# Is this deterministic or not? **URL:** \ **Category:** Ruby\ **Created:** [June 1, 2008, 8:00pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201 "2008-06-01T20:00:48Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 1, 2008, 8:00pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/1 "2008-06-01T20:00:48Z")
puts “Enter a sentence: " puts gets.split.sort\_by { rand }.join(” ") takes a sentence and scrambles the words. how is this implemented? i tried myself first with a for or while loop and tried if word is not in new\_sentence then new\_sentence += word etc. but that could go on for a long time and would be very slow for a large text. ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 1, 2008, 9:01pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/2 "2008-06-01T21:01:13Z")
slix wrote: > puts “Enter a sentence: " > puts gets.split.sort\_by { rand }.join(” ") > > takes a sentence and scrambles the words. how is this implemented? i > tried myself first with a for or while loop and tried if word is not > in new\_sentence then new\_sentence += word etc. > but that could go on for a long time and would be very slow for a > large text. I assume you already know how gets, split, and join work and the question in your subject refers to sort\_by. The sort\_by method sorts the enumerable (in this case, the array of words produced by split) using the keys generated by the block. That is, sort\_by calls the block once for each element in the array. The block usually (but not in this case) inspects the element and returns a key. You’re wanting a random sort, so you use the rand method to return a random number as the key. The sort\_by method sorts the array in increasing order by the random numbers. Since the keys are randomly generated, the array is randomly ordered. Is it deterministic? See my P.S. You can use srand to ensure that rand returns the same sequence of numbers. The doc for sort\_by is pretty good. Try “ri sort\_by”, and “ri Kernel#rand”. P.S. I’m sure there’s a lot of good things to be said about PRNGs and stuff but I’ll let somebody else type it in. ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 2, 2008, 11:38am UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/3 "2008-06-02T11:38:22Z")
Tim H. wrote: > P.S. I’m sure there’s a lot of good things to be said about PRNGs and > stuff but I’ll let somebody else type it in. I’ll just add that Ruby’s rand is deterministic, but it uses a very good PRNG algorithm (Mersenne twister). Unless you really need true randomness, it’s good enough to use out-of-the-box. ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 24, 2008, 10:55pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/4 "2008-06-24T22:55:48Z")
but is the function he is talking about deterministic? i mean rand could be but he could theoretically guess the same word over and over. os will this function-execution-time grow according to a formula or it depends on “how lucky” he is? ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 25, 2008, 11:45am UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/5 "2008-06-25T11:45:52Z")
2008/6/2 Dave B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Tim H. wrote: > > > P.S. I’m sure there’s a lot of good things to be said about PRNGs and > > stuff but I’ll let somebody else type it in. > > I’ll just add that Ruby’s rand is deterministic, but it uses a very good > PRNG algorithm (Mersenne twister). Unless you really need true > randomness, it’s good enough to use out-of-the-box. Plus, opposed to other libs and languages, there is a random element in seeding: [http://www.ruby-doc.org/core/classes/Kernel.html#M001086](http://www.ruby-doc.org/core/classes/Kernel.html#M001086) Kind regards robert ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 25, 2008, 11:01pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/6 "2008-06-25T23:01:01Z")
On 25.06.2008 19:34, Dave B. wrote: > Robert K. wrote: > > > Plus, opposed to other libs and languages, there is a random element in > > seeding > > Well, a combination of the time, the process id, and a sequence number > isn’t actually random! 😉 > > However, it’s certainly better than using the time on its own, as has > commonly been done in the past. Which in turn is even better than using a constant seed - which also has been done in the past. This was the case I was thinking of. Cheers robert ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 28, 2008, 5:55am UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/7 "2008-06-28T05:55:44Z")
everyoen is missing my point. is the complete method deterministic? not the rand… ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 28, 2008, 2:05pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/8 "2008-06-28T14:05:46Z")
On 28.06.2008 05:51, defn noob wrote: > everyoen is missing my point. is the complete method deterministic? > not the rand… What complete method? You presented two lines of code and in those the only potential source of indeterminism is #rand (apart from the user input), which is precisely why the discussion revolves around this. Cheers robert ---
**Author:** ![slix](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/8491ac/32.png) [@slix](https://www.ruby-forum.com/u/slix)\ **Post date:** [June 25, 2008, 7:36pm UTC](https://www.ruby-forum.com/t/is-this-deterministic-or-not/139201/9 "2008-06-25T19:36:23Z")
Robert K. wrote: > Plus, opposed to other libs and languages, there is a random element in > seeding Well, a combination of the time, the process id, and a sequence number isn’t actually random! 😉 However, it’s certainly better than using the time on its own, as has commonly been done in the past. Perl has a Math::TrulyRandom module. “The source of the randomness is from interrupt timing discrepancies.” > **[TrulyRandom - Perl interface to a truly random number generator function -...](https://metacpan.org/release/GARY/Math-TrulyRandom-1.0/view/TrulyRandom.pod)** > > Perl interface to a truly random number generator function