Adding an instance and classify it in Weka 3.6.5+

Question

I have a Train.arff file to which I want to add new instance (say "2,F,22222,1002-5,?") and then classify the last attributes. All my attributes are nominal.

@attribute age {2,3,4,5,6}
@attribute gender {F,M}
@attribute zipcode {22222,33333,11111}
@attribute race {1002-5,2028-9,2054-5,2076-8,2106-3}
@attribute service {H0018,H2034,H0004,H0009,H0006}

@data
2,F,22222,1002-5,H0018

1. Loaded Train.arff

2. Adding Instance

           Instance inst = new Instance(10);
           inst.setValue(trainData.attribute(0), age);
           inst.setValue(trainData.attribute(1), administrativeGenderCode);
           inst.setValue(trainData.attribute(2), zipCode);
           inst.setValue(trainData.attribute(3), race);
           inst.setValue(trainData.attribute(4), "H2034");
   
           // inst.setDataset(trainData);
   
           // add
           trainData.add(inst);
   

3. Building Classifier

   public String buildAndClassify() {
       //build model
       Logistic model = new Logistic();
       try {
           model.buildClassifier(trainData); <-- fails
   
           Instances labeled = new Instances(trainData);
           double clsLabel = model.classifyInstance(trainData.lastInstance());
           labeled.lastInstance().setClassValue(clsLabel);
   
           System.out.print(labeled.lastInstance().stringValue(7));
   
           return labeled.lastInstance().stringValue(7);
   
       } catch (Exception e) {
           e.printStackTrace();
       }
   
       return "";
   }   
   

   java.lang.NullPointerException at weka.filters.unsupervised.attribute.ReplaceMissingValues.batchFinished(ReplaceMissingValues.java:189) at weka.filters.Filter.useFilter(Filter.java:663) at weka.classifiers.functions.Logistic.buildClassifier(Logistic.java:546) at com.feisystems.bham.weka.LogisticRegression.buildAndClassify(LogisticRegression.java:70) at com.feisystems.bham.weka.LogisticRegression.(LogisticRegression.java:20) at com.feisystems.bham.weka.AttTest.main(AttTest.java:22)

I am using Weka 3.7 and don't see any example to correctly add instance to an existing file and classify one attribute of the instance.

As a work around I am currently creating another Test.arff file with all the matching attributes from Train.arff and adding instance to those attributes. I however want to add an instance to the same Train.arff file and classify it.

Answer 1

This works!

        Instance inst = new Instance(4); <-- Adjust number of instances you want to add.

        inst.setValue(trainData.attribute(0), age);
        inst.setValue(trainData.attribute(1), administrativeGenderCode);
        inst.setValue(trainData.attribute(2), zipCode);
        inst.setValue(trainData.attribute(3), race);
//      inst.setValue(trainData.attribute(4), "H2034"); <-- Do not add the instance you want to classify.