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floating point, step forward issue, Manipulate Appearance -> "Labeled"

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  • Subject: [mg108757] floating point, step forward issue, Manipulate Appearance -> "Labeled"
  • From: "Nasser M. Abbasi" <nma at 12000.org>
  • Date: Tue, 30 Mar 2010 05:02:32 -0500 (EST)

Consider the following:

Manipulate[Null,   {{a, -0.1, "a"}, -0.1, 0.1, 0.1,  Appearance -> 
"Labeled"}]

Now click on the little "+" button at the end of the slider near the number 
".1" to open the animation controls. Now click on the "play" button,  You 
should see nothing too exciting, just a sequence of number 
{-.1,0,.1,-.1,0,.1,....} as expected.

Now try the same as above, but change the range to be -0.3,0.3,0.1, as 
follows

Manipulate[Null,   {{a, -0.1, "a"}, -0.3, 0.3, 0.1,  Appearance -> 
"Labeled"}]

and run it again with the play button, now you'll see this output:

{-0.3,-0.2,-0.1, 8.32667*10^-17 ,0.1,0.2,0.3,.... and it repeats}. This is 
the same output one gets by doing
Range[-.3, .3, .1]

So, to avoid the floating point display problem on the label of the control, 
I changed the code to do a step by exact fraction values as follows:

Manipulate[Null,   {{a, -1/10, "a"}, -3/10, 3/10, 1/10,  Appearance -> 
"Labeled"}]

and when the above is played, you'll see that the exact values are stepped 
over as I wanted. No floating point hickups.

The problem, is that I do not want to display the values as fractions on the 
slider label, but as decimals.  I am talking about the label itself, on the 
end of the slide, which shows the control value of the control variable. Now 
this is shown as 1/10, 2/10, etc...

So, I changed the above to the following:

Manipulate[Null,{{a,-N[1/10],"a"},-N[3/10],3/10,1/10,Appearance->"Labeled"}]

And now, it displays as decimal points on the label, and also I do not get 
the floating point leftovers displayed anymore when it steps forward.

I was wondering if there is a better solution than having to throws N's here 
and there to get it to work as expected. I wanted to put one N[] around the 
control variable itself, like this:

Manipulate[Null,{{N[a],-1/10,"a"},-3/10,3/10,1/10,Appearance->"Labeled"}]

But I can't ofcourse. N[a] is not valid symbol anymore.  The better solution 
would have been is to capture what goes to the label, so I can format it as 
I want.  It would be nice to be able to do this. I.e. when one types

 Appearance -> "Labeled"

on a control, there is now no way to intercept this label, or tell 
Mathematica how to format it. At least, I did not find a way. If there was, 
I could have written something like this

Manipulate[Null,   {{a, -0.1, "a"}, -0.3, 0.3, 0.1,  Appearance -> 
{"Labeled", N[#]& }]   % not valid code


Thanks,
--Nasser 




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