windows - batch code not working when put inside a loop

This code works and picks a random file but when I put it inside the outer loop, I get empty "" instead.

rem scrambler
setlocal EnableDelayedExpansion

@echo off    
cd j:\target

rem for /R %%t in (*.mp3) do (

REM echo ********************
REM echo T folder is %%~dpt

cd j:\source
set n=0
for /R %%f in (*.mp3) do (

   set /A n+=1
   set "file[!n!]=%%f"
)
set /A "rand=(n*%random%)/32768+1"
rem copy "!file[%rand%]!" j:\target

echo "!file[%rand%]!"
cd j:\target
REM copy "!file[%rand%]!" %%~dpt
REM move "!file[%rand%]!" j:\old
rem )

PS. What I am trying to do is: identical source and target folders with mp3 files in them. Then I loop through the target files and for each target file, I overwrite them with a random mp3 file from the source folder. I (re)move the file from source and randomly pick another source file for the next target file, so I exhaust all files without any duplicates. In the end the target has same files and structure as source but they are now scrambled, that is if I can achieve it. I know the array takes some time so I want to optimize it too.

Answer

There are several flaws within your code

1. with nested for loops you'd need two levels of delayed expansion (what is possible)


2. For every iteration of the outer for /r you rebuild/overwrite the very same file[] array from source files.


3. if you build the array first there is no need to constantly switch between source and target (the folder is stored in the array anyway)


4. despite the name scrambler it's unclear what you want to achieve with two copy and one move, please elaborate.

---

:: Q:\Test\2019\01\06\SU_1391138.cmd
rem scrambler
@echo off    
setlocal EnableDelayedExpansion

cd j:\source

set n=0
for /R %%f in (*.mp3) do (
   set /A n+=1
   set "file[!n!]=%%f"
)

cd j:\target
for /R %%t in (*.mp3) do (
    set /A "rand=(n*!random!)/32768+1,m=n,n-=1"

rem following line demonstrates double delayedexpansion with a pseudo call
    call set "file=%%file[!rand!]%%"
    call set "file[!rand!]=%%file[!m!]%%"

    echo copy /B /Y "!file!" "%%t"
         copy /B /Y "!file!" "%%t"
)