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From |
wgould@stata.com (William Gould, StataCorp LP) |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Random seeder |

Date |
Wed, 29 Sep 2010 10:36:19 -0500 |

Antoine Terracol <Antoine.Terracol@univ-paris1.fr> writes, > some time ago, there was a discussion on Statalist about > getting a random seed for Stata's PRNG. Bill Gould suggested > that someone should write a little program to obtain (true) > random numbers from random.org > (see http://www.stata.com/statalist/archive/2010-08/msg00930.html) > > Based on R's random package, here's a first try, for anyone interrested I like it. In what follows, 1. I make some minor but important fixes to Antoine's program, and explain. 2. I improve and rename Antoine's program. The improvement deals with making sure the command fails if random.org someday changes their syntax or how they return results. Everything is below. I suspect from here on out, Antoine and I should work privately and just report the final result when we are done. In my version of Antoine's program, I renamed the command -setrngseed-, and I reversed Antoine's -setseed- option so that the option is now -nosetseed- and default is to reset the random-number seed. The command has syntax setrngseed [, nosetseed min(#) max(#)] The options are useful for debugging -setrngseed- and are probably not worth documenting. I suggest that when Antoine writes the documentation, or he makes me write it, we document the syntax as simply being setrngseed Here's what happens when you type it: . setrngseed (contacting http://www.random.org) (random-number seed set to 697429737) Antoine's program, fixed ------------------------ Here is Antoine's original program, updated, ----------------------------------------------------------------- program define truernd, rclass syntax [, min(integer 1) max(integer 1000000000) setseed] tempfile rndseed tempname myseed quietly copy "http://www.random.org/integers/?num=1&min=`min'&max=`max'&col=1&base=10&format=plain&rnd=new" "`rndseed'", replace file open `myseed' using "`rndseed'", read text file read `myseed' value file close `myseed' return scalar rndvalue=`value' di "The value returned by www.random.org is `value'" if "`setseed'"!="" { set seed `value' display "The seed was set to `value'" return scalar seed=`value' } end ----------------------------------------------------------------- Warning: the line in the middle that begins -quietly copy- is a single, long line and there are no blanks within the quioted string "http://...";. I made trhee small changes, two important and the third more a matter of style. 1. Antoine used the temporary filename `rndseed'.txt; I use `rndseed'. This is important. Stata assumes you use temporary file names in the form Stata supplied them. When your program concludes, Stata erases files with those names. When Antoine used `rndseed'.txt, Stata still attempted to erase `rndseed' when the program concluded, and never even attempted to erase `rndseed'.txt. 2. I enclose the temporary filename `rndseed' in double quotes everytime I use it; I code "`rndseed'" rather than `rndseed'. This is important because, on some computers, the system directory that contains the temporary files has blanks (spaces) in its name. 3. Antoine used -`value'- and -``value''-; I used -value- and -`value'-. -value- in Antoine's code, and my update, is a macro. Antoine put a temporary name in -value- and used macro `value' thereafter, meaning the contents of `value' were ``value''. That was unnecessary; macros are private anyway. setrngseed ---------- Here is the -setrngseed- code. ------------------------------------------------------------------------ program define setrngseed, rclass version 11 syntax [, MIN(integer 1) MAX(integer 1000000000) noSETseed] get_random_number `min' `max' local value "`r(result)'" if ("`setseed'"=="") { set seed `value' di as txt "(random-number seed set to `value')" } else { di as txt " random.org returns `value' (seed not set)" } return scalar seed = `value' end program get_random_number, rclass args min max tempfile rndseed tempname myseed display as txt "(contacting http://www.random.org)" quietly copy "http://www.random.org/integers/?num=2&min=`min'&max=`max'&col=1&base=10&format=plain&rnd=new" "`rndseed'" file open `myseed' using "`rndseed'", read text file read `myseed' value1 file read `myseed' value2 file close `myseed' check_integer_result `value1' check_integer_result `value2' if (`value1' != `value2') { return local result `value1' exit } di as err "{p 0 4 2}" di as err "random.org behaved unexpectedly{break}" di as err "random.org returned the same random" di as err "valiue twice, so the values are not" di as err "random or a very unlikely event occured." di as err "{p_end}" exit 674 end program check_integer_result args value capture confirm integer number `value' if (_rc) { di as err "{p 0 4 2}" di as err "random.org behaved unexpectedly{break}" di as err `"value returned was "`value'", which"' di as err "was not an integer." di as err "{p_end}" exit 674 } end ------------------------------------------------------------------------ The code looks different from Antoine's, but it's not much changed. It looks different because I made my additions by adding subroutines, which I think makes the code more readable. Here is what is substantively different: 1. I verify that the results returned by random.org really are integers. 2. I ask random.org for two random numbers rather than one. I verify that they are different. I'm worried that random.org might someday change their syntax or what they return. If random.org did that, Antoine's original program would probably break, but I wanted to make sure the program broke if results were not as expected. By the way, I tried to make another change, but failed. I tried changing syntax [, MIN(integer 1) MAX(integer 1000000000) noSETseed] to syntax [, MIN(integer 1) MAX(integer 2147483647) noSETseed] but when I did that, www.random.org reported an error. I then tried syntax [, MIN(integer 1) MAX(integer 2000000000) noSETseed] and random.org still reported an error. So I changed back to how Antoine had it. Antoine, do you know how many bits random.org bases their random number on? -- Bill wgould@stata.com * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Random seeder***From:*Nick Winter <nwinter@virginia.edu>

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