Fibonacci Code Golf

Answer 1

Ruby (30 characters):

def f(n)n<2?n:f(n-1)+f(n-2)end

Answer 2

Corrected after comments (thanks Sebastian), it wasn't a sequence solution, so here we go with 42 chars (includes the \n):

def f(a=0,b=1):
 while 1:yield a;a,b=b,a+b

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OLD post below

Python, 38 chars.

f=lambda n:n if n<2 else f(n-1)+f(n-2)

Not so short but the most readable in my opinion :P

EDIT: Here is the analytic way (if someone needs to see it in python :-)

f=lambda n:int(.5+(.5+5**.5/2)**n/5**.5)

Answer 3

33 characters in C:

F(n){return n<2?n:F(n-1)+F(n-2);}

Answer 4

22 characters with dc:

1[pdd5**v1++2/lxx]dsxx

Invoke with either:

dc -e'1[pdd5**v1++2/lxx]dsxx'

Or:

echo '1[pdd5**v1++2/lxx]dsxx' | dc

Note: not my work, poached from perlmonks.

Answer 5

Delphi Prism (Delphi for .net)

f:func<int32,int32>:=n->iif(n>1,f(n-1)+f(n-2),n)

49 chars

Answer 6

x86 (C-callable) realmode, 14 bytes.
Input is  n  on stack, returns  F_n  in AX.

59 31 C0 E3 08 89 C3 40 93 01 D8 E2 FB C3