Windows batch if-else not working - windows

Question

[simple program that recieves an integer as input and prints if that number is trivial or not]
when i run this i get an error "( was unexpected at this time"
#echo off
set /a i=2
set /p input="enter an integer: "
set /a n=input
set /a t=n/2
:loop1
if %t% LSS %i% (
goto trivial
) else (
set /a t0=n%i
if %t0%==0 (
goto notTrivial
) else (
set /a i=i+1
goto loop1
)
)
:trivial
echo %n% is trivial
goto endd
:notTrivial
echo %n% is not trivial
:endd
pause > nul
but when I remove else statement in loop1 (which is btw unnecessary (because of goto command in if block)) it works
:loop1
if %t% LSS %i% (
goto trivial
)
set /a t0=n%i
if %t0%==0 (
goto notTrivial
) else (
set /a i=i+1
goto loop1
)
(how) is this possible?

Answer 1

When you remove the else clause, the code inside it is now out of any block.
Why does it matter? Because in batch files, lines or blocks of lines (code inside parenthesis) are first parsed and then executed. While parsed variable read operations are removed, being replaced with the value inside the variable at parse time, before starting to execute the command (more here).
So, in this code
) else (
set /a t0=n%i
if %t0%==0 (
goto notTrivial
) else (
set /a i=i+1
goto loop1
)
)
you change the value of the variable t0, but you can not retrieve this changed value inside the same block. But if you remove the else clause the code is not inside a block and everything works as intended (except syntax errors, try with set /a "t0=n %% i").

Answer 2

Firstly, you need to state the modulo operator % as %% in batch files.
Secondly, just move the command set /a t0=n%%i up before the if block begins, then it will work:
:loop1
set /a t0=n%%i
if %t% LSS %i% (
goto trivial
) else (
if %t0% EQU 0 (
goto notTrivial
) else (
set /a i+=1
goto loop1
)
)
So the change of variable t0 is moved outside of a command block ().
Alternatively, you could also enable delayed expansion:
setlocal EnableDelayedExpansion
rem INITIAL CODE PORTION...
:loop1
if %t% LSS %i% (
goto trivial
) else (
set /a t0=n%%i
if !t0! EQU 0 (
goto notTrivial
) else (
set /a i+=1
goto loop1
)
)
rem REMAINING CODE PORTION...
endlocal
You will notice the !t0! type expansion which, in contrast to %t0%, will expand t0 at execution time rather than parse time.
See also setlocal /? and endlocal /? for more information about these commands.