I have a set like this:
set<vector<prmNode> > cammini
I need to take 2 random values (2 random vector) from this set. How could I do?? Thank you very much!
+1
A:
The simplest solution would be to do the following twice.
*(std::advance(cammini.begin(), rand() % cammini.size()))
You can do it more efficiently if you generate two indexes, advance by the smaller and then advance by the difference between them.
avakar
2010-03-06 14:19:46
Note that this is biased towards smaller indexes.
Joey
2010-03-06 14:22:02
@Johannes, I don't think so. Can you elaborate?
avakar
2010-03-06 14:22:30
Assuming `rand() % constant` is biased which it tends to be. The idea itself works, given non-biased random number generation, but a set is probably not a container of choice for picking random values out of.
UncleBens
2010-03-06 14:27:02
Ah, of course, `rand() % k` is biased (regardless of the quality of `rand` itself), but it's the best one can do without involving Boost.Random or writing a non-trivial piece of code.
avakar
2010-03-06 14:31:29
Unless `RAND_MAX` is an exact multiple of `cammini.size()` you will get the smaller indexes a little more often than the larger ones since you're using a modulus here. That's always the problem when using `%` to get a pseudo-random number up to *n*. It is less of a problem if `RAND_MAX` is larger but for some implementations it's ridiculously small. Basically you get (size/n) equal-sized intervals of *n* elements each and at the end of the `rand()` range one interval which is smaller. Every number falling into that last, smaller interval skews the distribution in your [0, *n*) interval.
Joey
2010-03-06 14:34:44
Well, for a fairly concise and elegant piece of code how to deal with it, you could take a look at java.util.Random.nextInt(int). But yes, omitting such an elemental piece of functionality from C and even C++ is a bit frustrating at times.
Joey
2010-03-06 14:35:51