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MathGroup Archive 2003

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Re: Random Trouble

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39224] Re: [mg39178] Random Trouble
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Tue, 4 Feb 2003 02:22:38 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

On Monday, February 3, 2003, at 09:42 AM, Donald A. Darling wrote:

>
>
>
> On Sunday, February 2, 2003, at 03:13 PM, Donald Darling wrote:
>
>> I have a program which uses a random number in several places - the
>> same
>> number in a given run of the program. When I implement the program
>> however
>> the number changes in every new call to it.  I've tried to overcome
>> this
>> using "Which", "Hold", "Verbatim", and others, all to no avail.  Any
>> help
>> greatly appreciated.
>>
>>
>>
>>
>>
>
> This is what SeedRandom is for, e.g.
>
>
> SeedRandom[5];Table[Random[Integer,{1,10}],{3}]
>
>
> {2,3,2}
>
> Now evaluating again:
>
>
> SeedRandom[5];Table[Random[Integer,{1,10}],{3}]
>
>
> {2,3,2}
>
>
> Andrzej Kozlowski
> Yokohama, Japan
> http://www.mimuw.edu.pl/~akoz/
> http://platon.c.u-tokyo.ac.jp/andrzej/
>
> Andrzej - Thanks for the tip on SeedRandom, which a few respondents
> suggested.  There is some difficulty.  If I try
>
> F[n_]= SeedRandom;Table[Random[Real,{-1,1}],{n}]
>
> with n=5, say, I get an error message.  I don't know how to fix this.
>
> Don Darling.
>
>
>
>

You need to use a "seed" with SeedRandom and you need to enclose the 
composite statement in parentheses:

F[n_] := (SeedRandom[5]; Table[Random[Real, {-1, 1}], {n}])

Now look:

In[6]:=
F[3]

Out[6]=
{0.573197,0.696679,0.225655}

In[7]:=
F[5]

Out[7]=
{0.573197,0.696679,0.225655,-0.203925,-0.523976}

Each "random" sequence will be an "extension" of shorter ones.



Andrzej Kozlowski
Yokohama, Japan
http://www.mimuw.edu.pl/~akoz/
http://platon.c.u-tokyo.ac.jp/andrzej/



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