batch-fileRandom In Batch Files

Random Numbers

Using the dynamic variable %Random%, we can get a random integer from 0 to 32767. For example:

echo %random%

This obviously, returns an integer from 0 to 32767. But sometimes we want it to be in a specific range, say from 1 to 100.


Generating Random Numbers Within Specific Range

The basic method to do so is listed below.

set /a result=(%RANDOM%*max/32768)+min

where max is the top number that can be generated, and min is the smallest number that can be generated. Note that you will not get any decimal numbers because set /a rounds down automatically. To generate a decimal random number, try this:

set /a whole=(%RANDOM%*max/32768)+min
set /a decimal=(%RANDOM%*max/32768)+min
echo %whole%.%decimal%

Generating Random Numbers larger than 32767

If you try

set /a whole=(%RANDOM%*65536/32768)+1

you will most likely get random numbers that are odd.

To generate numbers larger than 32767, here is a better method.

set /a result=%random:~-1%%random:~-1%%random:~-1%%random:~-1%%random:~-1%%random:~-1%

The previous code extracts the 1 character from each %random%. But this is done on purpose.

Since the random number could be one digit number, extracting the last 2 digit won't work. That's why we extract only the last character. In this case, we have 6 %random:~-1%, generating the maximum of 999999, and the minimum at 000000, you may need to adjust this to suit your needs.

Pseudorandom

cmd.exe generate the seed based on the time the cmd section started, so if you start mutliple section at the nearly same time, the result may not be 'random' enough.

Random Alphabets

Unfortunately, batch does not have a built-in method to generate alphabets, but using %random% and for loop, we can 'generate' alphabets.


This is a simple idea of how this works.

set /a result=(%random%*26/32768)+1
for /f "tokens=%result%" %%I in ("A B C D E F G H I J K L M N O P Q R S T U V W X Y Z") do (
    echo %%I
)
  • The first set /a statement generate a random number N between 1 to 26
  • The for /f statement picks the Nth item from a list of A to Z.
    • Return the result

One can put a total 31 items in 1 for loop, and practically unlimited items using [this method].(Batch - for loop parameter order

Pseudorandom And Uniform Random In Batch

Pseudorandom Distribution

Accorinding to this Stack Overflow answer, user CherryDT pointed out this code:

set /a num=%random% %% 100

does not give a uniform distribution.

The internal dynamic variable %random% does gives a uniform distribution, but the above code will not be a uniformed random. This code generates a random number between 0 ~ 99, but the result will not be uniform. 0 ~ 67 will occur more than 68 ~ 99 since 32767 MOD 100 = 67.

To generate a uniform distributed random using the above code, then 100 must be changed. Here is a method to get a number that creates a uniform distribution.

32767 mod (32767 / n)

where n is an integer, between 0 ~ 32767, the result may be decimal and may not work in batch.


Uniform Distribution

set /a result=(%RANDOM%*100/32768)+1

This method will generate a uniform distribution. It avoids using %, which is more like "remainder" then "modulus" in a batch script. Without using %, the result will be uniform.

Alternatively, here is an inefficient, but uniform method.

set /a test=%random%

if %test% geq [yourMinNumber] (
    if %test% leq [yourMaxNumber] (

        rem do something with your random number that is in the range.

   )
)

Change [yourMinNumber] and [yourMaxNumber] accordingly to your own values.