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Topic: Obtaining Random LIne from A file
Replies: 9   Last Post: Feb 21, 2013 5:46 AM

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David Bailey

Posts: 714
Registered: 11/7/08
Re: Obtaining Random LIne from A file
Posted: Feb 17, 2013 4:08 AM
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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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