Databases
StackOverflow2013
Postsdbo
POSeeking to middle of file in python
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POSeeking to middle of file in python
    primarykey
    data
    text
    <p>I'm writing a program to search for a specific line in a <em>very</em> large (unordered) file (so it would be <em>preferred</em> not to load the entire file into memory). </p> <p>I'm implementing multi threading to speed up the process. I'm trying to give a particular thread a particular part of the file i.e., the first thread would run through the first quarter of the file, the 2nd thread scans (simultaneously) from the endpoint of where the first thread stops and so on. </p> <p>So to do this I need to find byte location of different parts of the file for simplicity of the question lets say I just want to find the middle of the file. But the problem is each line has a different length so if I just do</p> <pre><code>fo.seek(0, 2) end = fo.tell() mid = end/2 fo.seek(mid, 0) </code></pre> <p>It could give me the middle of the line. So I need a way to seek to the next or previous newline. Also, note I dont want the <em>exact</em> middle just somewhere around it (since its a very large file).</p> <p>Heres what I was able to code, I'm not sure whether this loads the file into memory or not. And I would really like to avoid opening 2 instances of the same file (I did so in my program because I didnt want to worry about the offset changing when I read the file). </p> <p>Any modification (or a new program) which is faster would be appreciated.</p> <pre><code>fo = open(filename, "rw+") f2 = open(filename, "rw+") file_ = dict() fo.seek(0, 2) file_['end'] = fo.tell() file_['mid'] = file_['end'] / 2 fo.seek(file_['mid'], 0) f2.seek(file_['mid'], 0) line = f2.readline() fo.seek(f2.tell(), 0) file_['mid'] = f2.tell() fo.seek(file_['mid'], 0) print fo.readline() </code></pre>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USGrim Reaper
    1. USGrim Reaper
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POSeeking to middle of file in python
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POSeeking to middle of file in python
      1. This table or related slice is empty.
      1. VTUpMod
    1. COAs always, IO operations rarely gain speed improvements from multithreading. And I can't deceipher through your code, but I guess after `fo.seek(file_['mid']); fo.readline()` you're at a newline, i.e. isn't `fo.tell()` what you're looking for?
      singulars
      1. POSeeking to middle of file in python
      1. USalko
    2. CO@alko I thought that in python multithreading's sole purpose was to help with IO operations, and multiprocessing was to help with cpu operations...
      singulars
      1. POSeeking to middle of file in python
      1. USSteve P.
    3. COI don't see how reading from a file from several threads could possibly be faster. It could be interesting after the asker figures it out to compare to a simple "for line in open('myfile'):"
      singulars
      1. POSeeking to middle of file in python
      1. USBoo
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload