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Re: How to evaluate parts of an expression, but not other parts?


a = f[x];
b = g[x];
c = 2 f[x];
d = f[x] + 3 g[x];

expr = Hold[((a + b) + (c + d))];

ReplacePart[expr, {
  Position[expr, Literal[a]] -> a,
  Position[expr, Literal[b]] -> b,
  Position[expr, Literal[c]] -> c,
  Position[expr, Literal[d]] -> d}]

Hold[(f[x] + g[x]) + (2 f[x] + (f[x] + 3 g[x]))]


Bob Hanlon


On Sat, Nov 5, 2011 at 5:45 AM, Julian Francis
<julian.w.francis at gmail.com> wrote:
> Dear all,
>
> I'd like to use the TreePlot function to visualise the expression of a
> dynamic programming problem I am working on.
>
> If I have something like: ( (a+b) + (c+d )
>
> Mathematica helpfully simplifies this to: a + b + c + d
>
> But I'd prefer it to be in the original form.
>
> I can't write Hold[ ( (a+b) + (c+d) )] because I do want a,b,c & d to
> be evaluated.
>
> I want to write something like:
> Hold[ ( (Evaluate[a]+Evaluate[b]) + (Evaluate[c]+Evaluate[d]) ) ]
>
> But this just leaves the Evaluate expressions unevaluated.
>
> Any help greatly appreciated.
>
> Thanks,
> Julian.
>



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