On Fri, 14 May 2010 19:37:58 -0400, Walter Bright
<newshou...@digitalmars.com> wrote:
R.Tenton wrote:
At the very bottom of http://digitalmars.com/d/2.0/overview.html
there is an example implementation of the Eratosthenes' sieve.
That code is broken! It counts 1899 prime numbers, while there are only
1028
primes in the interval [1,8191]!
Are you sure? What's the mistake in the code?
I think the issue is that the expectation is that array index x represents
the number x. But it doesn't seem that way.
the i + i + 3 is very odd indeed.
If we consider each index, it means the first element represents 0 + 0 + 3
= 3;
The second element represents 1 + 1 + 3 = 5;
The third element represents 2 + 2 + 3 = 7;
So it looks like the numbers represented by the array actually go from 3
to (8190 + 8190 + 3) or 16383.
According to Wolfram Alpha, the number of primes in that list is 1899
http://www.wolframalpha.com/input/?i=primes+in+3+..+16383
A comment to explain the representation of the array may be good.
-Steve
