Re: Randomly selecting an element from an STL collection
On Jun 27, 2:45 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:
Generic Usenet Account wrote:
I had a need to randomly select an element from an STL collection. It
does not appear that this functionality is provided out-of-the-box
with STL. Here is my crude implementation. I am using advance and
distance in my approach. Is distance guaranteed to return an integral
value?
Not sure if it's guaranteed or not, but it's usually 'ptrdiff_t'.
It's guaranteed for the standard allocator.
If not, can anyone suggest improvements:
I could only say, make use of
iterator_traits<iterator>::difference_type.
I wonder if it's really necessary to bother. He's using rand()
to choose an element, and RAND_MAX can be (and far too often is)
as little as 32767. So if he has more elements that that, his
code isn't going to work. Also, he used the expression
rand() % distance to choose the element. This introduces a bias
toward the smaller elements; the closer distance is to RAND_MAX,
the larger the biais. Unless the number of elements is actually
very small, the biais will be noticeable. So we can conclude
that the actual number of elements will probably not be much
more than 10 or 20. And that a simple int will work just fine.
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