how use lowerBound with just sorting key, not complete values

[Daniel Davidson]

On Tuesday, 12 November 2013 at 15:51:53 UTC, bearophile wrote:
> Daniel Davidson:
>
>> Is there a way to search an array I know is ordered by date by 
>> only supplying date?
>
> You can use a map to perform a projection:
>
>
> import std.stdio, std.range, std.datetime, std.algorithm,
>        std.array;
>
> struct S {
>     Date date;
>     string foo;
> }
>
> void main() {
>     auto sarr = [S(Date(2000, 1, 1), "x"),
>                  S(Date(2000, 2, 1), "a"),
>                  S(Date(2000, 3, 1), "foo")];
>     assert(sarr.isSorted!q{ a.date < b.date });
>
>     auto needle = S(Date(2000, 2, 1));
>
>     sarr
>     .assumeSorted!q{ a.date < b.date }
>     .lowerBound(needle)
>     .writeln;
>
>     sarr.writeln;
>
>     sarr
>     .map!(s => s.date)
>     .assumeSorted
>     .lowerBound(Date(2000, 2, 1))
>     .writeln;
> }
>
>
> Output:
>
> [S(2000-Jan-01, "x")]
> [S(2000-Jan-01, "x"), S(2000-Feb-01, "a"), S(2000-Mar-01, 
> "foo")]
> [2000-Jan-01]
>
> Bye,
> bearophile

Yes, but that is only giving the dates. I want the actual array 
elements. Suppose S is a large object with lots of extra fields 
in addition to `string foo`. There should be a way to pull out 
the lower bound based on a date without creating a needle (S). 
Maybe lowerBound is not the right function?

Thanks
Dan