how use lowerBound with just sorting key, not complete values

[bearophile]

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