How batch script get substring by variable index

I know we could get substring like this in batch script:

SET a=abcdefgh
ECHO %a:~3,2%

But how can I get letter by a variable index? Kind like:

SET index=3
ECHO %a:~%index%,1%

>Solution :

Since the substring operation needs a constant it has to be called in a subprocess, that’s when CALL comes in handy:

SET _index=3
call set b=%%a:~%index%,1%%
echo (%b%)

See https://ss64.com/nt/syntax-substring.html for more details. SS64/nt is an excellent resource for anything batch related.