Interview Question

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    5
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    Hi All, Recently I came across an interesting problem. Following is the problem description. I would like to have your thoughts on the problem.

    Implement linux "tail -n" command in java:
    Restrictions:
    1) Should read file only from top to bottom.
    2) File size can range from 1KB to 1TB.
    3) System has only 32MB of RAM.

    - Girish. G on November 11, 2012 in India Report Duplicate | Flag
    Algorithm

Country: India
Interview Type: In-Person


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1
of 1 vote

We can use a n size queue.
Keep filling the queue. Once the queue is full then keep adding the next lines and remove the front element from the queue at the same time.
So, at any instance the queue will give the last n rows read from the file.

- Emmkay on November 11, 2012 | Flag Reply
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0
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What about the memory limitations? Perhaps you could flush 'chunks' of the queue to disk, deleting old chunks when they're no longer needed.

Another approach is you could iterate through the file first counting the number of lines. Then process the file knowing at which point in the file to start writing lines to the screen?

- CameronWills on November 11, 2012 | Flag
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0
of 0 vote

Yes we should free the memory of the front element when removing it from the queue. So, that only n rows are always present in the memory.

- Emmkay on November 12, 2012 | Flag Reply
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0
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One more approach will be similar to printing the last n node of a single linked list. Use 2 file pointers(fp1 and fp2). Move the first pointer(fp1) n rows or till the end-of-file whichever comes first. If there are more that n lines in the file then start moving the 2 file pointers till fp1 reaches the end of file. Once fp2 reaches end of file, then move fp1 till the end of file printing every line and this gives the last n lines in the file.

- Emmkay on November 12, 2012 | Flag Reply
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0
of 0 vote

fill an array of n Strings in cyclic way until you reach EOF.
then traverse from the cyclic beggining and print.

e.g. array[i % n] <= read
etc.

- avico81 on November 13, 2012 | Flag Reply


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