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MapReduce, mail # user - Reading fields from a Text line


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Re: Reading fields from a Text line
Mohammad Tariq 2012-08-02, 10:18
Thanks for the response Harsh n Sri. Actually, I was trying to prepare
a template for my application using which I was trying to read one
line at a time, extract the first field from it and emit that
extracted value from the mapper. I have these few lines of code for
that :

public static class XPTMapper extends Mapper<IntWritable, Text,
LongWritable, Text>{

public void map(LongWritable key, Text value, Context context)
throws IOException, InterruptedException{

Text word = new Text();
String line = value.toString();
if (!line.startsWith("TT")){
context.setStatus("INVALID LINE..SKIPPING........");
}else{
String stdid = line.substring(0, 7);
word.set(stdid);
context.write(key, word);
}
}

But the output file contains all the rows of the input file including
the lines which I was expecting to get skipped. Also, I was expecting
only the fields I am emitting but the file contains entire lines.
Could you guys please point out the the mistake I might have made.
(Pardon my ignorance, as I am not very good at MapReduce).Many thanks.

Regards,
    Mohammad Tariq
On Thu, Aug 2, 2012 at 10:58 AM, Sriram Ramachandrasekaran
<[EMAIL PROTECTED]> wrote:
> Wouldn't it be better if you could skip those unwanted lines
> upfront(preprocess) and have a file which is ready to be processed by the MR
> system? In any case, more details are needed.
>
>
> On Thu, Aug 2, 2012 at 8:23 AM, Harsh J <[EMAIL PROTECTED]> wrote:
>>
>> Mohammad,
>>
>> > But it seems I am not doing  things in correct way. Need some guidance.
>>
>> What do you mean by the above? What is your written code exactly
>> expected to do and what is it not doing? Perhaps since you ask for a
>> code question here, can you share it with us (pastebin or gists,
>> etc.)?
>>
>> For skipping 8 lines, if you are using splits, you need to detect
>> within the mapper or your record reader if the map task filesplit has
>> an offset of 0 and skip 8 line reads if so (Cause its the first split
>> of some file).
>>
>> On Thu, Aug 2, 2012 at 1:54 AM, Mohammad Tariq <[EMAIL PROTECTED]> wrote:
>> > Hello list,
>> >
>> >        I have a flat file in which data is stored as lines of 107
>> > bytes each. I need to skip the first 8 lines(as they don't contain any
>> > valuable info). Thereafter, I have to read each line and extract the
>> > information from them, but not the line as a whole. Each line is
>> > composed of several fields without any delimiter between them. For
>> > example, the first field is of 8 bytes, second of 2 bytes and so on. I
>> > was trying to reach each line as a Text value, convert it into string
>> > and using String.subring() method to extract the value of each field.
>> > But it seems I am not doing  things in correct way. Need some
>> > guidance. Many thanks.
>> >
>> > Regards,
>> >     Mohammad Tariq
>>
>>
>>
>> --
>> Harsh J
>
>
>
>
> --
> It's just about how deep your longing is!
>