public void reduce(LongWritable key, Iterable<Text> values, Context context) throws IOException, InterruptedException { LongWritable curNodeId = key; double previousPRValue = 1; double nextPRValue = 0; double localResidual = 0; String edgeListOfCurNode = ""; long localResidualTransformed = 0; for (Text value : values) { String[] inputInfo = value.toString().split("\\s+"); // incoming pagerank value if (inputInfo.length == 1) { nextPRValue += Double.parseDouble(inputInfo[0]); } // current node info else if (inputInfo.length == 3) { edgeListOfCurNode = inputInfo[2]; previousPRValue = Double.parseDouble(inputInfo[1]); } else if (inputInfo.length == 2) { previousPRValue = Double.parseDouble(inputInfo[1]); } else { System.out.println("ERROR: received unexpected TEXT in length"); } } if (previousPRValue == 1) System.out.println("No node info has been received by a reducer"); // calculate the pagerank value according to the given formula nextPRValue = pagerankFormula(nextPRValue); // should also iterate sink nodes list, add the evenly splitted value // reducer should store the updated node info(NPR) to output directory context.write(null, new Text(curNodeId + " " + nextPRValue + " " + edgeListOfCurNode)); // then compare PPR with NPR try { localResidual = Math.abs(previousPRValue - nextPRValue) / nextPRValue; localResidualTransformed = (long) (localResidual * 10000); // System.out.println("Make sure you got the right transformed residual : // "+localResidualTransformed); } catch (ArithmeticException e) { System.out.println("PPR is zero. Check where you get the value!"); } // assume there is a global counter called residualCounter; context.getCounter(myCounter.ResidualCounter.RESIDUAL_SUM).increment(localResidualTransformed); }