The Fibonacci numbers are the numbers in the following integer sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144

int fib(int n)
{
    if (n <= 1)
        return n;
    return fib(n - 1) + fib(n - 2);
}
 
int main()
{
    int n = 9;
    printf("%dth Fibonacci Number: %d", n, fib(n));
    return 0;
}

Uxntal

@on-reset ( -> )
	#0009 fib
	#010e DEO BRK

@fib ( num* -- numfib* )
	#0001 GTH2k ?{ POP2 JMP2r }
	SUB2k fib STH2 INC2
	SUB2 fib STH2r ADD2
	JMP2r

Titan

using std.manip
using std.cond
using std.out

fib == [: [1 - : 1 - fib ⟷ fib +] [] ⍉ 1 > ?]

9 fib .nl

Callisto

include "cores/select.cal"
include "std/io.cal"

func fib begin
	let cell n
	n !

	if n @ 1 <= then
		n @ return
	else
		n @ 1 - fib
		n @ 2 - fib
		+ return
	end
end

9 fib printdec new_line

Concata

uses io

fun Fib n[Uint] => 
    n 1 <= if n else n 1 - Fib n 2 - Fib +

9 Fib IO.PrintLn