• biocyberman (42/42) Jul 09 2017 Following is the code for a more generalized Fibonacci range.
• ag0aep6g (36/46) Jul 09 2017 As you've only got an input range, you have to popFront the 24 values

biocyberman <biocyberman gmail.com> writes:

Following is the code for a more generalized Fibonacci range.

Questions:

1. How do I get only the value of the Nth (i.e. N = 25) element 
in an idiomatic way?

2. Can I set constraints in the range so that user gets warning 
if he asks for Nth element greater than a limit, say N> 30; or if 
the actually range value at N is greater than datatype limit 
(e.g. max long)? Maybe this should be done outside of the range, 
i.e. do check before accessing the range?


     import std.stdio : writeln;
     long multifactor = 4;
     int elemth = 25;

     struct FibonacciRange
     {
       long first = 1;
       long second = 1;

       bool empty() const  property
       {
        // how to stop at n = 30?
         return false;
       }

       void popFront()
       {
         long tmp = 0;
         tmp = first*multifactor + second;
         first = second;
         second = tmp;
       }

       long front() const  property
       {
         return first;
       }
     }

     void main()
     {
         import std.range : take;
         import std.array : array;

         FibonacciRange fib;

         auto fib10 = take(fib, elemth);
         long[] the10Fibs = array(fib10);
     }

ag0aep6g <anonymous example.com> writes:

On 07/09/2017 11:51 PM, biocyberman wrote:
 Following is the code for a more generalized Fibonacci range.
 
 Questions:
 
 1. How do I get only the value of the Nth (i.e. N = 25) element in an 
 idiomatic way?

As you've only got an input range, you have to popFront the 24 values 
that come before. You can use std.range.drop:

----
import std.range : drop;
auto twentyfifth = fib.drop(24).front;
----

But if you're not sure that the range actually has 25 elements, you 
should check `empty`, of course:

----
import std.range : popFrontN;
fib.popFrontN(24); /* or fib = fib.drop(24); */
if (!fib.empty)
{
     auto twentyfifth = fib.front;
}
----

 2. Can I set constraints in the range so that user gets warning if he 
 asks for Nth element greater than a limit, say N> 30;

You can keep track of N in FibonacciRange and when it hits 30 you throw 
an exception or print a message or just set `empty` to true.

You can't make it a compilation warning/error as far as I can tell.

 or if the actually 
 range value at N is greater than datatype limit (e.g. max long)?

You can use std.experimental.checkedint to detect it at run time:

----
private bool _empty = false;
bool empty() const  property { return _empty; }

void popFront()
{
     import std.experimental.checkedint : checked, Throw;
     long tmp = 0;
     try tmp = (checked!Throw(first)*multifactor + second).get;
     catch (Throw.CheckFailure e) _empty = true;
     first = second;
     second = tmp;
}
----

(There may be a smarter way than making the operation throw an exception 
and catching that.)