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Re: Dynamic changing of variables
*To*: mathgroup at smc.vnet.net
*Subject*: [mg96344] Re: Dynamic changing of variables
*From*: Albert Retey <awnl at gmx-topmail.de>
*Date*: Thu, 12 Feb 2009 06:34:07 -0500 (EST)
*References*: <gmu8rl$gn3$1@smc.vnet.net>
Hi,
> DynamicModule[{a, b = 0},
> a = Dynamic[Sin[b]];
> Column[{
> Dynamic[a],
> Slider[Dynamic[b], {0, 2*Pi}]
> }]
> ]
> In order to update "a" with the actual value of Sin[b] I need
> Dynamic around it. Unfortunately, now the variable "a" is invisibly
> wrapped and completely useless for further calculations. I'm not able to
> calculate even a+1 inside the DynamicModule.
>
> DynamicModule[{a, b = 0},
> a = Dynamic[Sin[b]];
> Column[{
> Dynamic[a+1],
> Slider[Dynamic[b], {0, 2*Pi}]
> }]
> ]
>
> If I'm not just too stupid and this behaviour is intended, then I'm
> wondering whether this doesn't lead to problems when you have more
> complex constructs with several dynamic variables.
You need to understand that Dynamic is something that only starts its
magic when it is shown within the FrontEnd. So this:
a = Dynamic[Sin[b]];
is assigning the Dynamic (actually a DynamicBox) to a, something that
only makes sense for the FrontEnd, not the Kernel. What you really want
is not to assign a to Dynamic[Sin[b]] but you want to assign Sin[b] to a
dynamically, which could be done with:
Dynamic[a=Sin[b]]
but again, this would only work if it appears visible within the
FronEnd, if you put a ; after it, it will not appear in the output, so
it will not do anything. I think this is more close to what you want:
DynamicModule[{a,b = 0},
Column[{
Dynamic[a = Sin[b]],
Dynamic[a + 1],
Slider[Dynamic[b], {0, 2*Pi}]
}]
]
If you don't want the 'intermediate' result to be shown in the FrontEnd,
you can wrap everything and suppress output:
DynamicModule[{a,b = 0},
Column[{
Dynamic[
a = Sin[b];
a + 1
],
Slider[Dynamic[b], {0, 2*Pi}]
}]
]
Another very useful mechanism to assign a value to a whenever b changes
is to use the second argument in Dynamic, like this:
DynamicModule[{a,b = 0},
Column[{
Dynamic[a + 1],
Slider[Dynamic[b, (b = #; a = Sin[b]; #) &], {0, 2*Pi}]
}]
]
hth,
albert
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