How can I run a command with a variable? /// How can I produce a random number with with a variable?

Question

I'm aware that set zeroThroughNine=%Random%*9/32768 followed by echo %zeroThroughNine% will produce a a random number between and including 0 and 9. But it seems the interpreter doesn't evaluate the contents of the variable every time it is called, and as such, echo %zeroThroughNine% produces, for example, 7 every time.

I looked up a method for running commands using variables so that I could try to force it to work. I liked the question because it was very basal in its approach; something along the lines of "How can I run commands using variables?", tagged appropriately. I didn't much care for the answer because it was very narrow. The highest voted and selected answer was:

Simple. Just run set commandVar=echo "Hello world.", followed by echo %commandVar%.

Of course the truth is that only works for the echo command. >: [

Anyway I'll stop complaining. This is what I've tried:

set zeroThroughNine=set /a number=%Random%*9/32768 & echo %number% followed by echo %zeroThroughNine%

Unfortunately the & echo %number% section of my SET command runs immediately, producing "%number%" as output --and using echo %zeroThroughNine% produces "set /a number=8436*9/32768", for example, as output.

So two questions: How can I universally achieve running commands with the use of variables (or some alternative method), and perhaps more pressing, how can I achieve producing a new random number at the command line with each new command calling?

Answer 1

You should set number before you set zeroThroughNine to the command, like so:

set /a number=%Random%*9/32768
set zeroThroughNine=echo %number%
%zeroThroughNine%

Also, since zeroThroughNine already is an echo command, you don't need to add the extra echo before it.

EDIT:

Taking into account your Random calculation is needlessly complicated, the final code should be something like this (1 - 10 exclusive):

set /a number=%Random% %% 10
set zeroThroughNine=echo %number%
%zeroThroughNine%

Important thing is, rather than trying to do it all on one line, it is much more readable by separating it into two.

Answer 2

The CALL SET syntax allows a variable substring to be evaluated, the CALL page has more detail on this technique, in most cases a better approach is to use Setlocal EnableDelayedExpansion.

Command line (note that all % percent signs are escaped as ^% and that > and & characters are escaped within a pair of " double quotes:

set "zeroThroughNine=call set /a number=^%Random^% ^% 10>nul & call echo number=^%number^%"
%zeroThroughNine%
for /L %G in (1, 1, 10) do @%zeroThroughNine%

Batch script, CALL method (note that all % percent signs are escaped as %%):

@echo OFF
SETLOCAL
set "_zeroThroughNine=call set /a _number=%%Random%% %%%% 10 & call echo number=%%_number%%"
echo check variables 
set _
echo output
%_zeroThroughNine%
for /L %%G in (1,1,10) do %_zeroThroughNine%
echo check variables after evaluating 
set _
ENDLOCAL

Batch script, EnableDelayedExpansion only for output:

@echo OFF
SETLOCAL EnableExtensions DisableDelayedExpansion
set "_zeroThroughNine=set /a _number=!Random! %% 10 & echo Number=!_number!"

SETLOCAL EnableDelayedExpansion
echo check variables 
set _
echo output
%_zeroThroughNine%
for /L %%G in (1,1,10) do %_zeroThroughNine%
echo check variables after evaluating 
set _
ENDLOCAL
ENDLOCAL

Batch script, EnableDelayedExpansion script wide (note that ! exclamation sign is escaped as ^!):

@echo OFF
SETLOCAL EnableExtensions EnableDelayedExpansion
set "_zeroThroughNine=set /a _number=^!Random^! %% 10 & echo NUMBER=^!_number^!"
echo check variables 
set _
echo output
%_zeroThroughNine%
for /L %%G in (1,1,10) do %_zeroThroughNine%
echo check variables after evaluating 
set _
ENDLOCAL

Answer 3

Check out this question. Basically, it's an issue with how the %random% environment variable works...

EDIT:

To elaborate, the reason your random value was always 7 is because of how cmd's pseudo-random number generator works, not because of how the variables are interpreted. The matter is explained very well in this answer.

Essentially, in repeated runs of a batch file, %RANDOM% will produce a value very close to the previous run. Thus, the expression %RANDOM%*9/32768 produces the same result in each separate run because of the random value.

If I understand correctly, the question you're asking is how to better generate a random value 0 - 9 inclusive, which would be by using the following expression:

set /a zeroThroughNine=%RANDOM% %% 10