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This question already has an answer here:

Dynamic[x=7]; does not make x dynamic! But Dynamic[x=7;] makes x dynamic with initial value 7. The problem with the later is the presence of Null in the output. My question is: how can I assign a (big) value (or a huge list, etc.) to a dynamic variable AND suppress any possible output (including Null)?

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marked as duplicate by Mr.Wizard Mar 5 '13 at 18:33

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

What's the point of Dynamic then ? – b.gatessucks Mar 5 '13 at 17:39
Have you investigated DynamicWrapper? – John Fultz Mar 5 '13 at 17:40
Read the tutorials on Dynamic. There's no such thing as a dynamic variable. There's only stuff that shows as the dynamically updated "current" value of an expression, and in the process of calculating that current value, it might update other "non-dynamic" variables – Rojo Mar 5 '13 at 17:58
I closed this question as a duplicate. (It is a common problem therefore "too localized" is inaccurate.) If anyone has a suggestion for a better duplicate (on Mathematica.SE) let me know. – Mr.Wizard Mar 5 '13 at 18:34
@Mr.Wizard Well, actually it's a duplicate of… – Ajasja Mar 5 '13 at 19:11

Dynamic doesn't work the way you think it does. See this answer for a full explanation.

In short, Dynamic doesn't do anything until it is actually displayed on screen.

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Dynamic is only updated if it is displayed. Dynamic[]; is never displayed so it is never updated. Dynamic[...;] returns a dynamic that always returns Null, but is never the less updated.

Here is an example that demonstrates the problem:

x = .5;
Dynamic[Print["will not print:", x]];
Dynamic[Print["Should print:", x];]
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