Created
October 22, 2021 01:35
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Inverted Index using MapReduce
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| package my.index; | |
| import java.io.*; | |
| import java.util.*; | |
| import org.apache.hadoop.fs.Path; | |
| import org.apache.hadoop.io.*; | |
| import org.apache.hadoop.conf.Configuration; | |
| import org.apache.hadoop.mapreduce.Job; | |
| import org.apache.hadoop.mapreduce.Mapper; | |
| import org.apache.hadoop.mapreduce.Reducer; | |
| import java.util.ArrayList; | |
| import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; | |
| import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; | |
| import org.apache.hadoop.io.Text; | |
| public class InvertedIndex { | |
| public static class Map extends Mapper<LongWritable, Text, Text, Text> { | |
| private final Text keyOutput = new Text(); | |
| private final Text valueOutput = new Text(); | |
| public void map(LongWritable key, Text value, Context context) throws | |
| IOException, InterruptedException { | |
| /* | |
| Remember that what is inside this method is run for every single line | |
| in the text file. | |
| */ | |
| //1. Read line | |
| //2. Transform to string and split by "," | |
| //3. Convert each string to tokens | |
| //4. Loop over the tokens | |
| //5. context.write() -> Reduce output: ({pancake,tweet1}, {day,tweet2}, ... ) | |
| String line = value.toString(); | |
| String[] lineArray = line.split(","); | |
| String valueTmp = lineArray[0]; | |
| StringTokenizer tokenizer = new StringTokenizer(lineArray[1]); | |
| while (tokenizer.hasMoreTokens()) { | |
| keyOutput.set(tokenizer.nextToken()); | |
| valueOutput.set(valueTmp); | |
| context.write(keyOutput, valueOutput); | |
| } | |
| } | |
| } | |
| public static class Reduce extends Reducer<Text, Text, Text, Text> { | |
| private final Text keyOutput = new Text(); | |
| private final Text valueOutput = new Text(); | |
| public void reduce(Text key, Iterable<Text> values, Context context) throws | |
| IOException, InterruptedException{ | |
| /* | |
| Remember you are given a unique key and all its associated values. You | |
| loop over each "key:{all values}" once. | |
| */ | |
| //1. Create ArrayList | |
| //2. Loop over values and add them to ArrayList | |
| //3. context.write() -> Reduce output: | |
| // {pancake, [tweet1, tweet23, twee87]}, {sun, [tweet2,..]} | |
| List<String> listValues = new ArrayList<>(); | |
| for (Text val : values) { | |
| listValues.add(val.toString()); | |
| } | |
| keyOutput.set(key); | |
| valueOutput.set(listValues.toString()); | |
| context.write(keyOutput, valueOutput); | |
| } | |
| } | |
| public static void main(String[] args) throws Exception { | |
| Configuration conf = new Configuration(); | |
| Job job = Job.getInstance(conf, "invertedIndex"); | |
| job.setJarByClass(InvertedIndex.class); | |
| job.setOutputKeyClass(Text.class); | |
| job.setOutputValueClass(Text.class); | |
| job.setMapperClass(InvertedIndex.Map.class); | |
| job.setReducerClass(InvertedIndex.Reduce.class); | |
| FileInputFormat.addInputPath(job, new Path(args[0])); | |
| FileOutputFormat.setOutputPath(job, new Path(args[1])); | |
| job.waitForCompletion(true); | |
| } | |
| } |
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