Batch /a doesn't store a value - variables

Question

Code:
#echo off
set /a x=5
set /a TNUM=%x%-3
echo %TNUM%
pause
And the output is:
ECHO is off
So, how do I fix this?
Here's the full code:
#echo off
setlocal enableextensions
set /a COLUMNS=60
set /a ROWS=40
mode con: cols=%COLUMNS% lines=%ROWS%
:main
set /a xpos=5
set /a ypos=5
call:displayScreen
pause >nul
goto :eof
::FUNCTIONS::
:displayScreen
set /a TEMPR=%ROWS%-3
for /l %%c in (1,1,%COLUMNS%) do echo|set /p=#
for /l %%r in (1,1,%TEMPR%) do (
echo|set /p=#
if %%r==%ypos% (
set /a TNUM=%xpos%-3
echo %TNUM%
for /l %%t in (1,1,%TNUM%) do echo|set /p=A
echo|set /p=o
set /a TNUM=%COLUMNS%-%xpos%-1
for /l %%t in (1,1,%TNUM%) do echo|set /p=A
) else (
set /a TNUM=%COLUMNS%-2
for /l %%t in (1,1,%TNUM%) do (
echo|set /p=A
)
)
echo|set /p=#
echo.
)
for /l %%c in (1,1,%COLUMNS%) do echo|set /p=#
The main problem is in the display screen function. It is supposed to print the screen and have the x at the two coordinates I gave it. I put in echo %TNUM% so I could see what was in it, and it always amounts to "".

Answer 1

There are mainly three ways to obtain the indicated behaviour
1 - Problems with variable expansion. In batch files, lines or blocks of lines (a set of lines enclosed in parenthesis), are parsed, executed and then the operation is repeated on the next line/block.
During the parse phase all read operations on variables (any %var%) are removed from code, replacing them with the value stored in the variable before starting to execute the line/block of code.
So, if inside the line/block the value of a variable is changed, this value can not be readed/retrieved inside the same line/block during execution phase. Why? Because there are no read operations in the line/block to retrieve the value of the variable. This code
set x=0
if 1==1 (
set x=1
set y=%x%
)
echo x=%x% y=%y%
will echo to console x=1 y=0 Why? When the if block of code is parsed (the full block), the read operations (=%x%) where replaced at parse time with the values inside the variables at parse time and the code executed is
if 1==1 (
set x=1
set y=0 <- Read operation in x replaced with its value at parse time
)
that is, the changed value stored in x can not be retrieved.
This case is usually solved with the usage of delayed expansion. It is a behaviour that can be activated with command setlocal enabledelayedexpansion and that allow the programmer to change, where needed, the syntax to access variable content from %var% to !var!, indicating to the parser that the expansion of the variable (the replacement of the read operation with the value of the variable) must be delayed until the execution phase. In the proposed sample it should be
setlocal enabledelayedexpansion <- delayed expansion enabled
set x=0
if 1==1 (
set x=1
set y=!x! <- Read operation delayed
)
echo x=%x% y=%y%
2 - Spaces. An habitual problem with variable declaration is
set /a TNUM = %x%-3
^.......This value is included in the variable name
The space is included in the variable name, and echoing %TNUM% fails as this is not the name of the variable, it is named %TNUM %.
BUT as i forgot and #jeb corrects in comments, while set and set /p exhibit this behaviour, this will NOT happen when set /a is used. In this case a parser with different rules is used (for example, it allows variables references without previous expansion) and the indicated space will be supressed.
3 - You machine has extensions disabled by default. If extensions are disabled, set /a will not work and no value will be assigned to the variable
#echo off
setlocal enableextensions
set /a x=5
set /a TNUM=x-3
echo %TNUM%
Summarizing, to execute the line echo %TNUM% and obtain the indicated ECHO is off it is necessary that when the line is parsed TNUM is not defined or it only contain spaces or tabs.
As, a priori, when the line set /a TNUM=%x%-3 is executed, a value is assigned to the variable (at least the -3), it seems your problem is the case 1 (the echoed value was replaced before the execution) or the case 3 (you have no value to echo as the set operation failed).
edited With all the code posted, the problem is delayed expansion.
#echo off
setlocal enableextensions enabledelayedexpansion
set /a COLUMNS=60
set /a ROWS=40
mode con: cols=%COLUMNS% lines=%ROWS%
:main
set /a xpos=5
set /a ypos=5
call :displayScreen
pause >nul
goto :eof
::FUNCTIONS::
:displayScreen
set /a TEMPR=%ROWS%-3
for /l %%c in (1,1,%COLUMNS%) do echo|set /p=#
for /l %%r in (1,1,%TEMPR%) do (
echo|set /p=#
if %%r==%ypos% (
set /a TNUM=%xpos%-3
for /l %%t in (1,1,!TNUM!) do echo|set /p=A
echo|set /p=o
set /a TNUM=%COLUMNS%-%xpos%
for /l %%t in (1,1,!TNUM!) do echo|set /p=A
) else (
set /a TNUM=%COLUMNS%-2
for /l %%t in (1,1,!TNUM!) do (
echo|set /p=A
)
)
echo|set /p=#
)
for /l %%c in (1,1,%COLUMNS%) do echo|set /p=#

Answer 2

It looks like it isn't waiting to evaluate %x%. So you can add ENABLEDELAYEDEXPANSION so that it will wait to evaluate x so that it will be initialized during the math operation.
#echo off
setlocal ENABLEDELAYEDEXPANSION
set /a x=5
set /a TNUM=!x!-3
echo !TNUM!
endlocal
pause

Answer 3

Within a block statement (a parenthesised series of statements), the entire block is parsed and then executed. Any %var% within the block will be replaced by that variable's value at the time the block is parsed - before the block is executed.
Since the for...%%r... loop executes a block of code (from the ( following the do through to the matching ) ) then any %tnum% appearing within that block will be replaced by the value of tnum when the loop was parsed. Tnum has no value when the block is started, so that's what is shown.
tnum is being set within the loop, so since you have enabledelayedexpansion set to ON in the setlocal enabledelayedexpansion statement at the start of the program, you need to use !tnum! in place of %tnum% within the loop to access the value of tnum as it changes.