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Re: Obtaining Random LIne from A file

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  • Subject: [mg129860] Re: Obtaining Random LIne from A file
  • From: David Bailey <dave at>
  • Date: Tue, 19 Feb 2013 00:59:46 -0500 (EST)
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On 18/02/2013 10:59, awnl wrote:
> Am 17.02.2013 10:08, schrieb David Bailey:
>> 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 he just wants to get an arbitrary line that's not true: just choosing
> a position in the file at random and searching e.g. the previous and
> next linebreak would also result in picking a random line. Of course the
> probability of choosing longer lines would be larger than that for
> shorter lines, but it isn't clear from the question whether that would
> be a problem for what the OP tries to do...
>> 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!)
> ... and slightly (?) higher memory requirements...
> hth,
> albert
Well yes - but I assume that by 'arbitrary' he means a specific record 
somewhere in the file!

David Bailey

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