digitalmars.D.learn - Lazy range, extract only Nth element, set range size constraint?

• 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?

#!/usr/bin/env rdmd
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);
}
```
Jul 09 2017
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.)
```
Jul 09 2017