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MathGroup Archive 1998

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Re: Structure modification and related questions

  • To: mathgroup at
  • Subject: [mg13890] Re: [mg13857] Structure modification and related questions
  • From: Jon Prudhomme <prudhomj at>
  • Date: Sun, 6 Sep 1998 02:55:27 -0400
  • Sender: owner-wri-mathgroup at


A.1)	You should be able to alter parts of a list x  by using x[[3]]=0; 
I have never had any trouble with that.

In[1]:= x=Table[i,{i,4}]
Out[1]= {1,2,3,4}

In[2]:= x[[3]]=0
Out[2]= 0

In[3]:= x
Out[3]= {1,2,0,4}

A.2)	The function ReplacePart[list, newelement,position] will replace
the  element list[[position]] with newelement.

In[4]:= ReplacePart[x,5,3]
Out[4]= {1,2,5,4}

A.3)	Structures with variables in them like x={1,y,4} will change when 
y changes.

In[5]:= x[[3]]=y
Out[5]= y

In[6]:= y={2,3};x
Out[6]= {1,2,{2,3},4}

In[7]:= y={2,5};x
Out[7]= {1,2,{2,5},4}

Hope that helps.

Jon Prudhomme
prudhomj at

On Wed, 2 Sep 1998, Roberto Moriyon wrote:

> Hello,
> I am working with a big structure (actually, it is a list), like
> x=Table[i, {i,10000}]
> Q.1) Can I modify x[[3]], let's say, without having to create another
> list of 10000 elements? (the obvious thing for me to try, x[[3]]=0,
> does not work).
> Q.2) In the same spirit, are there any functions similar to Insert and
> Delete, that modify directely the expression affected by them, without
> taking a whole copy?
> Q.3) Related to Q.1&2, is it possible in Mathematica that two structures
> (or lists, for that sake) have some parts in common? What I mean by
> this is that, for example, I would like to have the variable x equal to
> the second element of {1,{2,3},4}, so that in case the 3 is changed by
> a 5 in one of them, it is also changed in the other one.
> Thank you for any help you can give me on this
> Roberto Moriyon
> --
> Roberto Moriyon
> Departamento de Ingenieria Informatica Universidad Autonoma de Madrid
> 28049 Madrid
> Phone: +34-91-397-4350
> Fax: +34-91-397-5277

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