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

[John Colvin via Digitalmars-d-learn]

On Wednesday, 11 June 2014 at 11:22:08 UTC, Andrew Brown wrote:
> Hi there,
>
> The problem this question is about is now solved, by writing my 
> own binary search algorithm, but I'd like to ask it anyway as I 
> think I could learn a lot from the answers.
>
> The problem was, given an array of numbers, double[] numbers, 
> and an ordering from makeIndex size_t[] order, I want to count 
> how many numbers are less than a number N. The obvious way 
> would be to use lowerBound from std.range, but I can't work out 
> how to pass it a predicate like "numbers[a] < b". Could someone 
> explain the template:
>
> (SearchPolicy sp = SearchPolicy.binarySearch, V)(V value) if 
> (isTwoWayCompatible!(predFun, ElementType!Range, V));
>
> The alternative way I thought to do it was to combine map with 
> lowerBound, i.e.:
>
> map!(a => numbers[a])(order).assumeSorted
>                             .lowerBound(N)
>                             .length
>
> 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)();