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Finding the number of palindromes in a text corpus using Hadoop
import java.io.IOException;
import java.util.StringTokenizer;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;
public class Palindrome {
public static class PalindromeMapper
extends Mapper<Object, Text, Text, IntWritable> {
public void map(Object key, Text value, Context context)
throws IOException, InterruptedException {
// Format: K1,V1,K2,V2
// So key here probably is the byte offset
// Value is the line
// Context is the stream to which you will write
// Loads of customizations are possible, to get the
// key-value format that you want.
Text word = new Text();
IntWritable one = new IntWritable(1);
StringTokenizer tokens = new StringTokenizer(value.toString());
while(tokens.hasMoreTokens()) {
String token = tokens.nextToken();
// Check if palindrome
boolean isPalindrome = true;
for(int i = 0; i < token.length(); i++)
if(token.charAt(i) != token.charAt(token.length()-i-1))
{
isPalindrome = false;
break;
}
if(isPalindrome)
{
word.set(token);
context.write(word, one);
}
}
}
}
public static class PalindromeReducer
extends Reducer<Text,IntWritable,Text,IntWritable> {
private IntWritable result = new IntWritable();
public void reduce(Text key, Iterable<IntWritable> values, Context context)
throws IOException, InterruptedException {
int sum = 0;
for(IntWritable val : values) {
sum += val.get();
}
result.set(sum);
context.write(key, result);
}
}
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
Job job = new Job(conf, "palindrome");
job.setJarByClass(Palindrome.class);
job.setMapperClass(PalindromeMapper.class);
job.setReducerClass(PalindromeReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
@surabhi26
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surabhi26 commented Oct 6, 2015

this code doesnot give correct palindrome output,it shows the word is palindrome even if only first and last letters matches ..pls help

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