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]!
What is the outermost for loop good for?
This example is just screwed up!
And this was the first D source I ever encountered...

I am referring to the following:

/* Sieve of Eratosthenes prime numbers */

import std.stdio;

bool[8191] flags;

int main()
{   int i, count, prime, k, iter;

    writefln("10 iterations");
    for (iter = 1; iter <= 10; iter++)
    {	count = 0;
	flags[] = 1;
	for (i = 0; i < flags.length; i++)
	{   if (flags[i])
	    {	prime = i + i + 3;
		k = i + prime;
		while (k < flags.length)
		{
		    flags[k] = 0;
		    k += prime;
		}
		count += 1;
	    }
	}
    }
    writefln("%d primes", count);
    return 0;
}

D2 code, the 8191 too is counted:

import std.stdio: writeln;

int sieve(int m) {
    auto isPrime = new bool[m + 1];
    isPrime[] = true;
    int count;

    foreach (i; 2 .. isPrime.length) {
        if (isPrime[i]) {
            count++;
            for (int k = i * 2; k < isPrime.length; k += i)
                isPrime[k] = false;
        }
    }

    return count;
}

void main() {
    writeln(sieve(8191));
}


Bye,
bearophile

The sieve algorith was not my problem. I wanted to point out that
the example is totally screwed up, so it would be corrected.
Or is this the wrong place to ask?

R.Tenton:
> Or is this the wrong place to ask?

The right place to report errors in the docs is the bugzilla...
But if you don't want to subscribe some people here can do it for you.

Bye,
bearophile

That would be nice.

R.Tenton:
> That would be nice.

http://d.puremagic.com/issues/show_bug.cgi?id=4164

Thanks!

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?

> What is the outermost for loop good for?

It is a translation of the C benchmark that was common in the 1980's. The outermost loop is to make it take longer so the test can be timed better.

The original:

http://home.iae.nl/users/mhx/nsieve.c

The more common later version:

http://www.paxcompiler.com/js_sieve.htm

Walter Bright:
> Are you sure? What's the mistake in the code?

This is the code in the Overview, it prints 1899: http://codepad.org/lzRtggEL

This is the code I have suggested in bugzilla, it prints 1027: http://ideone.com/D9ZqQ

Wolfram Alpha says they are 1027 (I have left out the last number because Alpha uses a closed interval) http://www.wolframalpha.com/input/?i=primes+in+2+..+8190

Bye,
bearophile

bearophile wrote:
> Walter Bright:
>> Are you sure? What's the mistake in the code?
> 
> This is the code in the Overview, it prints 1899:
> http://codepad.org/lzRtggEL
> 
> This is the code I have suggested in bugzilla, it prints 1027:
> http://ideone.com/D9ZqQ
> 
> Wolfram Alpha says they are 1027 (I have left out the last number because Alpha uses a closed interval)
> http://www.wolframalpha.com/input/?i=primes+in+2+..+8190


It's been printing 1899 primes since the 80's. Just for fun, google [sieve of eratosthenes 1899 primes]