(41) Round To The Nearest 1/n
04-12-2018, 01:10 PM
Post: #1
 Eddie W. Shore Senior Member Posts: 730 Joined: Dec 2013
(41) Round To The Nearest 1/n
Original Blog Entry: https://edspi31415.blogspot.com/2018/04/...arest.html

The program RNEAR rounds a number x to the nearest 1/n. For example, to round x to the nearest 10th, n = 10. To round to the nearest 16th, n = 16.

Keystrokes: x [ENTER] y [XEQ] [ALPHA] RNEAR [ALPHA]
Code:
 01 LBL^T REAR 02 FIX 0 03 STO 01 04 X<>Y 05 STO 02 06 RFC 07 * 08 RND 09 RCL 01 10 / 11 RCL 02 12 INT 13 + 14 FIX 4 15 RTN

Examples:

x = π
n = 10, result: 3.10 (nearest 10th)
n = 1000, result: 3.142 (nearest 1000th)
n = 16, result: 3.125 (nearest 16th)
04-12-2018, 06:12 PM (This post was last modified: 04-12-2018 07:04 PM by hth.)
Post: #2
 hth Member Posts: 212 Joined: Mar 2014
RE: (41) Round To The Nearest 1/n
What is the magic instruction RFC? I am not familiar with it.

Håkan
04-12-2018, 06:57 PM (This post was last modified: 04-12-2018 07:12 PM by Dieter.)
Post: #3
 Dieter Senior Member Posts: 2,134 Joined: Dec 2013
RE: (41) Round To The Nearest 1/n
(04-12-2018 06:12 PM)hth Wrote:  What is the magic instruction RFC? I do not familiar with it.

I think that's supposed to be a "FRC".

But, Eddie: why do you split x into integer and fractional part? Why should this be required?

Hint re. your blog post: int(x+0,5) rounds to the nearest integer. This even works on a TI-59. ;-)
But there is also a way to round a number to display precision: EE INV EE does it.

Dieter
04-13-2018, 06:27 PM
Post: #4
 Dieter Senior Member Posts: 2,134 Joined: Dec 2013
RE: (41) Round To The Nearest 1/n
(04-12-2018 08:24 PM)Mike (Stgt) Wrote:  I get the same this way:
Code:
 01*LBL "RNR"  02 X<>Y  03 ST+ X  04 1/X  05 +  06 RCL X  07 LASTX  08 ST+ X  09 MOD  10 -  11 .END.

Wow, that's a quite ...creative solution. I'm not sure if I understand completely how this works. For a more straightforward approach see below.

(04-12-2018 08:24 PM)Mike (Stgt) Wrote:  Note: Do not round "twice", rounding a result to a new n-th.
Example: round $$\pi$$ to next 32th results $$\frac{101}{32}$$, rounding this to next 16th results $$\frac{51}{16}$$. In contrast, $$\pi$$ to next 16th is $$\frac{50}{16}$$, a difference of a full 16th.

Sure. You once round $$\pi$$ and once $$\frac{101}{32}$$. So the results are different.

(04-12-2018 08:24 PM)Mike (Stgt) Wrote:  Dieter's hint int(x+0,5) would take two bytes less.

Right – 9 bytes without LBL and END.
But here x and n must have identical signs:

Code:
01 LBL "RNR" 02 STO Z 03 x 04 ,5 05 + 06 INT 07 X<>Y 08 / 09 END

But with 2 bytes more there is a way to handle also different signs:

Code:
01 LBL "RNR" 02 STO Z 03 x 04 ENTER 05 SIGN 06 2 07 / 08 + 09 INT 10 X<>Y 11 / 12 END

Finally, here is a TI-59 version.
Data entry is done "the HP way": x [x<>t] n. ;-)

Code:
000 LBL 001 A 002 x 003 x<>t 004 + 005 OP 006 10 007 / 008 2 009 = 010 INT 011 / 012 x<>t 013 = 014 RTN

Or with the already mentioned EE INV EE rounding method:

Code:
000 LBL 001 A 002 x 003 x<>t 004 = 005 FIX 006 0 007 EE 008 INV 009 EE 010 INV 011 FIX 012 / 013 x<>t 014 = 015 RTN

Of course real TI-59 programmers would avoid "=" and use parentheses instead. ;-)

Dieter
04-14-2018, 06:31 PM (This post was last modified: 04-14-2018 06:34 PM by Dieter.)
Post: #5
 Dieter Senior Member Posts: 2,134 Joined: Dec 2013
RE: (41) Round To The Nearest 1/n
(04-14-2018 08:18 AM)Mike (Stgt) Wrote:  About the sign, I am not sure if this is correct (BTW, I discarded the X<>Y previously in line 2):
$$\pi$$, 16, XEQ "RNR" -> 3,125
$$-\pi$$, 16, XEQ "RNR" -> -3,125
$$\pi$$, -16, XEQ "RNR" -> 3,125
$$-\pi$$, -16, XEQ "RNR" -> -3,125

I'd say this is exactly the way it's supposed to be. If you enter $$-\pi$$ the output of course also has to be negative.
The approximation of $$-\pi$$ is –3,125 and not +3,125. So the signs of input and output should match.

Dieter
04-14-2018, 09:03 PM
Post: #6
 Joe Horn Senior Member Posts: 1,300 Joined: Dec 2013
RE: (41) Round To The Nearest 1/n
(04-14-2018 08:09 PM)Mike (Stgt) Wrote:  $$\pi$$, 16, XEQ "RNR" -> 3,125 -- agreed
$$\pi$$, -16 (negative!), XEQ "RNR" -> also 3,125 -- positive! even though n is negative. How come?

IMHO that's the correct answer because, of all the infinitely many multiples of 1/(-16), the one nearest to pi is in fact 3.125 ... and rounding x to the nearest multiple of 1/n is the program's stated purpose.

X<> c
-Joe-
04-15-2018, 04:13 PM (This post was last modified: 04-15-2018 04:15 PM by Dieter.)
Post: #7
 Dieter Senior Member Posts: 2,134 Joined: Dec 2013
RE: (41) Round To The Nearest 1/n
(04-14-2018 08:09 PM)Mike (Stgt) Wrote:  For positive n do doubt. Same for negative n? Sure? How did you prove that?

Simple. We are looking for a value of z for which z/n is as close to x as possible. The program then returns z/n. If n<0 then z has to have the opposite sign of x.

In simple words: z has to be an integer, but it doesn't have to be a natural number.
In our example $$\pi$$ rounded to the nearest –1/16 is 3,125, which is –49/–16. Or z=–49.
The essential point here: $$z\in\mathbb{Z}$$ and not only $$z\in\mathbb{N}$$.

On the other hand –3,125 cannot be the correct answer as it isn't the closest –1/16 at all. Even 0 is closer to $$\pi$$ than –3,125. ;-)

Dieter
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