在Java中使用weka：创建一个ARFF文件

下面的代码产生一个实例对象，作为ARFF文件输出到标准流。

产生如下类型的属性：

• numeric
• nominal
• string
• date
• relational

AttTest.java

import weka.core.Attribute;
import weka.core.FastVector;
import weka.core.Instance;
import weka.core.Instances;

/**
 * Generates a little ARFF file with different attribute types.
 *
 * @author FracPete
 */
public class AttTest {
  public static void main(String[] args) throws Exception {
    FastVector      atts;
    FastVector      attsRel;
    FastVector      attVals;
    FastVector      attValsRel;
    Instances       data;
    Instances       dataRel;
    double[]        vals;
    double[]        valsRel;
    int             i;

    // 1. set up attributes
    atts = new FastVector();
    // - numeric
    atts.addElement(new Attribute("att1"));
    // - nominal
    attVals = new FastVector();
    for (i = 0; i < 5; i++)
      attVals.addElement("val" + (i+1));
    atts.addElement(new Attribute("att2", attVals));
    // - string
    atts.addElement(new Attribute("att3", (FastVector) null));
    // - date
    atts.addElement(new Attribute("att4", "yyyy-MM-dd"));
    // - relational
    attsRel = new FastVector();
    // -- numeric
    attsRel.addElement(new Attribute("att5.1"));
    // -- nominal
    attValsRel = new FastVector();
    for (i = 0; i < 5; i++)
      attValsRel.addElement("val5." + (i+1));
    attsRel.addElement(new Attribute("att5.2", attValsRel));
    dataRel = new Instances("att5", attsRel, 0);
    atts.addElement(new Attribute("att5", dataRel, 0));

    // 2. create Instances object
    data = new Instances("MyRelation", atts, 0);

    // 3. fill with data
    // first instance
    vals = new double[data.numAttributes()];
    // - numeric
    vals[0] = Math.PI;
    // - nominal
    vals[1] = attVals.indexOf("val3");
    // - string
    vals[2] = data.attribute(2).addStringValue("This is a string!");
    // - date
    vals[3] = data.attribute(3).parseDate("2001-11-09");
    // - relational
    dataRel = new Instances(data.attribute(4).relation(), 0);
    // -- first instance
    valsRel = new double[2];
    valsRel[0] = Math.PI + 1;
    valsRel[1] = attValsRel.indexOf("val5.3");
    dataRel.add(new Instance(1.0, valsRel));
    // -- second instance
    valsRel = new double[2];
    valsRel[0] = Math.PI + 2;
    valsRel[1] = attValsRel.indexOf("val5.2");
    dataRel.add(new Instance(1.0, valsRel));
    vals[4] = data.attribute(4).addRelation(dataRel);
    // add
    data.add(new Instance(1.0, vals));

    // second instance
    vals = new double[data.numAttributes()];  // important: needs NEW array!
    // - numeric
    vals[0] = Math.E;
    // - nominal
    vals[1] = attVals.indexOf("val1");
    // - string
    vals[2] = data.attribute(2).addStringValue("And another one!");
    // - date
    vals[3] = data.attribute(3).parseDate("2000-12-01");
    // - relational
    dataRel = new Instances(data.attribute(4).relation(), 0);
    // -- first instance
    valsRel = new double[2];
    valsRel[0] = Math.E + 1;
    valsRel[1] = attValsRel.indexOf("val5.4");
    dataRel.add(new Instance(1.0, valsRel));
    // -- second instance
    valsRel = new double[2];
    valsRel[0] = Math.E + 2;
    valsRel[1] = attValsRel.indexOf("val5.1");
    dataRel.add(new Instance(1.0, valsRel));
    vals[4] = data.attribute(4).addRelation(dataRel);
    // add
    data.add(new Instance(1.0, vals));

    // 4. output data
    System.out.println(data);
  }
}

对于丢失的数据

默认新的数组被初始化为0。如果你想在某个位置设置缺失值，通过weka.core.Instance类中的missingValue()方法来明确的设置。在3.7.1版本后的weka中的Instance 是一个接口，所以missingValue()被移到了weka.core.Utils中。如果你已经有一个存在的weka.core.Instance对象，那么你可以使用setMissing(int)方法在给定位置设置缺失值。下面给出在第三个属性设置缺失值的例子。

• double array:

double[] vals = ...  // from somewhere, e.g., from AttTest.java example
 vals[2] = Instance.missingValue(); // or ... = Utils.missingValue() for Weka > 3.7.1

• weka.core.Instance object:

double[] vals = ... // from somewhere, e.g., from AttTest.java example
 Instance inst = new Instance(1.0, vals);
 inst.setMissing(2);