Partial evaluation
- To: mathgroup at smc.vnet.net
- Subject: [mg21094] Partial evaluation
- From: "Kevin Jaffe" <kj0 at mailcity.com>
- Date: Sun, 12 Dec 1999 23:52:01 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
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/