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 method preserve the order of original lines.

@echo off
setlocal EnableDelayedExpansion

rem Generate an array of line numbers with all file lines
for /F "delims=:" %%a in ('findstr /N "^" input.txt') do (
   set "n[%%a]=%%a"
   set "lines=%%a"
)

rem Delete half the numbers in the array in random order
set /A halfLines=lines/2
for /L %%n in (%lines%,-1,%halfLines%) do (
   set /A rnd=!random!*%%n/32768+1
   set /A n[!rnd!]=n[%%n]
   set "n[%%n]="
)

rem Reorder the resulting elements
for /F "tokens=2,3 delims=[]=" %%i in ('set n[') do (
   set "l[%%j]=1"
   set "n[%%i]="
)

rem Copy such lines
(for /F "tokens=1* delims=:" %%a in ('findstr /N "^" input.txt') do (
   if defined l[%%a] (
      echo(%%b
      set "l[%%a]="
   )
)) > output.txt