wekaAddingListToInstances.java

import weka.core.Instances;
import weka.core.converters.ConverterUtils.DataSource;
import weka.filters.Filter;
import weka.filters.unsupervised.attribute.Add;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class wekaDev {

        public static void main(String[] args) throws Exception {

            // read in data
            DataSource source = new DataSource("C:\\Program Files\\Weka-3-8\\data\\iris.arff");
            Instances data = source.getDataSet();

            // look at data before
            System.out.println("\n====== BEFORE ======\n");
            printHead(data,5);

            // make a list of random doubles
            List<Double> list = new ArrayList<>(data.numInstances());
            Random rand = new Random();
            for (int i=0;i<=data.numInstances();i++) {
                list.add(rand.nextDouble());
            }

            // get list of numbers into Instances object as new attribute
            Add filter = new Add();
            filter.setAttributeIndex("last"); // add as last so we know where it is
            filter.setAttributeName("randomDouble"); // give it a name
            filter.setInputFormat(data); // set the filter
            data = Filter.useFilter(data, filter); // use the filter
            // now update all the values
            for (int i = 0; i < data.numInstances(); i++) {
                data.instance(i).setValue(data.numAttributes()-1, list.get(i)); // need to -1 as zero based index
            }

            // look at data after
            System.out.println("\n====== AFTER ======\n");
            printHead(data,5);

        }

        public static void printHead(Instances data, int n) {
            int numCols = data.numAttributes();
            for (int i=0; i<=(numCols-1); i++){
                if (i < (numCols-1)) {
                    System.out.print(data.attribute(i).name() + ",");
                }
                else {
                    System.out.print(data.attribute(i).name() + "\n");
                }
            }
            for (int i=0; i<=(n-1); i++){
                System.out.println(data.instance(i));
            }
        }
}