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);