       Re: "If" syntax question

• To: mathgroup at smc.vnet.net
• Subject: [mg105147] Re: "If" syntax question
• From: Michael Greene <mgreene at csumb.edu>
• Date: Sun, 22 Nov 2009 06:10:46 -0500 (EST)
• References: <6029eb910911210735s5be59d99wf16925fd4cdc7302@mail.gmail.com>

```Oops. I corrected the expression to use == instead of = in the conditional.
Still don't get what I'm trying for which is a single 3d point coordinate.

r3=If[{s3==0},RotationTransform[-s2*Pi, {0, 0, 01}, {00, -r - h/2,
0}],RotationTransform[-s2*Pi, {0, 0, 01}, {00, -r - h/2, 0}]]

On Sat, Nov 21, 2009 at 7:35 AM, Michael Greene <mgreene at csumb.edu> wrote:

> I downloaded the cylinder net demonstration code to show my middle school
> class. I thought it was pretty good but needed a few enhancements before I
> used it in class.
>
> One thing I wanted the animation to show was the circle radius rotating
> when the circles were left in their flat orientation. So I modified the
> demonstration and came up with:
>
>  Manipulate[With[{da = 2 Pi/30}, With[
>    {
>     r1 = RotationTransform[s1*Pi/2, {1, 0, 0}, {0, h/2, 0}],
>     r2 = RotationTransform[-s1*Pi/2, {1, 0, 0}, {0, -h/2, 0}],
>     r3 = RotationTransform[-s2*Pi, {0, 0, 01}, {00, -r - h/2, 0}]
>     },
>    With[{
>      d1 =
>       Table[(r1[{r Cos[a], r Sin[a] + h/2 + r, 0}]), {a, 0, 2 Pi,
>         da}], d2 =
>       Table[r2[{r Cos[a], r Sin[a] - h/2 - r, 0}], {a, 0, 2 Pi, da}],
>      l1 = {r3[{0, -h/2, 0}], {0, -r - h/2, 0}}},
>     Graphics3D[{Opacity[.5], EdgeForm[],
>       {EdgeForm[{Blue}], Polygon[d1]}, {EdgeForm[{Blue}],
>        Polygon[d2]}, Line[r3[l1]], Table[
>        Polygon[{
>          {-a, +h/2, 0},
>          {-a + r da, h/2, 0},
>          {-a + r da, -h/2, 0},
>          {-a, -h/2, 0}}],
>        {a, 0, +(1 - s2) (r 2 Pi) - r da, r da}],
>       Take[Table[Polygon[{
>           {r Cos[a], +h/2, r + r Sin[a]},
>           {r Cos[a + da], h/2,
>            r + r Sin[a + da]}, {r Cos[a + da], -h/2,
>            r + r Sin[a + da]}, {r Cos[a], -h/2,
>            r + r Sin[a]}}], {a, -Pi/2, -Pi/2 + (2 Pi - 2 Pi/60),
>          2 Pi/60}], Round[s2 60]]}, ImageSize -> {500, 400},
>      Boxed -> False, SphericalRegion -> True,
>      PlotRange -> {Automatic, {-3, 3}, {0, 2}}]]]], {{r, 1/2,
>    "radius"}, .1, 1}, {{h, 1, "Height"}, .1,
>   1}, {{s1, 0, "rotate caps"}, 0, 1, ControlType -> Trigger,
>   DefaultDuration -> 1}, {{s2, 0, "wrap"}, 0, 1,
>   ControlType -> Trigger, DefaultDuration -> 2}]
>
> which does what I wanted when you click just the "Wrap" button. The circle
> clearly makes a single rotation as the rectangle rolls "up" into a
> cylinder.
>
> I have a separate modification that shows the radius sticking to the end
> caps as they get rotated into position when you click the "rotate caps"
> button. Both modifications to the original involve the third rotation
> operator (function?), r3. They differ only in the parameters being passed to
> the third RotationTransform call. My first attempt at melding the two
> disparate calls was to modify the r3 definition with the following does
> nothing code:
>
> r3=If[{s3=0},RotationTransform[-s2*Pi, {0, 0, 01}, {00, -r - h/2,
> 0}],RotationTransform[-s2*Pi, {0, 0, 01}, {00, -r - h/2, 0}]]
>
> I say "does nothing" because the true and false outputs are identical. I
> was just testing Mathematica to see if that usage was OK. The parser didn't
> like that modification and the code quite working.
>
> My question is, why didn't the r3=If... syntax work? What kind of thing is
> r3 that it can't be one function under one condition and a separate function
> under another? Perhaps there is some syntactical clue I have to give the
> parser that indicates my intention here?
>
>
>

--
Michael Greene - Programmer Analyst
Information Systems, michael_greene at csumb.edu
California State University, Monterey Bay
Corporation Bldg - RM 201 - 100 Campus Center, Seaside, CA 93955-8001
831.582.3567 Fax 831.582.4796

```

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