[SOLVED] Commenting via REM and :: doesn't work in batch script

Commenting via REM and :: doesn't work in batch script

I just want to leave a reminder about the %~ symbol combination, but my running my batch file outputs this:

C:\Users\batch\batch>add_one2The following usage of the path operator in batch-parameter
substitution is invalid: %~n' is passed argument
For valid formats type CALL /? or FOR /?
C:\Users\batch\batch>

Here's the script:

@echo off
REM '%~n' is passed argument
goto :main

:add_one
setlocal
    echo  Running 'add_one'...
endlocal & set /a %~1=%~2+1
goto :eof

:main
setlocal
    set /a x=1
    set /a y=50
    
    echo Created variable X and set it to %x%
    echo.
    call :add_one y %y%
    echo.
    echo The value of X is now %x%
endlocal
goto :eof

I tried ::, REM, and escaping in quotes, but it didn't work either.

Solution

Parameter expansion happens before other commands (see How does the Windows Command Interpreter (CMD.EXE) parse scripts? for much more detail), so the interpreter is trying to expand %~n before the REM command tells it that that line is a comment. Because argument-style variables must contain a digit, you will always get an error when you only use a letter.

For better information, use the actual variables in the function header documentation:

@echo off
goto :main

REM ===================================================
REM Arguments: %1 - The name of the variable to store
REM                 the value in
REM            %2 - The value to add
REM ===================================================
:add_one
setlocal
    echo  Running 'add_one'...
endlocal & set /a %~1=%~2+1
goto :eof

:main
setlocal
    set /a x=1
    set /a y=50
    
    echo Created variable X and set it to %x%
    echo.
    call :add_one y %y%
    echo.
    echo The value of X is now %x%
endlocal
goto :eof