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

• To: mathgroup at smc.vnet.net
• Subject: [mg122694] Re: How to evaluate parts of an expression, but not other parts?
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Sun, 6 Nov 2011 05:56:17 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201111050945.EAA10308@smc.vnet.net> <144C59E7-489B-4015-B060-2985B69D3E08@mimuw.edu.pl>

```On 5 Nov 2011, at 13:22, Andrzej Kozlowski wrote:

>
> 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

A slightly simpler way (using the same idea) is:

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

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

```

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