兜兜米

for /l %%a in (1,1,20) do (
    for /l %%b in (1,1,9) do (
          set /a a=!random!%%36
          call,set "var=!var!%%str:~!a!,1%%"
     )
    set /a b=!random!%%5+1&call :lp
    set "var="
)
goto :eof
:lp
for /f "tokens=%b%" %%c in ("%code%") do echo.>!var!.%%c
能否解释一下这段代码的意思，特别是set "var=!var!%%str:~!a!,1%%"