passing predicates to lowerBound, or alternatively, how lazy is map?

[Andrew Brown via Digitalmars-d-learn]

>>
>> My question about this is how lazy is map? Will it work on 
>> every value of order and then pass it to lowerBound, or could 
>> it work to evaluate only those values asked by lowerBound? I 
>> guess probably not, but could a function be composed that 
>> worked in this way?
>>
>> Thank you very much
>>
>> Andrew
>
> map is fully lazy.
>
> However, if you've already got the sorted indices in `order`, I 
> would do this:
>
> auto numLessThanN = numbers.indexed(order).countUntil!((x) => x
> >= N)();

Thanks for the reply, I'm going to have a lot of numbers though. 
I guess compared to the time it will take me to sort them, it 
makes no difference, but is it right that countUntil will take 
linear time? If I can figure out lowerBound, then I have my 
answer in log(n) time?

Best

Andrew