speeding up + ctfe question

[jerro]

On Saturday, 26 May 2012 at 18:40:53 UTC, maarten van damme wrote:
> well, I was thinking about using a sieve but when you were to 
> request
> prime 400_000 you're sieve would blow up in size.

Because you only need primes up to sqrt(n) to check if n is prime,
you can make a sieve based range that only uses O(sqrt(n)) memory.
I've put an example at https://gist.github.com/2795954 .It could
be optimized further (skipping even numbers, keeping track of the
multiples of small primes...), but it is already much faster
than a range that does not use a sieve.

>That's why I
> opted
> for something else (and I don't know if it was the right thing 
> to do
> though). (Ab)using opIndex like that is indeed a bit wrong but 
> what
> would be the alternative? remove slicing and introduce getPrime?
> Wouldn't that be the same but without the great syntactic sugar?

Syntax sugar isn't great if it confuses people and causes bugs.
You can use r.drop(n).front to get nth element of a range.
If you need to do it often, you could just write a nth() function.