wp34s  FIB problem (DBLON)

11222018, 07:59 PM
Post: #1




wp34s  FIB problem (DBLON)
That's a minor problem, actually. In double precision mode FIB returns a nearinteger for most arguments. For example:
10 FIB > 54.99999999999999999999999999999995 A simple workaround is 10 FIB CEIL > 55 I found it when computing the Reciprocal Fibonacci constant on the wp34s: 0001 **LBL A 0002 RCL X 0003 # 002 0004 MOD 0005 STO+ X 0006 DEC X 0007 # 000 0008 ENTER[^] 0009 STOS 00 0010 R[^] 0011 FIB 0012 RCL L 0013 INC X 0014 FIB 0015 RCL 03 0016 STO+ X 0017 INC X 0018 FIB 0019 RCL L 0020 INC X 0021 FIB 0022 RCL 02 0023 STOS 04 0024 **LBL 000 0025 RCL 07 0026 1/x 0027 STO+ 00 0028 RCL L 0029 STO 06 0030 x[<>] 06 0031 STO 07 0032 RCL L 0033 x[^2] 0034 RCL[times] 02 0035 RCL 05 0036 RCL+ 01 0037 STO/ Y 0038 y[<>] 01 0039 RCL L 0040 STO 04 0041 x[<>] 04 0042 STO 05 0043 DSE 03 0044 GTO 000 0045 RCL+ 01 0046 1/x 0047 RCL+ 00 0048 END In theory 9 iterations would have sufficed for correct 34 significant digits, but the last three digits went wrong: 9 A > 3.359885666243177553172011302918861 After CEILing and a few necessary changes, the program gets all of them right: 3.359885666243177553172011302918927 

11232018, 03:23 AM
Post: #2




RE: wp34s  FIB problem (DBLON)
FIB takes real arguments in addition to integers so CEIL isn't a general solution.
Pauli 

11232018, 08:08 AM
Post: #3




RE: wp34s  FIB problem (DBLON)
(11222018 07:59 PM)Gerson W. Barbosa Wrote: That's a minor problem, actually. In double precision mode FIB returns a nearinteger for most arguments. For example: Is it possible for the result of FIB (for a whole number) to be slightly larger than a whole number? If so, ROUNDI would be better than CEIL here. — Ian Abbott 

11232018, 10:32 AM
Post: #4




RE: wp34s  FIB problem (DBLON)
(11232018 08:08 AM)ijabbott Wrote:(11222018 07:59 PM)Gerson W. Barbosa Wrote: That's a minor problem, actually. In double precision mode FIB returns a nearinteger for most arguments. For example: I'd examined FIB(23) down to FIB(1) prior to choosing CEIL, but you're right, ROUNDI is a safer alternative. Here's my modified program: 0001 **LBL A 0002 RCL X 0003 # 002 0004 MOD 0005 STO+ X 0006 DEC X 0007 # 000 0008 ENTER[^] 0009 STOS 00 0010 R[^] 0011 FIB 0012 CEIL 0013 RCL 03 0014 INC X 0015 FIB 0016 CEIL 0017 RCL 03 0018 STO+ X 0019 INC X 0020 RCL X 0021 FIB 0022 CEIL 0023 x[<>] Y 0024 INC X 0025 FIB 0026 CEIL 0027 RCL 02 0028 STOS 04 0029 **LBL 000 0030 RCL 07 0031 1/x 0032 STO+ 00 0033 RCL L 0034 STO 06 0035 x[<>] 06 0036 STO 07 0037 RCL L 0038 x[^2] 0039 RCL[times] 02 0040 RCL 05 0041 RCL+ 01 0042 STO/ Y 0043 y[<>] 01 0044 RCL L 0045 STO 04 0046 x[<>] 04 0047 STO 05 0048 DSE 03 0049 GTO 000 0050 RCL+ 01 0051 1/x 0052 RCL+ 00 0053 END 9 A > 3.359885666243177553172011302918927 

11232018, 10:45 AM
Post: #5




RE: wp34s  FIB problem (DBLON)
(11232018 03:23 AM)Paul Dale Wrote: FIB takes real arguments in addition to integers so CEIL isn't a general solution. From Walter's Blue Book, page 223 of 244: "In DECM, FIB returns the extended Fibonacci number" So the only problem is it is not accurate enough in double precision mode for certain applications. Well, not really a problem once we are aware of that. Gerson. 

11242018, 08:37 PM
Post: #6




RE: wp34s  FIB problem (DBLON)
(11232018 10:45 AM)Gerson W. Barbosa Wrote: So the only problem is it is not accurate enough in double precision mode for certain applications. Well, not really a problem once we are aware of that. We deliberately made no guarantees about accuracy in double precision. That the FIB function is in XROM and working at double precision pretty much guarantees it will be off in the last place or so. Flash was full so I had to pull the C implementation that worked in higher precision. I agree ROUNDI is better for your purposes. Pauli 

11242018, 11:58 PM
Post: #7




RE: wp34s  FIB problem (DBLON)
(11242018 08:37 PM)Paul Dale Wrote:(11232018 10:45 AM)Gerson W. Barbosa Wrote: So the only problem is it is not accurate enough in double precision mode for certain applications. Well, not really a problem once we are aware of that. The accuracy of the FIB function in double precision is indeed very good. By "not accurate enough" I meant it failed giving the exact integer results I expected for integer arguments. I am sorry I haven't made myself clear about that. Gerson. 

10142021, 03:25 AM
Post: #8




RE: wp34s  FIB problem (DBLON)
Just a sizeoptimized version, without forcing integer results for FIB in double precision mode. As a result this will never get the last two or three digits right.
Code:


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