How to make random number variable with variables within it? - batch-file

Question

So I've been trying to make the following code:
set /a num1=10
set /a num2=%random% %%60 +%num1%
echo %num2%
(This is simplified)
For this code I need the +%num1% to be a variable because I need to be able to change the lowest number.
For some reason, instead of giving me a random number it gives a totally unrelated number, that isn't random either, but the same every time. My first thought was it was perhaps adding the second variable instead of making a randomizer. That is not the case though, and I'm not sure how to fix this issue.
I have also tried the following code:
set /a num1=10
set /a num2=(%random%*60/32768)+%num1%
echo %num2%
The issue with this code is it never seems to work as randomizer for me even without the variable.
Any help is appreciated.

Answer 1

Here's some examples to assist you:
Set "num1=10"
Set /A "num2 = (%RANDOM% %% 60) + num1"
Echo(%num2%
Set "num1=10"
Set /A "num2 = (%RANDOM% * 60 / 32768) + num1"
Echo(%num2%
Please note, that we have only been provided with a very small portion of your batch file, so if this code is part of a parenthesized block, you may need to enable delayed expansion and use !RANDOM! and possibly !num2! instead of %RANDOM% and %num2% respectively.

Answer 2

In the first form you are have to account for modulus bias, as described further in this question Why do people say there is modulo bias when using a random number generator?
(Note: the accepted answer there has a flaw which I pointed out and provide a solution to in my answer to that question.)
In both the first and the second form they present an incorrect range of values the way it is written. More info can be found here How to use random in BATCH script?
Long story Short:
In both forms, if you want your range to start at 10 and go to 60 you will need to adjust it to be using a modulus of 50, in the first form you must account for modulus bias in your calculations causing some numbers to appear more often.
The first form can be fixed using this method:
SET "Min=10"
SET "Max=60"
SET /A "Discard= 32768 - ( ( ( 32768 %% (Min-Max) ) + 1 ) %% (Min-Max) )"
:Rand
SET /A "num2=%random%"
IF %num2% GTR %Discard% GOTO :Rand
SET /A "num2= num2 %% (Min-Max) + Min"
echo=%num2%
The second form can be fixed using this method:
SET "Min=10"
SET "Max=60"
SET /A "num2= %random% * ( Max - Min + 1 ) / 32768 + Min "
echo=%num2%
Example of Full script:
#(SETLOCAL
ECHO OFF
)
CALL :Main
( ENDLOCAL
EXIT /B
pause
)
:Main
SET "Min=10"
SET "Max=60"
SET /A "Discard= 32768 - ( ( ( 32768 %% (Min-Max) ) + 1 ) %% (Min-Max) )"
ECHO=Form1:
CALL :Form1
ECHO=Form2:
CALL :Form2
pause
GOTO :EOF
:Form1
SET "num2=%random%"
IF %num2% GTR %Discard% GOTO :Form1
SET /A "num2= num2 %% ( Min - Max ) + Min"
echo=%num2%
GOTO :EOF
:Form2
SET /A "num2= %random% * ( Max - Min + 1 ) / 32768 + Min "
echo=%num2%
GOTO :EOF
Results:
C:\WINDOWS\system32>C:\Admin\SE\testrandom.cmd
Form1:
10
Form2:
31
Press any key to continue . . .
C:\WINDOWS\system32>