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MathGroup Archive 2007

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Re: How show Locator point and its image under a function?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg79915] Re: How show Locator point and its image under a function?
  • From: Albert <awnl at arcor.net>
  • Date: Wed, 8 Aug 2007 05:01:26 -0400 (EDT)
  • References: <f990t4$c2d$1@smc.vnet.net>

Murray Eisenberg wrote:
> The following code displays a fixed point and its image under a 
> specified function f:
> 
>    f[{x_,y_}]:={2x+1,y+2}
>    Module[{pt = {-1, 0.5}},
>       Row[{
>         Framed@Graphics[Point[pt], PlotRange -> 5],
>         Framed@Graphics[Point[f[pt]], PlotRange -> 5]
>       }]
>     ]
> 
> Now I want to have the point in the left-hand graphic to be dynamic, 
> controlled by a Locator, and to have its changed image under the 
> function to be shown in the right-hand graphic.
> 
> The idea is something like the following (WRONG!) code:
> 
>    DynamicModule[{pts={{0,0}}},
>      Row[{
>        LocatorPane[Dynamic[pts],Graphics[{},PlotRange->5]],
>        Graphics[Dynamic[f/@pts],PlotRange->5]
>      }]
>    ]
> 
> That code produces error messages, "1 is not a Graphics primitive or 
> directive", "2 is not a Graphics primitive or directive".

this is because f/@pts just gives a List of Numbers, which is not a 
Graphics primitive.

> How can it actually be done?

make the list of coordinates a list of Points:

DynamicModule[{pts = {{0, 0}}},
  Row[{
    LocatorPane[Dynamic[pts], Graphics[{}, PlotRange -> 5]],
    Graphics[Dynamic[Point /@ f /@ pts], PlotRange -> 5]
    }]
  ]


hth,

albert


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