Using an if statement inside a for loop - batch-file

Question

I am trying to make a batch file to solve the first Project Euler problem, http://projecteuler.net/problem=1, but I need an if statement inside my loop to check if n modulo 3 or 5 is 0. And the sum has suddenly stopped working.
My code:
echo off
set sum=0
for /l %%n in (1,1,999) do (
set a/ sum+=%%n *(only add if n%%3 == 0 or n%%5 == 0)*
)
echo %sum%
pause

Answer 1

Here is a very efficient solution, though it is a bit obfuscated:
#echo off
setlocal
set /a sum=0
for /l %%N in (1 1 999) do set /a "sum+=%%N/!((%%N%%5)*(%%N%%3))" 2>nul
echo %sum%
The expression (%%N%%5)*(%%N%%3) yields zero if %%N is divisible by 3 or 5, or non-zero if it is not divisible by either. The ! takes the inverse logical value, so 0 becomes 1, and non-zero becomes 0. Dividing %%N by that expression yields either %%N or a division by zero error. So simply add that entire expression to the sum, and redirect error messages to nul.
Final result - only numbers divisible by 3 or 5 are added :-)

Answer 2

#ECHO OFF
SETLOCAL
set /A sum=0
for /l %%n in (1,1,999) do (
CALL :modulo %%n
IF DEFINED addme set /a sum+=%%n
REM CALL echo %%n %%sum%% %%addme%%
)
echo %sum%
GOTO :EOF
:modulo
:: set addme if %1 %% 3 == 0 or %1 %% 5 == 0
SET /a addme = %1 %% 3
IF %addme%==0 GOTO :EOF
SET /a addme = %1 %% 5
IF %addme%==0 GOTO :EOF
SET "addme="
GOTO :eof
Simply pass each value to the :modulo routine in turn and set a flag value to (clear or not)
OR
#ECHO OFF
SETLOCAL enabledelayedexpansion
set /A sum=0
for /l %%n in (1,1,999) do (
SET /a mod=%%n %% 3
IF NOT !mod!==0 SET /a mod=%%n %% 5
IF !mod!== 0 set /a sum+=%%n
rem CALL echo %%n %%sum%%
)
echo %sum%
GOTO :EOF
which does the same thing using delayedexpansion.

Answer 3

And the sum has suddenly stopped working.
I think your sum stopped working because your set needs to have the slash in front of the 'a', and not behind it, like this:
SET /A sum+=%%n
Also, there isn't an OR operator in DOS Batch, so you'll need to use a nested IF for that. This worked for me:
echo off
SETLOCAL ENABLEDELAYEDEXPANSION
set sum=0
for /l %%n in (1,1,999) do (
SET /A mod3=%%n%%3
SET /A mod5=%%n%%5
IF !mod3!==0 (
SET /A sum+=%%n
) ELSE (
IF !mod5!==0 (
SET /A sum+=%%n
)
)
)
echo %sum%
ENDLOCAL
If you need more help, check out Rob van der Woude's Scripting pages. Specifically, here is a link to his page on performing mathematical operations in DOS batch files.