Why is the array filled with "m" - windows

Question

I'm fairly new to batch and I've been trying to write some simple sorting programs. This program uses the most basic sorting system, and the code(from what I can see) seems to be error-free. Yet when I run it, a random list is generated, and it seems as if some sorting is going on, then the array is filled with the letter "m". I don't see why this is occurring, so if somebody could point me in the correct direction I would greatly appreciate it.
My code:
#echo off
color b
title sorting
set ar=0
set num=0
set check=0
set checknum=0
set totalnumber=500
set randmax=5000
:array
if %num% LSS %totalnumber% (
set /A a[%num%]=%random% %% %randmax%
set /A num=%num%+1
goto array
)
if %num% EQU %totalnumber% (
goto echo1
)
:echo1
for /F "tokens=2 delims==" %%s in ('set a[') do echo %%s
echo sort initialized
goto sort
)
:sort
set n=0
:sortloop
set /A m=%n%+1
if %n% EQU %totalnumber% (
goto check
)
if %a[%n%]% GTR %a[%m%]% (
set hold=%a[%m%]%
set a[%m%]=%a[%n%]%
set a[%n%]=%hold%
set /A n=%n%+1
goto sortloop
)
if %a[%n%]% LSS %a[%m%]% (
echo a[%n%] check
set /A n=%n%+1
goto sortloop
)
:check
set check=0
set checknum=0
:checkloop
set /A checknumplus=%checknum%+1
if %check% EQU %totalnumber% (
goto complete
)
if %checknum% EQU %totalnumber% (
set n=0
goto sort
)
if %a[%checknum%]% LSS %a[%checknumplus%]% (
set /A check=%check%+1
set /A checknum=%checknum%+1
goto checkloop
)
:complete
for /F "tokens=2 delims==" %%s in ('set a[') do echo %%s
for /F "tokens=2 delims==" %%s in ('set a[') do echo %%s > sortedlist.txt

Answer 1

When you need to use variables inside of variables in batch (most commonly when working with arrays), you need to use delayed expansion.
Right now, your code says set hold=%a[%m%]%. The interpreter is treating this value as the variable %a[% (which doesn't exist, so it using nothing), the literal character m, and the variable %]% (which also doesn't exist and is therefore empty).
To get around this, put setlocal enabledelayedexpansion at the top of your code and then change your set statement to set hold=!a[%m%]! (and do the same thing with the other lines that are using it).