Modifying a global variable within a Manipulate expression

Question

I am trying to change a variable using Manipulate, like this:

var = something;
var = 
  Manipulate[f[var, {a, b, c}],
    {{a, .8}, -1, 1},
    {{b, 0}, -1, 1},
    {{c, 0}, -1, 1}];

var

I read the Manipulate and the Dynamic tutorial in the Wolfram documentation, but I still can't figure it out.

Does anyone got any clue? Other than, this ?

It appears that a function $f$ is used but not defined in the example. — Leucippus, Oct 31 '18 at 17:38
Manipulate[var = f[var,{a,b,c}],{{a,.8},-1,1},{{b,0},-1,1},{{c,0},-1,1}]; is that what you want? — Kuba, Oct 31 '18 at 22:06
@Kuba. Don't you think what you suggest might lead to run-away recursion? — m_goldberg, Nov 01 '18 at 01:16
@m_goldberg you are right. Probably can be fixed with TrackedSymbols. I was trying to confirm I get the question right. — Kuba, Nov 01 '18 at 08:16
@EmilianoTonizzo an example that is not clear. Is f supposed to modify var or only use it to return a value you will later use to assign to var? — Kuba, Nov 01 '18 at 08:18

score 3 · Accepted Answer · edited Nov 01 '18 at 08:17

3

You could do something like this.

var = 42;

SetAttributes[f, HoldFirst]
f[v_Symbol, {a_, b_, c_}] := (v = (a + b)/(2 + c))

Manipulate[f[var, {a, b, c}],
  {{a, .8}, -1, 1, Appearance -> "Labeled"},
  {{b, 0}, -1, 1, Appearance -> "Labeled"},
  {{c, 0}, -1, 1, Appearance -> "Labeled"}]

Dynamic @ var

0.49

The dynamic evaluation of var is made to show that the global value var is updated as the sliders move.

edited Nov 01 '18 at 08:17

Kuba

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answered Nov 01 '18 at 01:37

m_goldberg

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@Kuba. Good edit. I should have remembered, that since Set is also HoldFirst, I didn't need the Unevaluated. – m_goldberg Nov 01 '18 at 13:01
Yeah thanks, didn't know about SetAttributes – Emiliano Tonizzo Nov 01 '18 at 14:33

score 1 · Answer 2 · answered Oct 31 '18 at 19:11

Try this:

Manipulate[
 Plot[a*x^2 + b*Sqrt[x] - x, {x, 0, 10}],
 {a, 1, 4}, {b, 0, 10}
 ]

Where you may call var=a*x^2 + b*Sqrt[x] - x in your post. You should be able to get the plot of this function var.

your post has other problems such as var is something and something is assigned to Manipulate. Many thing are mixed up in your question.

Modifying a global variable within a Manipulate expression

2 Answers2