[文本处理] 统计字符串长度的批处理，诡异的错误，却找不到原因

@echo off
setlocal enabledelayedexpansion
set /p str=<a.txt

::[方法1]
set mx=50
for /l %%a in (0,1,%mx%) do if "!str:~%%a,1!"=="" set num=%%a &goto ok
:ok
echo 长度=%num% 
::pause

::[方法2]
set /a st=0
:loop
if "!str:~%st%,1!" neq "" set /a st+=1&goto loop
echo,len=%st%
::pause

echo.[方法3]
set /a n=8192*2,max=1
set "var="
pause
for /l %%a in (1 1 14) do (
  if defined var set /a n=var
  echo n=%n% 
  pause
 )
复制代码

@echo off
setlocal enabledelayedexpansion
set /p str=<a.txt

::[方法1]
set mx=50
for /l %%a in (0,1,%mx%) do if "!str:~%%a,1!"=="" set num=%%a &goto ok
:ok
echo 长度=%num% 
::pause

::[方法2]
set /a st=0
:loop
if "!str:~%st%,1!" neq "" set /a st+=1&goto loop
echo,len=%st%
::pause

echo.[方法3]
set /a n=8192*2,max=1
set "var="
pause
for /l %%a in (1 1 14) do (
  if defined var set /a n=var
  echo n=%n% 
  pause
  set /a n/=2
  for %%i in (!n!) do (
    if "!str:~%%i,1!=="" (set /a var=n) else (
	set str=!str:~%%i!
	set /a max+=%%i,var-=%%i	
    )
   )
)
endlocal 
echo 长度: %max%
pause  	
复制代码