Prime numbers in batch - batch-file

Question

I've been working on a little project using batch files and I've ran into a problem. As far as I'm aware there's no way to run a check to see if a certain variable is a prime number, if I'm wrong would anyone please inform me of how to do so, otherwise, can anyone think of a workaround I could use (like checking if a number is equal to a number on a list of prime numbers on a txt file or whatever).
Thanks ^^
(Also it's worth noting I'm not very knowledgeable with batch files so please excuse any idiocy I may present..)

Answer 1

If you have a text file of prime numbers, 1 per line (obviously up through some limit), then the solution is trivial - just use FINDSTR.
Assuming you have a NUMBER variable containing a number, then
>nul findstr /x %NUMBER% "primes.txt" && (
REM prime actions go here
echo %NUMBER% is prime
) || (
REM not prime actions go here
echo %NUMBER% is NOT prime
)
UPDATE
Here is a native batch script that can test any valid integer supported by batch (signed 32 bit ints) to see if it is prime. Performance is much better than I thought possible.
::testPrime Number
::
:: Computes whether Number is a prime or not.
:: The result is printed to stdout.
::
:: ERRORLEVEL is also set to indicate the result:
:: 0 = Prime
:: 1 = Not Prime
:: 2 = Error
::
:: Number = Any valid integral expression supported by SET /A
::
#echo off
if "%~1"=="test" (
setlocal enableDelayedExpansion
for /l %%N in (3 2 0x7fffffff) do (
set /a "test1=num %% %%N, test2=%%N*%%N"
if !test1! equ 0 exit 1
if !test2! gtr !num! exit 0
)
)
setlocal disableDelayedExpansion
2>nul set /a "num=%~1" || (
>&2 echo invalid number: %1
exit /b 2
)
if %num% leq 1 (
echo %num% is NOT prime
exit /b 1
)
if %num% leq 3 (
echo %num% is prime
exit /b 0
)
2>nul set /a "1/(num %% 2)" || (
echo %num% is NOT prime
exit /b 1
)
(
cmd /c "%~f0" test
) && (
echo %num% is prime
exit /b 0
) || (
echo %num% is NOT prime
exit /b 1
)
exit /b
The test is actually split into 2 parts, the 2nd of which is actually run in a new CMD instance. The 2nd part actually appears at the top of the script. This is done for performance reasons. It is the only way I can break out of a FOR /L loop immediately without terminating the batch script.
You can integrate your code with the script easily enough. For example:
#echo off
::----------------------------------------------------
:: This 2nd part of :testPrime must be at top of script
::
if "%~1"=="test" (
setlocal enableDelayedExpansion
for /l %%N in (3 2 0x7fffffff) do (
set /a "test1=num %% %%N, test2=%%N*%%N"
if !test1! equ 0 exit 1
if !test2! gtr !num! exit 0
)
)
:: End of 2nd part of :testPrime
::-----------------------------------------------------
:: Your code goes here
:: I'll just call the test with some representative values
::
setlocal disableDelayedExpansion
for %%N in (
1 2 3 4 100001 100003 5000009 5000011 0x7fffffff-2 0x7fffffff
) do >nul call :testPrime %%N && (
rem prime number actions go here
echo %%N is prime!
) || (
rem non-prime number actions go here
echo Not prime (%%N^)
)
exit /b
::----------------------------------------------------
:: Here is the 1st part of :testPrime
::
:testPrime
2>nul set /a "num=%~1" || (
>&2 echo invalid number: %1
exit /b 2
)
if %num% leq 1 (
echo %num% is NOT prime
exit /b 1
)
if %num% leq 3 (
echo %num% is prime
exit /b 0
)
2>nul set /a "1/(num %% 2)" || (
echo %num% is NOT prime
exit /b 1
)
(
cmd /c "%~f0" test
) && (
echo %num% is prime
exit /b 0
) || (
echo %num% is NOT prime
exit /b 1
)
exit /b
The output for the above looks like this:
Not prime (1)
2 is prime!
3 is prime!
Not prime (4)
Not prime (100001)
100003 is prime!
Not prime (5000009)
5000011 is prime!
Not prime (0x7fffffff-2)
0x7fffffff is prime!
Finally, just for yucks, I wrote a variation that lists the next prime >= or <= a given number.
::nextPrime [/less] Num
::
:: List the minimum prime number >= Num
::
:: The /L option lists the maximum prime number <= Num
::
:: The ERRORLEVEL is set to the found prime number
::
:: Num = Any valid integral expression supported by SET /A
::
#echo off
setlocal enableDelayedExpansion
if "%~1"=="test" (
for /l %%N in (3 2 0x7fffffff) do (
set /a "test1=%2 %% %%N, test2=%%N*%%N"
if !test1! equ 0 exit 1
if !test2! gtr %2 exit 0
)
)
if "%~1"=="prev" (
if !num! lss 2 exit 0
set /a "test=num%%2"
if !test! equ 0 set /a num-=1
for /l %%N in (!num! -2 2) do cmd /c "%~f0" test %%N && exit %%N
exit 0
)
if "%~1"=="next" (
if !num! lss 2 exit 2
set /a "test=!num!%%2"
if !test! equ 0 set /a num+=1
for /l %%N in (!num! 2 0x7fffffff) do cmd /c "%~f0" test %%N && exit %%N
exit 0
)
set "cmd=next"
if /i "%~1" equ "/L" (
set "cmd=prev"
shift /1
)
2>nul set /a "num=%~1" || exit /b 0
cmd /c "%~f0" %cmd% || echo !errorlevel!
And here is a demonstration of usage with output:
D:\test>nextPrime 10000000
10000019
D:\test>nextPrime /l 10000000
9999991

Answer 2

All of those scripts seem awfully (and unnecessarily) large to me.
An easier way to do this is using... I believe the term I'm looking for is either modulo or modulus expressions (I think modulo is the plural or modulus).
#echo off & setlocal enabledelayedexpansion
:a
cls
set /p num=Type a number to be checked:
cls
set num2=%num%-1
if %num% leq 2 goto yes
for /l %%i in (2,1,%num2%) do (
set rem=%num% %% %%i
if %rem% neq 0 goto no
)
:yes
echo %num% is a prime number.
pause
goto a
:no
echo %num% is not a prime number.
pause
goto a
Basically, it gets a user-defined variable and checks, when divided by a number, if the remainder (rem) is 0 or not.
This way is a little slow, but the shortest code. You can make it a bit shorter by putting another if statement before the for loop that checks if the number has a remainder when divided by two.
Hope it helps.

Answer 3

Another prime lister, this one does'nt use a file and can reach 64,000,000 if you have patience. Keeps a list of prime divisors in an environment variable. If i had a batch integer square root routine i could make it faster.
#echo off
::batch prime list up to 64M by Antoni Gual
:: does not use files!!
setlocal enabledelayedexpansion
set bitmap=
set n=Y
set /a test=3,npri=3
echo 1th prime is 2 & echo 2th prime is 3
:nextpri
set /a test+=2,index=0,div=3
if %test% LSS 8000 set bitmap=%bitmap%%n%
if %test% gtr 64000000 exit /b
:nextest
if "!bitmap:~%index%,1!"=="N" goto nextdiv
set /a resi=!test!%%!div!
if %resi% equ 0 set n=N& goto nextpri
:nextdiv
set /a index+=1, div+=2
set /a div2=div*div
if %div2% gtr %test% (
set n=Y
echo %npri%th prime is %test%
set /a npri+=1
goto nextpri)
goto nextest

Answer 4

The following script does not perform a primality test. Rather, it generates prime numbers up to a specific bound (the hard-coded 1000 in this particular case). You could generate the list once and then use it in your script(s):
#echo off
echo 2
echo 3
echo 2 > primenos.txt
echo 3 >> primenos.txt
set current=3
:numbercalc
set tim=3
set /a max=%current%/5
:try
set /a t=%current%/%tim%
set /a u=%t%*%tim%
if %u% EQU %current% goto noprime
set /a tim+=2
if %tim% GTR %max% goto endtry
goto try
:endtry
echo %current%
echo %current% >> primenos.txt
:noprime
set /a current+=2
if %current% GTR 1000 goto end
goto numbercalc
:end
pause
Taken from here...

Answer 5

Assuming you have a text file with prime numbers (each number in a single line) you could do it like this:
#echo off
if "%1"=="" (echo Syntax: %~nx0 number & exit /b 2)
for /F "tokens=*" %%p in (primes.txt) do (
if %%p EQU %1 (
echo %1 is prime!
exit /b 0
)
)
echo %1 is not prime!
exit /b 1
Example call: isprime.cmd 2 would give you 2 is prime!.

Answer 6

Here is what I made. It finds all prime numbers up to 214748 but it is accurate and fast and outputs the results to a file called "pn.txt" and the current number to "cn.txt" (this was to let me know what its limit was):
(#echo off)&((set n=3)&((set tn=%n%)&((set d=2)&((set d2=)&((set m=100)&((echo.prime-numbers>pn.txt)&((echo. >>pn.txt)&((echo.1>>pn.txt)&((echo.2>>pn.txt)&((echo.1)&((echo.2)&(goto a))))))))))))
:a
(echo.%n%cn.txt)&((set tn=%n%)&(set tn=%n:~-1%))
(if %tn%==2 ((set /a n=%n%+1)&((set d=2%d2%)&(goto a))))&((if %tn%==4 ((set /a n=%n%+1)&((set d=2%d2%)&(goto a))))&((if %tn%==6 ((set /a n=%n%+1)&((set d=2%d2%)&(gotoa))))&((if %tn%==8 ((set /a n=%n%+1)&((set d=2%d2%)&(goto a))))&((if %tn%==0 ((set /a n=%n%+1)&((set d=2%d2%)&(goto a))))&(goto b)))))
:b
((set /a tn=%d%*%d%)&(if /i %tn% equ %n% ((set /a n=%n%+1)&((set d=2%d2%)&(goto a)))))&((if /i %tn% gtr %n% ((set tn=%n%)&((set l=%d%)&((if /i %n% equ 10000 (set d2=0))&((if /i %n% equ 100000 set (d2=00))&((set d=2%d2%)&((set m=100%d2%)&(goto d)))))))&((set /a d=%d%+1)&(goto b))
:d
(title verifing %n% at %d%)&(set tn=%n%)
set /a tn=%tn%*%m%
set /a tn=%tn%/%d%
set tn=%tn:~-2%
(if /i %tn% equ 00%d2% ((set /a n=%n%+1)&((set d=2%d2%)&(goto a))))&(((set /a tn=%n%-1)&(if %d%==%tn% ((echo.%n%)&((echo.%n%>pn.txt)&(((set /a n=%n%+1)&((set d=2%d2%)&(goto a)))))))&(if %d%==%l% ((echo.%n%)&((echo.%n%>pn.txt)&((set /a n=%n%+1)&((set d=2%d2%)&(goto a)))))))&((set /a d=%d%+1)&(goto d)))

Answer 7

#echo off
::PRIMES
set multiple2=1
set add=1
set counter=1
color f0
set /p range=Computer primes 0-?:
set /a limit=(range/2)+1
set ut=3
mkdir prime0-%range%
cd prime0-%range%
echo >>2
:opipe
echo >>%ut%
set /a ut=ut+2
if %ut% GEQ %range% goto next
goto opipe
:next
set /a multiple2=multiple2+2
if %multiple2% GEQ %limit% goto end
set /a add=add+2
set /a multiple=multiple2
:process
set /a multiple=multiple+add
del %multiple%
if %multiple% GEQ %range% goto next
goto process
:end
CD ..
echo 2 >>ALLprime0-%range%.txt
:offx
set /a counter=counter+2
if exist prime0-%range%\%counter% echo %counter% >>ALLprime0-%range%.txt
if %counter% GEQ %range% goto down
goto offx
:down
echo Computation Succesful
pause
exit
::RMDIR /S /Q prime0-%range%

Answer 8

Please excuse me for beating a dead horse. Here is a radically differnt approach to generating primes in pure batch that overperforms everything else I have found .
It's based in Keeping a list of multiples of previous found primes around the window we're checking. The numbers not found in the list are primes. For keeping the list I use environment variables of the form #composite=next_increment .It uses a single loop, much faster and degrading slower than the nested double loop used in other algorithms. Unforunately a FOR and an auxiliar subrputine are required to overcome the impossiblilty of having repeated keys in environment, as sometimes multiples of different primes clash.
BTW the idea comes from an exerciste in Knuth's TAOCP Vol 3 page 617.
:: prime table using rotating multiples stored in environment
#echo off
setlocal enabledelayedexpansion
mode con cols=90
set #25=-10
set /a num=7,inc1=4,cnt=0,inc2=0,num1=0, maxprime=10000
set lin= 0:
call:line 2 & call:line 3 & call:line 5
:nextnum
if defined #%num% (
for %%i in (!#%num%!) do (
if %%i lss 0 (set /a num1=%num%-%%i,"inc2=-%%i<<1") else (set /a num1=%num%+%%i,"inc2=-(%%i>>1)")
call :aux !num1! !inc2!
)
set #%num%=
) else (
call :line %num%
set /a num1= num * num
if %inc1% equ 4 (set/a "inc2=num<<2") else (set /a "inc2=-(num<<1)")
if !num1! leq %maxprime% set #!num1!=!inc2!
)
set /a num+=inc1, inc1=6-inc1
if %num% lss %maxprime% goto nextnum
echo %lin%
pause & goto:eof
:aux
if %1 leq %maxprime% set #%1=%2 !#%1!
goto:eof
:line formats output in 10 right aligned columns
set num2= %1
set lin=%lin%%num2:~-8%
set /a cnt+=1,res1=(cnt%%10)
if %res1% neq 0 goto:eof
echo %lin%
set cnt1= %cnt%
set lin=%cnt1:~-5%:
goto:eof

Answer 9

#echo off
:top
set /p number=number:
set /a check=1
:work
set /a check=%check%+1
set /a result=%number%/%check%
set /a abc=%result%+1
if %abc% LSS %check% goto end
SET /a modulo=%number% %% %check%
IF %modulo%==0 (goto factor
) ELSE (goto notfactor
)
:factor
echo %check%
set /a other= %number%/%check%
echo %other%
echo.
goto work
:notfactor
goto work
:end
echo If you see 2 numbers this its a prime otherwise its composit
echo.
echo i am too lazy to code it further and have already spend way too much time trying to make this
pause
cls
goto top
this will also tell the prime factors if any.