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MathGroup Archive 2005

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Re: Elegant syntax for multiple conditional assignment?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54444] Re: Elegant syntax for multiple conditional assignment?
  • From: Peter Pein <petsie at arcor.de>
  • Date: Sun, 20 Feb 2005 00:10:39 -0500 (EST)
  • References: <cv6q9a$65c$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Scott Hemphill wrote:
> Dear Mathematica aficionados:
> 
> I have several state variables, each of which is a vector of "n" elements.
> I would like to assign each element in each vector based on a condition
> which is in the corresponding position in a condition vector.  To be more
> specific:
> 
>   Each of a, c, x and y is a List of Length n.
>   cond is a List of Length n, containing True and False values
> 
> What I have is
> 
>   old = {a, c, x, y};
> 
>   code which changes a, c, x, and y
> 
>   new = {a, c, x, y};
> 
> What I want to do is assign to each element of a, c, x and y the old value
> of a, c, x or y if the corresponding element of cond is False, and the new
> value of a, c, x or y if the corresponding element of cond is True.
> 
> I've got a method that works:
> 
> t=Transpose;
> {a, c, x, y} = t[If[#[[1]],#[[2]],#[[3]]]& /@ t[{cond,t[new],t[old]}]];
> 
> All the transpositions and array indexing make me wonder if there's a more
> elegant way of expressing this.
> 
> Any takers?  Thanks in advance.
> 
> Scott
Something like the following?

In[1]:=
   n=5;
   old=Table[10i+j,{i,4},{j,n}];
   new=-old;
   cond=Table[Random[]>.5,{n}]
Out[4]=
   {True,False,True,True,True}
In[5]:=
   If[Or@@cond,
     ReplacePart[old,new,#,#]&@Position[Table[cond,{4}],True],
     old]
Out[5]=
   {{-11,12,-13,-14,-15},
    {-21,22,-23,-24,-25},
    {-31,32,-33,-34,-35},
    {-41,42,-43,-44,-45}}

-- 
Peter Pein
Berlin


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