Difference between revisions of "Talk:Lucas number"
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:::: I especially like the <syntaxhighlight lang="pascal" enclose="none">relativePosition</syntaxhighlight> in the [[Fibonacci number#iterative implementation|iterative implementation of <syntaxhighlight lang="pascal" enclose="none">fibonacci</syntaxhighlight>]] producing IMHO really readable and comprehensible code. [[User:Kai Burghardt|Kai Burghardt]] ([[User talk:Kai Burghardt|talk]]) 00:40, 12 November 2018 (CET) | :::: I especially like the <syntaxhighlight lang="pascal" enclose="none">relativePosition</syntaxhighlight> in the [[Fibonacci number#iterative implementation|iterative implementation of <syntaxhighlight lang="pascal" enclose="none">fibonacci</syntaxhighlight>]] producing IMHO really readable and comprehensible code. [[User:Kai Burghardt|Kai Burghardt]] ([[User talk:Kai Burghardt|talk]]) 00:40, 12 November 2018 (CET) | ||
:::::No, I don't agree (talking about the use of NativeInt here). It makes the example unnecessarily complex IMHO. But it's perfectly fine to disagree on that. | :::::No, I don't agree (talking about the use of NativeInt here). It makes the example unnecessarily complex IMHO. But it's perfectly fine to disagree on that. | ||
+ | :::::: Yeah, I see, it isn't really part of the problem, of the task: “calculate n-th Lucas number”. On the other hand I wanna write examples, promote code, which you actually would see ''in a production program''. I don't wanna put Pascal in ''teaching bubble'' as its reputation used to be. I'd leave it as it is, although isn't explained on the page. [[User:Kai Burghardt|Kai Burghardt]] ([[User talk:Kai Burghardt|talk]]) 22:34, 15 November 2018 (CET) | ||
Revision as of 23:34, 15 November 2018
return type
Q: Why not use UInt64 as return type for the function. It should be the same on all platforms.
Remark: it seems a bit strange to me to derive Lucas(n) from Fib(n), because the Fibonacci sequence is jus a special case of the Lucas numbers (with starting values 1,1). --Bart (talk) 19:15, 10 November 2018 (CET)
Here's an example of what I mean:
{
A more general form of the Lucas series, where the 2 starting values
can be set using parameters
}
function LucasGen(L1, L2, N: UInt64): UInt64;
var
i, LucMin1, LucMin2: UInt64;
begin
if (N = 0) then
Raise ERangeError.Create('Lucas function is undefined for 0.');
if (N = 1) then
Exit(L1)
else if (N = 2) then
Exit(L2)
else
begin
LucMin1 := L2;
LucMin2 := L1;
i := 2;
while (i <> N) do
begin
Inc(i);
Result := LucMin2 + LucMin1;
LucMin2 := LucMin1;
LucMin1 := Result;
end;
end;
end;
function Lucas(N: UInt64): UInt64;
begin
Result := LucasGen(2,1,N);
end;
function Fib(N: UInt64): UInt64;
begin
Result := LucasGen(1,1,N);
end;
--Bart (talk) 19:25, 10 November 2018 (CET)
I tried to add the lookuptables here, but the wiki insists that is spam ;-)
- A: Yeah, I have a general preference for the CPU's native integer size. Unlike Delphi or GPC, FPC's
integer
/cardinal
is always of fixed width, so I usednativeUInt
. [However, according to its documentation it's only either 32 or 64 bits, no more, no less, so I can't put code in there for other case.] I just wanna raise awareness for such issues (although it is potentially confusing [hence your question]). I of course would have writtenuInt64
, too, specificallyqword
, since the former isn't mentioned in the Reference Guide but the latter is. - Don't ask me, but Djzepi created both Fibonacci number and Lucas number. However, I guess it's the other way around: I regard Fibonacci as a general case, and Lucas being a specialization. He introduced calculation of Lucas numbers based on Fibonacci in the page's initial version.
- Regarding your code: I want to keep it simple, the examples in the wiki. This implies:
- I don't wanna assume one is in
{$mode objFPC}
(extended syntax ofexit
routine) - has
{$modeSwitch exceptions on}
(implicitly set by{$mode objFPC}
or{$mode Delphi}
), and wants them - has
{$modeSwitch result on}
(implicitly set by{$mode objFPC}
or{$mode Delphi}
).
- I don't wanna assume one is in
- Therefore I regard my implementation as *the best*. [Surprise!] No. Kidding aside, but seriously, I think it is important to embrace robustness of routines, a general notion I derive from Pascal's strictness. You can discern this in
lucasLeftInverseRange
. In conjunction with{$rangeChecks on}
an out-of-range error can be easily detected (during development). - Kai Burghardt (talk) 20:25, 11 November 2018 (CET)
- OK, I see. I consulted Wikipedia. I stand corrected. You're right, Lucas sequences being the generic case, and Fibonacci a special case. However,
if (N = 0) … 'Lucas function is undefined for 0.'
, well, hell yeah it's defined [math]\displaystyle{ n=0 }[/math]. It's been all implementations oflucas
like that. Kai Burghardt (talk) 20:38, 11 November 2018 (CET)- I'm off by one then. --Bart (talk) 23:05, 11 November 2018 (CET)
- You otherwise agree? My answer isn't like a set in stone argument, but a “plea” for a certain policy.
- I especially like the
relativePosition
in the iterative implementation offibonacci
producing IMHO really readable and comprehensible code. Kai Burghardt (talk) 00:40, 12 November 2018 (CET)- No, I don't agree (talking about the use of NativeInt here). It makes the example unnecessarily complex IMHO. But it's perfectly fine to disagree on that.
- Yeah, I see, it isn't really part of the problem, of the task: “calculate n-th Lucas number”. On the other hand I wanna write examples, promote code, which you actually would see in a production program. I don't wanna put Pascal in teaching bubble as its reputation used to be. I'd leave it as it is, although isn't explained on the page. Kai Burghardt (talk) 22:34, 15 November 2018 (CET)
- No, I don't agree (talking about the use of NativeInt here). It makes the example unnecessarily complex IMHO. But it's perfectly fine to disagree on that.
- I'm off by one then. --Bart (talk) 23:05, 11 November 2018 (CET)
- OK, I see. I consulted Wikipedia. I stand corrected. You're right, Lucas sequences being the generic case, and Fibonacci a special case. However,
OK, so here's the new implementation. It's zero based, allows for negative indices.
type
LucasRange = -90..90;
FibRange = -92..92;
function LucasGen(L0, L1, N: Integer; AllowNegativeIndex: Boolean = False): Int64;
var
i: Integer;
LucMin1, LucMin2: Int64;
IsNegative: Boolean;
begin
IsNegative := (N < 0);
if (not AllowNegativeIndex) and IsNegative then
Raise ERangeError.Create('Range check error: to allow for negative indexes in LucasGen(), you must set the AllowNegativeIndex parameter to TRUE.');
N := Abs(N);
if (N = 0) then
begin
Result := L0
end
else if (N = 1) then
begin
Result := L1
end
else
begin
LucMin1 := L1;
LucMin2 := L0;
i := 1;
while (i <> N) do
begin
Inc(i);
Result := LucMin2 + LucMin1;
LucMin2 := LucMin1;
LucMin1 := Result;
end;
end;
if IsNegative and Odd(N) then
Result := -Result; //(Lucas(-N) = (-1^N)*Lucas(N);
end;