• Ramon (20/20) Nov 24 2014 what range/algorithm allows me to make projections from one
• bearophile (22/35) Nov 24 2014 Here I have defined ProjectionWanted as a struct.
• Ramon (2/2) Nov 24 2014 What is the difference between lazy and eager ranges?
• bearophile (6/8) Nov 24 2014 The lazy returns a range that once iterated gives the results one

"Ramon" <midiway midi.rs> writes:

what range/algorithm allows me to make projections from one 
sequence to another?


(http://msdn.microsoft.com/en-us/library/vstudio/bb548891%28v=vs.100%29.aspx)



class ProjectionWanted
{
   public int field1 { get; set; }
   public int field2 { get; set; }
   public int field3 { get; set; }
}

void Foo(List<ProjectionWanted> list)
{
   var list_projected = list.Select(l => new { l.field1, l.field2 
});
   // list_projected elements now contain only field1 and field2
}

So how would I make it in D? First yet, does D supports creating 
anonymous type for the projection? Or maybe I need to declare the 
class/struct that I want to be projected?

"bearophile" <bearophileHUGS lycos.com> writes:

Ramon:



 class ProjectionWanted
 {
   public int field1 { get; set; }
   public int field2 { get; set; }
   public int field3 { get; set; }
 }

 void Foo(List<ProjectionWanted> list)
 {
   var list_projected = list.Select(l => new { l.field1, 
 l.field2 });
   // list_projected elements now contain only field1 and field2
 }

Here I have defined ProjectionWanted as a struct.

//--------------
import std.stdio, std.algorithm, std.typecons;

struct ProjectionWanted {
     public int a, b, c;
}

auto foo(ProjectionWanted[] seq) pure nothrow  safe  nogc {
     return seq.map!(p => tuple(p.a, p.b));
}

void main() {
     [ProjectionWanted(1, 2, 3), ProjectionWanted(4, 5, 6)]
     .foo
     .writeln;
}
//--------------

Note that foo() returns a lazy range. If you need an eager one 
you can append an ".array":

     return seq.map!(p => tuple(p.a, p.b)).array;

And you have to import std.array too.

Bye,
bearophile

"Ramon" <midiway midi.rs> writes:

What is the difference between lazy and eager ranges?

(I guess, the lazy one has not yet queried the elements)

"bearophile" <bearophileHUGS lycos.com> writes:

On Monday, 24 November 2014 at 15:44:02 UTC, Ramon wrote:
 What is the difference between lazy and eager ranges?

 (I guess, the lazy one has not yet queried the elements)

The lazy returns a range that once iterated gives the results one 
at a time (so the function allocates no heap memory).
The eager version creates an array of the results in heap memory.

Bye,
bearophile