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MathGroup Archive 2007

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Re: modify elements of arb.dimensionality matrices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77351] Re: [mg77218] modify elements of arb.dimensionality matrices
  • From: Carl Woll <carlw at wolfram.com>
  • Date: Thu, 7 Jun 2007 03:48:16 -0400 (EDT)
  • References: <200706051104.HAA02287@smc.vnet.net>

alexxx.magni at gmail.com wrote:

>I was able to write an interface to deal with matrices having
>dimension defined at runtime (see
>http://groups.google.it/group/comp.soft-sys.math.mathematica/browse_frm/thread/f286f5bdafba4195/c85eaca115f18a46?lnk=st&q=+%22Alessandro+Magni%22&rnum=2&hl=it#c85eaca115f18a46).
>
>My last problem - more a performance problem - is modify access to
>those matrices:
>
>reading access  at a given index location i (where i is the location
>{x,y,z,...}) is made with:
>
>  
>
>>Extract[h, i]
>>    
>>
>
>while to increment h[[]] at i I use:
>
>  
>
>>h = ReplacePart[h, i ->  Extract[h, i] + 1]
>>    
>>
>
>but I really miss the possibility to write h[[x,y]]=h[[x,y]]+1, and
>I'm user there are serious performance issues with my solution.
>  
>
Why not use Increment or AddTo? For example:

In[1]:= h = ConstantArray[0, {3, 3}];

Using Increment:

In[2]:= h[[1, 2]]++
Out[2]= 0

In[3]:= h
Out[3]=
0    1    0
0    0    0
0    0    0

Using AddTo:

In[4]:= h[[2, 3]] += 10
Out[4]= 10

In[5]:= h
Out[5]=
0    1    0
0    0    10
0    0    0

Carl Woll
Wolfram Research

>The nearest I came to a solution was with:
>
>  
>
>>h[[##]] & @@ i = "x"
>>    
>>
>
>but I immediately get:
>
>  
>
>>Set::write: Tag Apply in (s[[##1]]&)@@{1,2,1} is Protected. >>
>>    
>>
>
>Do you have any idea on this?
>
>thank you!
>
>Alessandro Magni
>
>  
>



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