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 October 12th, 2011, 06:02 PM #1 Member   Joined: Aug 2009 From: Copenhagen, Denmark Posts: 45 Thanks: 0 Mathematica: Applying rules to random integer generation Hello I have a problem in Mathematica. In the range 1-30 I will like Mathematica to choose five almost random integers. I can do so by writing: RandomSample[Range[30], 5] The thing is how to apply some rules, restricting the random command. Like this: 1) At most 2 integers =<12 2) At least one integer >=25 3) Each of the integers 3,4 and 7 can't be a divisor for more than 3 of the random chosen integers (so e.g. 3-12-15-30 or 7-21-28-14 is not possible) 4) At most one "pair of integers" (if both e.g. 3 and 4 are chosen, I call it a "pair"). Anyone with suggestions on how to solve this in Mathematica language?
 October 12th, 2011, 08:11 PM #2 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 937 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Mathematica: Applying rules to random integer generation Write a function that checks if the conditions are met, then define a function that stores a random number in a variable and checks if the conditions are met; if so return the variable's value, if not recurse.
October 13th, 2011, 05:42 PM   #3
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by CRGreathouse Write a function that checks if the conditions are met, then define a function that stores a random number in a variable and checks if the conditions are met; if so return the variable's value, if not recurse.
I am not totally sure on how to understand this. But I have made a try:

chosen = Sort[RandomSample[Range[36], 7], Less];
less12 = Count[Select[chosen, # <= 12 &], _Integer];
great31 = Count[Select[chosen, # > 31 &], _Integer];
divisor = Count[Select[chosen, Divisible[#, {3, 8}] &], _Integer];
test = If[less12 == 2,
If[great31 == 1, If[divisor <= 5, chosen, False], False], False]

If the random generation is False, then the elements in the list either contains more than two integers <12, no integers >31, or more than five elements with 3-8 as a divisable integer. If every condition is met, I get the list generated in "chosen". The thing is - I now want to loop the procedure, so I don't have to run it manually a lot of times to get anything other than False. I have thought about the while command. It will have to be something like:

1) Check if "test" is true
2) If yes -> print list. If no-> make a new random sample and repeat the test.

Any input on this?

October 13th, 2011, 09:41 PM   #4
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by thesixmax Any input on this?
Once you have a function that will generate one random number of the sort you want, just use Table or the like to come up with a bunch of them.

October 14th, 2011, 05:28 AM   #5
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by CRGreathouse Once you have a function that will generate one random number of the sort you want, just use Table or the like to come up with a bunch of them.
Okay, I have found the right way to produce a loop, so that Mathematica will list a random list generation based on the conditions mentioned before:

Code:
list = Sort[RandomSample[Range[36], 7]];
f[x_] := If[Count[Select[list, # <= 12 &], _Integer] == 2,
If[Count[Select[list, # > 31 &], _Integer] == 1,
If[Count[Select[list, Divisible[#, {3, 8}] &], _Integer] <= 5,
list, False], False], False]
While[f[x] == False, list = Sort[RandomSample[Range[36], 7], Less];
If[list == f[x], a]]
f[x]
First, a random list is made. Then f checks if the conditions are met. If that is the case, f equals list, False otherwise. The "While" makes a loop. In the case f=False, a new list is produced. This procedure is made until the list equals f, and f is printed.
Now the last challenge: This procedure only produces one result. How to make the first 5 results appear at one time?

October 14th, 2011, 05:51 AM   #6
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by thesixmax First, a random list is made. Then f checks if the conditions are met. If that is the case, f equals list, False otherwise. The "While" makes a loop. In the case f=False, a new list is produced. This procedure is made until the list equals f, and f is printed.
That's not what I suggested, but I suppose it works. (You can cut out a whole step by replacing "False" with f[x] -- then you wouldn't need the While loop.) Also I'd drop the dummy variable x, which you don't use. If I properly understand your code it's doing a huge amount more work than it needs to.

Quote:
 Originally Posted by thesixmax Now the last challenge: This procedure only produces one result. How to make the first 5 results appear at one time?
Table.

October 14th, 2011, 06:02 AM   #7
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by CRGreathouse Table.
You are right about the function. I tried not to include the loop, but I got a huge amount of errors, so I decided to include it anyway. You mention table. I am not sure how you would use it? How could I make sure that different results appear? Table have to execute the function 5 times, but I can't understand how to do that.

 October 14th, 2011, 06:55 AM #8 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 937 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Mathematica: Applying rules to random integer generation This is the most basic possible use of the function. If you want a 5 random numbers as generated by g[], you just write List[g[], 5].
October 14th, 2011, 07:10 AM   #9
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Re: Mathematica: Applying rules to random integer generation

Quote:
 Originally Posted by CRGreathouse This is the most basic possible use of the function. If you want a 5 random numbers as generated by g[], you just write List[g[], 5].
I now have the code:

Code:
list = RandomSample[Range[36], 7];
f := If[1 <= Count[Select[list, # <= 12 &], _Integer] <= 2,
If[Count[Select[list, # > 31 &], _Integer] >= 1,
If[Count[Select[list, Divisible[#, {2, 7}] &], _Integer] <= 5,
list, False], False], False];
While[f == False,
list = RandomSample[Range[36], 7];
If[list == f, f]]
f
Example output is:
Code:
{1, 7, 15, 16, 25, 28, 34}
. Then I want to list the first 5 runs. I try something like:

Code:
Table[Sort[f], {i, 5}]
but the thing I get is
Code:
{1, 7, 15, 16, 25, 28, 34},
{1, 7, 15, 16, 25, 28, 34},
{1, 7, 15, 16, 25, 28, 34},
{1, 7, 15, 16, 25, 28, 34},
{1, 7, 15, 16, 25, 28, 34}
I think the problem is that Table does not ask the loop to run 5 times. The loop itself does not come up with a value, but defines the value for my original function f. I can't figure out how to solve that problem.

 October 14th, 2011, 09:08 AM #10 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 937 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Mathematica: Applying rules to random integer generation You need tro wrap everything that generates the random numbers into a single function and run that repeatedly. If you generate it with one call and simply test it it will be the same each time.

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