Comments on: LINQ to prime numbers

By: Jacob

Jacob — Mon, 20 Oct 2008 15:33:58 +0000

Yeah; the prime generator in the post will calculate primes virutally infinitely. LINQPad only shows the first 1000 results in a sequence, when calling .Dump().

By calling .Count(), you’re trying to count the number of prime numbers the “primes” sequence contains.

By: Fowl

Fowl — Sun, 19 Oct 2008 11:22:52 +0000

I’m not very adept at LINQ, but this had me puzzled.
var odds =
from n in Enumerable.Range(0, int.MaxValue)
select 3 + (long)n * 2;
var primes = (new[] { 2L }).Concat(
from p in odds
where ! odds.TakeWhile(odd => odd * odd p % odd == 0)
select p);
primes.Dump();
Runs in about 200ms for me, however when I change the last line to:
primes.Count().Dump();
It takes at least 5 minutes (that’s when I gave up), at full CPU!

By: Sam Jack

Sam Jack — Mon, 21 Apr 2008 08:40:48 +0000

I’ve written up a slightly more efficient prime generator that checks divisibility against previously generated primes rather than odd numbers: it uses a variant of the Unfold operation.
http://blog.functionalfun.net/2008/04/project-euler-problem-7-and-10.html

By: Todd White

Todd White — Fri, 28 Mar 2008 21:40:19 +0000

53 chars is my best so far.

new int[1000].Select((n,i)=>i%3==0||i%5==0?i:n).Sum()

By: jacobcarpenter

jacobcarpenter — Thu, 27 Mar 2008 19:38:35 +0000

Oh; it also looks like your answer is off by 999 (which is divisible by 3).

The lame thing about Enumerable.Range is that the second argument is a sequence length (“count”), rather than an ending number. So your sequence starts at zero and contains 999 elements (ending at 998).

It sure seems like .Take() could have satisfied the need to limit a range to a specific length, but oh well.

By: jacobcarpenter

jacobcarpenter — Thu, 27 Mar 2008 19:24:34 +0000

Well, for one, you’ve got so much unnecessary whitespace.

Here’s mine:

Enumerable.Range(1,999).Where(n=>n%3==0||n%5==0).Sum()

By: Judah

Judah — Thu, 27 Mar 2008 19:03:04 +0000

54 characters? The best I can do is:

(from i in Enumerable.Range(0, 999)
where i % 3 == 0 || i % 5 == 0
select i).Sum()

What do you ahve?