand the basic concepts in MATLAB. But I still dont understand how we can use it with image processing.??
    Can you please help me.

    Thanks and Regards

    1. Hello Richa,
      Did you have a particular image processing problem in mind?

      One simple example would be if you had an image with some object in it you wanted to detect. Say you then labeled each pixel in the image as 0 or 1 where a 0 represents the background, and a 1 represents the object. You could then train a Random Forest by giving the pixel values (i.e. the intensities) as features (the variable in the above example) and the 0 or 1 label as the classLabels (see variable in the above example).

      Then you could take a new image, and pass in ONLY the pixel values (the newData1 variable in the example above) to the Random Forest. The Random Forest would then try and predict what the correct label (0 or 1) of the pixels should be based only on the image features.

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