Use FOR /L to iterate a range of numbers - much faster than SET /A with GOTO loop.
Use SETLOCAL to localize variables so no need to undefine them. The EnableDelayedExpansion option does just that so that you can use !var! within a FOR loop.
A minor optimization - it is faster to combine multiple math computations into one SET /A.
@echo off
setlocal enableDelayedExpansion
for /l %%N in (1 1 100) do (
set /a "fizzbuzz=%%N%%15, buzz=%%N%%5, fizz=%%N%%3"
if !fizzbuzz! == 0 (
echo FizzBuzz
) else if !buzz! == 0 (
echo Buzz
) else if !fizz! == 0 (
echo Fizz
) else echo %%N
)
You might consider parameterizing the start and end conditions of the loop.
@echo off
REM %1=start %2=end
setlocal enableDelayedExpansion
for /l %%N in (%1 1 %2) do (
set /a "fizzbuzz=%%N%%15, buzz=%%N%%5, fizz=%%N%%3"
if !fizzbuzz! == 0 (
echo FizzBuzz
) else if !buzz! == 0 (
echo Buzz
) else if !fizz! == 0 (
echo Fizz
) else echo %%N
)
You could even parameterize the two divisors, but then a different algorithm is needed. Here is a fully parameterized solution that is efficient, though perhaps a bit obfuscated. I still use a modulo operation to test if divisible, but instead of an IF statement, I intentionally divide by zero to raise an error and trigger the conditional execution of the SET statement. Of course I redirect error messages to NUL to avoid unwanted error messages..
@echo off
REM %1=start %2=end %3=divisor1 %4=divisor2
setlocal enableDelayedExpansion
for /l %%N in (%1 1 %2) do (
set "val="
set /a "1/(%%N %% %3)" 2>nul || set "val=Fizz"
set /a "1/(%%N %% %4)" 2>nul || set "val=!val!Buzz"
if defined val (echo !val!) else echo %%N
)