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


On 5 Nov 2011, at 10:45, Julian Francis 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.
>


Well, there may be a simpler way, but here is one that comes to my mind:

a = x; b = y; c = z; d = w;

HoldForm @@ Apply[Hold, Hold[(a + b) + (c + d)], {2}] /. p_Hold :> With[{w = Plus @@ p}, w /; True]

HoldForm[(x + y) + (w + z)]

Andrzej Kozlowski=



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