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POCustom hadoop key and value : How to write CompareTo() Method
 

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    <p>I need to emit a 2D double array as key and value from mapper. There are questions posted in Stack Overflow, but they are not answered.</p> <p>I am doing some of the matrix multiplication in a given dataset, and after that I need to emit the value of <code>A*Atrns</code> which will be a matrix as key and <code>Atrans*D</code> which will also be a matrix as value. So how to emit these matrices from mapper. And the value should be corresponding to the key itself.</p> <pre><code>ie key -----&gt; A*Atrans---------&gt;after multiplication the result will be a 2D array which is declared as double (matrix) lets say the result be Matrix "Ekey"(double[][] Ekey) value ------&gt; Atrans*D ---------&gt; after multiplication the result will be Matrix "Eval" (double[][] Eval). After that I need to emit these matrix to reducer for further calculations. So in mapper: context.write(Ekey,Eval); Reducer: I need to do further calculations with these Ekey and Eval. </code></pre> <p>I wrote my class: </p> <p><strong>UPDATE</strong></p> <pre><code> public class MatrixWritable implements WritableComparable&lt;MatrixWritable&gt;{ /** * @param args */ private double[][] value; private double[][] values; public MatrixWritable() { // TODO Auto-generated constructor stub setValue(new double[0][0]); } public MatrixWritable(double[][] value) { // TODO Auto-generated constructor stub this.value = value; } public void setValue(double[][] value) { this.value = value; } public double[][] getValue() { return values; } @Override public void write(DataOutput out) throws IOException { out.writeInt(value.length); // write values for (int i = 0; i &lt; value.length; i++) { out.writeInt(value[i].length); } for (int i = 0; i &lt; value.length; i++) { for (int j = 0; j &lt; value[i].length; j++) { out.writeDouble(value[i][j]); } } } @Override public void readFields(DataInput in) throws IOException { value = new double[in.readInt()][]; for (int i = 0; i &lt; value.length; i++) { value[i] = new double[in.readInt()]; } values = new double[value.length][value[0].length]; for(int i=0;i&lt;value.length ; i++){ for(int j= 0 ; j&lt; value[0].length;j++){ values[i][j] = in.readDouble(); } } } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + Arrays.hashCode(value); return result; } /* (non-Javadoc) * @see java.lang.Object#equals(java.lang.Object) */ @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (!(obj instanceof MatrixWritable)) { return false; } MatrixWritable other = (MatrixWritable) obj; if (!Arrays.deepEquals(value, other.value)) { return false; } return true; } @Override public int compareTo(MatrixWritable o) { // TODO Auto-generated method stub return 0; } public String toString() { String separator = "|"; StringBuffer result = new StringBuffer(); // iterate over the first dimension for (int i = 0; i &lt; values.length; i++) { // iterate over the second dimension for(int j = 0; j &lt; values[i].length; j++){ result.append(values[i][j]); result.append(separator); } // remove the last separator result.setLength(result.length() - separator.length()); // add a line break. result.append(","); } return result.toString(); } } </code></pre> <p>I am able to emit a value as matrix from mapper</p> <pre><code>context.write(...,new MatrixWritable(AAtrans)); </code></pre> <p>How to emit matrix AtransD as key from mapper?</p> <p>For that I need to write compareto() method, right?</p> <p>What should be included in that method? </p>
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    1. CODuplicate: http://stackoverflow.com/questions/19674314/empty-value-in-write-method-custom-datatype-in-hadoop-mapreduce?noredirect=1#comment29230037_19674314
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