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From Shi Yu <sh...@uchicago.edu>
Subject Re: Automatic line number in reducer output
Date Fri, 10 Jun 2011 16:46:51 GMT
Yes, it works perfectly. Actually didn't realize the flexibility to 
employ different classes in combiner and reducer. In that case it would 
have a three layer architecture, think that would be interesting and useful.

Shi

On 6/10/2011 9:31 AM, Robert Evans wrote:
> In this case you probably want two different classes.  You can have the base Reducer
class that adds in the line count, and then subclass it for the combiner, that sets a flag
to not output the line numbers.
>
> --Bobby
>
>
> On 6/9/11 12:57 PM, "Shi Yu"<shiyu@uchicago.edu>  wrote:
>
> Hi,
>
> Thanks for the reply. The line count in new API works fine now, it was a
> bug in my code.  In new API,
>
> Iterator  is changed to Iterable,
>
> but I didn't pay attention to that and was still using Iterator and hasNext(), Next()
method. Surprisingly, the wrong code still ran and got output, but the line number count did
not work and I think it was null value. After fixing that Iterable mistake, the code works
fine.
>
> The remaining problem is when combiner and reducer are both implemented, the output is
like
>
> 00000   00000   value1
> 00001   00000   value2
> 00002   00000   value3
> 00003   00001   value4
> 00004   00001   value5
>
> The first column are counts from reducer, the second column are counts from combiner.
I want to avoid the line counter in combiner, so my plan is to create another class which
is almost the same as Reducer, but without the line count. I think it is doable to set Combiner
and Reducer to different classes in jobconf, but I haven't tried it yet.
>
> Best,
>
> Shi
>
>
> On 6/9/2011 8:49 AM, Robert Evans wrote:
>
>> What exactly is linecount being output as in the new APIs?
>>
>> --Bobby
>>
>> On 6/7/11 11:21 AM, "Shi Yu"<shiyu@uchicago.edu>   wrote:
>>
>> Hi,
>>
>> I am wondering is there any built-in function to automatically add a
>> self-increment line number in reducer output (like the relation DB
>> auto-key).
>>
>> I have this problem because in 0.19.2 API, I used a variable linecount
>> increasing in the reducer like:
>>
>>     public static class Reduce extends MapReduceBase implements
>> Reducer<Text, IntWritable, Text,IntWritable>{
>>            private long linecount = 0;
>>
>>            public void reduce(Text key, Iterator<IntWritable>   values,
>> OutputCollector<Text, IntWritable>   output, Reporter reporter) throws
>> IOException {
>>
>>            //.....some code here
>>            linecount ++;
>>            output.collect(new Text(Long.toString(linecount)), var);
>>
>>           }
>>
>> }
>>
>>
>> However, I found that this is not working in 0.20.2 API, if I write the
>> code like:
>>
>> public static class Reduce extends
>> org.apache.hadoop.mapreduce.Reducer<Text, IntWritable, Text, IntWritable>{
>>           private long linecount = 0;
>>
>>           public void reduce (Text key, Iterator<IntWritable>   values,
>> org.apache.hadoop.mapreduce.Reducer.Context context) throws IOException,
>> InterruptedException {
>>
>>           //some code here
>>           linecount ++;
>>           context.write(new Text(Long.toString(linecount)),var);
>>          }
>> }
>>
>> but it seems not working anymore.
>>
>>
>> I would also like to know if there are combiner and reducer implemented,
>> how to avoid that line number being written twice (cause I only want it
>> in reducer, not in combiner). Thanks!
>>
>>
>> Shi
>>
>>
>>
>>
>
>


-- 
Postdoctoral Scholar
Institute for Genomics and Systems Biology
Department of Medicine, the University of Chicago
Knapp Center for Biomedical Discovery
900 E. 57th St. Room 10148
Chicago, IL 60637, US
Tel: 773-702-6799


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