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Re: Obtaining Random LIne from A file

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  • Subject: [mg129863] Re: Obtaining Random LIne from A file
  • From: Ramiro <ramiro.barrantes at gmail.com>
  • Date: Tue, 19 Feb 2013 01:00:46 -0500 (EST)
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Thank you so much for the reply.  My files are 50MB each, I don't think ReadList would work for my purposes, it would be too slow.  I am actually doing an MCMC simulation, doing (hopefully if I have time) millions of iterations and in each one I need to read a random line from one of many files, thus requiring this reading to happen as quickly as possible. Any suggestions? Each line is pretty much the same length.

Thanks,
Ramiro

On Sunday, February 17, 2013 4:08:27 AM UTC-5, David Bailey wrote:
> On 16/02/2013 06:07, Ramiro Barrantes wrote:
>
> > Hello,
>
> >
>
> > I would like to get a random line from a file, I know this can be done
>
> > with Mathematica but I am playing with using sed to see if it goes
>
> > faster, say I want to get line 1000
>
> >
>
> > In mathematica it would be:
>
> >
>
> > <<"! sed -n p1000 filename.txt"
>
> >
>
> > However, I am trying to put the filename as a variable, say
>
> >
>
> > filename="hugefile.txt"
>
> >
>
> > cmd="! sed -n p1000 "<>filename
>
> > <<cmd
>
> >
>
> > does not work.
>
> >
>
> > How can I do this?
>
> >
>
> > Lastly, I am getting a randomline using mathematica doing:
>
> >
>
> > getRandomLine[file_, n_] :=
>
> >    Block[{i = RandomInteger[{1, n}], str = OpenRead[file], res},
>
> >     Skip[str, "String", i];
>
> >     res = Read[str, Expression];
>
> >     Close[str];
>
> >     res[[2]]
>
> >     ]
>
> >
>
> > However, it is very slow so I was going to try with sed.Any suggestions?
>
> >
>
> > Thanks in advance,
>
> > Ramiro
>
> >
>
> >
>
> I would stick with Mathematica to do this job! How big is the file
>
> (number of lines and number of bytes)? If it will fit inside Mathematica
>
> comfortable, I'd see how it works to read it all in as a list of strings
>
> and pick the one you want:
>
>
>
> xx=ReadList["C:\\some file",String];//Timing
>
>
>
> Then you have an array of strings, and you can select what you want
>
> directly.
>
>
>
> Remember, the basic problem with reading at an arbitrary position in a
>
> text file, is that if the line lengths are not the same, any algorithm
>
> has to read every line before the one you want! If you create this file,
>
> you should consider packing the lines to make them all the same length -
>
> then you could access what you want very efficiently (but with a little
>
> more coding!)
>
>
>
> David Bailey
>
> http://www.dbaileyconsultancy.co.uk




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