How To Get The Reducer To Emit Only Duplicates

I have a Mapper that is going through lots of data and emitting ID numbers as keys with the value of 1. What I hope to accomplish with the MapReduce job is to get a list of all IDs

Solution 1:

You have committed mistake in few places.

1. This code:

   if last_key and last_key != key:
       sys.stdout.write("%s\t%s\n" % (last_key,tot_cnt))
   

   should be changed to:

   if last_key != key:
       if(tot_cnt > 1):
           sys.stdout.write("%s\t%s\n" % (last_key, tot_cnt))
   

   You were not checking for tot_cnt > 1.

2. Last 2 lines:

   if last_key:
       sys.stdout.write("%s\t%s\n" % (last_key, tot_cnt))
   

   should be changed to:

   if last_key and tot_cnt > 1:
       sys.stdout.write("%s\t%s\n" % (last_key, tot_cnt))
   

   Here again, you were not checking for tot_cnt > 1.

Following is the modified code, which works for me:

import sys
import codecs

sys.stdout = codecs.getwriter('utf-8')(sys.stdout)
inData = codecs.getreader('utf-8')(sys.stdin)

(last_key, tot_cnt) = (None, 0)
for line in inData:
    (key, val) = line.strip().split("\t")
    if last_key != key:
        if(tot_cnt > 1):
            sys.stdout.write("%s\t%s\n" % (last_key, tot_cnt))
        (last_key, tot_cnt) = (key, int(val))
    else:
        (last_key, tot_cnt) = (key, tot_cnt + int(val))

if last_key and tot_cnt > 1:
    sys.stdout.write("%s\t%s\n" % (last_key, tot_cnt))

I get following output, for your data:

abc     2