Re: Partial evaluation

• To: mathgroup at smc.vnet.net
• Subject: [mg21172] Re: Partial evaluation
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Fri, 17 Dec 1999 01:22:54 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <8320dg\$glm@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Kevin,

In[]:=A = {{1, 1}, {2, 2}};

In[]:={{Hold[A], {2, 2}},
val} /. {{Hold[a_], index_}, v_} :> {Part[a, Sequence @@
index] = v}

In[]:=A

Out[]={{1, 1}, {2, val}}

works nice.

Hope that helps
Jens

Kevin Jaffe wrote:
>
> I'm sure this is a FAQ, but I haven't found the answer.
>
> First we define:
>
>     In[1]:= A = {{0, 0}, {0, 0}}
>
>     Out[1]= {{0, 0}, {0, 0}}
>
>
> Then the following assignment works:
>
>     In[2]:= Part[A, 1, 1] = 1
>
>     Out[2]= 1
>
>     In[3]:= A
>
>     Out[3]= {{1, 0}, {0, 0}}
>
> But this one doesn't:
>
>     In[4]:= Part@@{A, 1, 1} = 2
>
>     Set::write: Tag Apply in Part @@ {{{1, 0}, {0, 0}}, 1, 1} is Protected.
>
>     Out[4]= 2
>
>     In[5]:= A
>
>     Out[5]= {{1, 0}, {0, 0}}
>
> And this is even worse, of course:
>
>     In[6]:= Evaluate[Part@@{A, 1, 1}] = 2
>
>     Set::setraw: Cannot assign to raw object 1.
>
>     Out[6]= 2
>
>     In[7]:= A
>
>     Out[7]= {{1, 0}, {0, 0}}
>
> I understand the reasons for the error messages, but I don't know how
> to get around the problem.  What I need is to partially evaluate the
> left-hand side of In[4], but I don't know how to do this.
>
> The reason I want to do things like "Part@@{A, 1, 1} = 2" is that I'm
> trying to write a module that must modify entries in an n_1 x n_2 x
> ... x n_k array A of arbitrary dimension k.  I can compute the desired
> multi-index v = {i_1, i_2,..., i_k} for the array without knowing the
> array's dimension, but I can't figure a way to assign to the
> location A[[i_1, i_2, ..., i_k]].
>
> Is there any way to do what I'm trying to do?
>
> Thanks!
>
> KJ
>
> LYCOShop is now open. On your mark, get set, SHOP!!!
> http://shop.lycos.com/
> .

```

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