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## Re: modify coord-rotate to get exact values for (sin PI) etc (issue 2695

 From: thomasmorley65 Subject: Re: modify coord-rotate to get exact values for (sin PI) etc (issue 269530043 by address@hidden) Date: Sun, 25 Oct 2015 13:27:38 +0000

```On 2015/10/21 09:32:07, dak wrote:

https://codereview.appspot.com/269530043/diff/40001/scm/lily-library.scm
```
```File scm/lily-library.scm (right):
```
```

https://codereview.appspot.com/269530043/diff/40001/scm/lily-library.scm#newcode721
```
```scm/lily-library.scm:721: (moved-angle (- angle (/ (* quadrant PI)
```
```2)))
```
```Ok, let's just tackle this as a theoretical exercise since I remain
```
```unconvinced
```
```that we are doing ourselves much of a favor straying from degrees
```
```except at the
```
```final step.  Then angle-0-2pi is basically an independent source of
```
```numerical
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```ambiguity we want to avoid.  So I'd probably do it as follows:
```
```
```
```(let* ((quadrant
(inexact->exact (round (/ angle PI/2))))
(moved-angle (- angle (* quadrant PI/2)))
(s (sin moved-angle))
(c (cos moved-angle))
(x (coord-x coordinate))
(y (coord-y coordinate)))
((0) % -45 .. 45
(cons ...))
((1) % 45 .. 135
(cons ...))
((2) % 135 .. 225
(cons ...))
((3) % 225 .. 315
(cons ...))))
```
```
```
```Note that this retains the problem that (* quadrant PI/2) might just
not be properly representable which can make the whole exercise end up
still not delivering perfect values for 3*PI/2 or 5*PI/2 or similar,
depending on just how those values were arrived at.  However,
```
```
```
```(* exact-integer PI/2) likely has about the best chance to match
values that the user arrived with using similar expressions of his
own.  I'd also don't make two separate case statements for cos and sin
since that's likely a construct that is a bit expensive.
```
```
Up to now I found no drawback.

https://codereview.appspot.com/269530043/diff/40001/scm/lily-library.scm#newcode751
```
```scm/lily-library.scm:751: (cyclic-base-value (+ value cycle) cycle))
On 2015/10/20 23:06:58, thomasmorley651 wrote:
> what are those red dots??
> Never seen them before.
```
```
```
```Spaces before tabs would be my guess.  Or just generally a spacing
```
```change.
```
```either of the two.
```
```
Must be spacing change then

https://codereview.appspot.com/269530043/

```