Difference between revisions of "Fibonacci number"
(blow up) |
(implementation: generation vs. lookup) |
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The idea is to add the two last numbers in order to produce the next value. | The idea is to add the two last numbers in order to produce the next value. | ||
− | == recursive implementation == | + | == generation == |
+ | === recursive implementation === | ||
<syntaxhighlight lang="pascal"> | <syntaxhighlight lang="pascal"> | ||
{** | {** | ||
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</syntaxhighlight> | </syntaxhighlight> | ||
− | == iterative implementation == | + | === iterative implementation === |
This one is preferable for its run-time behavior. | This one is preferable for its run-time behavior. | ||
<syntaxhighlight lang="pascal"> | <syntaxhighlight lang="pascal"> | ||
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end; | end; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
+ | |||
+ | == lookup == | ||
+ | While calculating the Fibonacci number every time it is needed requires almost no space, it takes a split second of time. | ||
+ | Applications heavily relying on Fibonacci numbers definitely want to use a lookup table instead. | ||
+ | And yet in general, do not calculate what is already a known fact. | ||
+ | Since the Fibonacci sequence doesn't change, actually calculating it is a textbook demonstration but not intended for production use. | ||
+ | |||
+ | Nevertheless an actual implementation is omitted here, since everyone wants to have it differently, a different flavor. | ||
== see also == | == see also == |
Revision as of 15:40, 8 November 2018
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The Fibonacci Sequence is the series of numbers:
0, 1, 1, 2, 3, 5, 8, 13, 21, …
The idea is to add the two last numbers in order to produce the next value.
generation
recursive implementation
{**
implements Fibonacci sequence recursively
\param n the index of the Fibonacci number to retrieve
\returns the Fibonacci value at n
}
function fibonacci(const n: byte): qword;
begin
// optimization: then part gets executed most of the time
if n > 1 then
begin
fibonacci := fibonacci(n - 2) + fibonacci(n - 1);
end
else
begin
// since the domain is restricted to non-negative integers
// we can bluntly assign the result to n
fibonacci := n;
end;
end;
iterative implementation
This one is preferable for its run-time behavior.
{**
implements Fibonacci sequence iteratively
\param n the index of the Fibonacci number to calculate
\returns the Fibonacci value at n
}
function fibonacci(const n: longword): qword;
type
/// more meaningful identifiers than simple integers
relativePosition = (previous, current, next);
var
/// temporary iterator variable
i: longword;
/// holds preceding fibonacci values
f: array[relativePosition] of qword;
begin
f[previous] := 0;
f[current] := 1;
// note, in Pascal for-loop-limits are inclusive
for i := 1 to n do
begin
f[next] := f[previous] + f[current];
f[previous] := f[current];
f[current] := f[next];
end;
// assign to previous, bc f[current] = f[next] for next iteration
fibonacci := f[previous];
end;
lookup
While calculating the Fibonacci number every time it is needed requires almost no space, it takes a split second of time. Applications heavily relying on Fibonacci numbers definitely want to use a lookup table instead. And yet in general, do not calculate what is already a known fact. Since the Fibonacci sequence doesn't change, actually calculating it is a textbook demonstration but not intended for production use.
Nevertheless an actual implementation is omitted here, since everyone wants to have it differently, a different flavor.