RosettaCodeData/Task/Fibonacci-sequence/True-BASIC/fibonacci-sequence.basic

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FUNCTION fibonacci (n)
LET n1 = 0
LET n2 = 1
FOR k = 1 TO ABS(n)
LET sum = n1 + n2
LET n1 = n2
LET n2 = sum
NEXT k
IF n < 0 THEN
LET fibonacci = n1 * ((-1) ^ ((-n) + 1))
ELSE
LET fibonacci = n1
END IF
END FUNCTION
PRINT fibonacci(0) ! 0
PRINT fibonacci(13) ! 233
PRINT fibonacci(-42) !-267914296
PRINT fibonacci(47) ! 2971215073
END