Fibonacci number

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Revision as of 14:40, 8 November 2018 by Kai Burghardt (talk | contribs) (implementation: generation vs. lookup)

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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.


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;
	// optimization: then part gets executed most of the time
	if n > 1 then
		fibonacci := fibonacci(n - 2) + fibonacci(n - 1);
		// since the domain is restricted to non-negative integers
		// we can bluntly assign the result to n
		fibonacci := n;

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;
	/// more meaningful identifiers than simple integers
	relativePosition = (previous, current, next);
	/// temporary iterator variable
	i: longword;
	/// holds preceding fibonacci values
	f: array[relativePosition] of qword;
	f[previous] := 0;
	f[current] := 1;
	// note, in Pascal for-loop-limits are inclusive
	for i := 1 to n do
		f[next] := f[previous] + f[current];
		f[previous] := f[current];
		f[current] := f[next];
	// assign to previous, bc f[current] = f[next] for next iteration
	fibonacci := f[previous];


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