Batch file to delete half the lines of a text file at random?

Question

I need a way to use batch to look at every line of a text file, and delete half of the lines in that text file, choosing which lines to delete at random.

This is to simu

Answer 1

This is a native Windows batch script using Jscript to randomise the lines of a text file and print half of them.

The size of the file is limited to the RAM that Jscript is able to request.

@if (@X)==(@Y) @end /* harmless hybrid line that begins a JScript comment
:batch file portion
@echo off
if "%~1"=="" (
echo(Purpose: Randomises a text file - prints every 2nd random line
echo(
echo(Syntax: "%~0" "inputfile.txt" ^> "outputfile.txt"
echo(
pause
goto :EOF
)

:: Randomises a file
::
:: Reads the lines of file %1 into a jscript sparse array
:: with a random number and space before each line.
:: The array is sorted using the random numbers,
:: and the randomised lines are printed - prints every 2nd random line


@echo off
cscript //E:JScript //nologo "%~f0" %* 
:pause
exit /b


************ JScript portion ***********/

  e=0;
  var array = new Array();
  fso = new ActiveXObject("Scripting.FileSystemObject");
  input=fso.OpenTextFile((WScript.Arguments(0)),1);

  while (!input.AtEndOfStream) {
    array[e]=Math.random()+" "+input.ReadLine();
    e++;
  }
  input.close();

  array.sort();

  var re = new RegExp(".*? (.*)");

  c=0;
  while (c