How to print out the predicted class after cross-validation in WEKA

Question

Once a 10-fold cross-validation is done with a classifier, how can I print out the prediced class of every instance and the distribution of these instances? ``` J48 j48 = new J48(); Evaluation eval = new Evaluation(newData); eval.crossValidateModel(j48, newData, 10, new Random(1)); ``` When I tried something similar to below, it said that the classifier is not built. ``` for (int i=0; i<data.numInstances(); i++){ System.out.println(j48.distributionForInstance(newData.instance(i))); } ``` What I'm trying to do is the same function as in the WEKA GUI wherein once a classifier is trained, I can click on `Visualize classifier error" > Save`, and I will find the predicted class in the file. But now I need it in to work in my own Java code. I have tried something like below: ``` J48 j48 = new J48(); Evaluation eval = new Evaluation(newData); StringBuffer forPredictionsPrinting = new StringBuffer(); weka.core.Range attsToOutput = null; Boolean outputDistribution = new Boolean(true); eval.cro…

Accepted Answer

The `crossValidateModel()` method can take a `forPredictionsPrinting` `varargs` parameter that is a `weka.classifiers.evaluation.output.prediction.AbstractOutput` instance.

The important part of that is a `StringBuffer` to hold a string representation of all the predictions. The following code is in untested `JRuby`, but you should be able to convert it for your needs.

```
j48 = j48.new
eval = Evalution.new(newData)
predictions = java.lange.StringBuffer.new
eval.crossValidateModel(j48, newData, 10, Random.new(1), predictions, Range.new('1'), true)
# variable predictions now hold a string of all the individual predictions
```

How to print out the predicted class after cross-validation in WEKA

Solution

Problem