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Re: Beginner: List Indexing and Assignment

  • To: mathgroup at smc.vnet.net
  • Subject: [mg94517] Re: Beginner: List Indexing and Assignment
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Mon, 15 Dec 2008 07:46:32 -0500 (EST)
  • References: <gi2ut4$a5h$1@smc.vnet.net>

Hi,

a = {{1, 2, 3}, {4, 5, 6}};

Map[If[# > 2, -1, #] &, a, {2}]

??

Regards
   Jens


C Rose wrote:
> Hi
> 
> I am moving from another system to Mathematica and have a few simple questions about indexing and altering lists. I've been able to find Mathematica equivalents to some of the other system's  idioms, but as a Mathematica neophyte they're not very elegant. I'd be very grateful if someone could tell me the Mathematica equivalents---or point me to a suitable Rosetta stone (Google didn't easily turn one up).
> 
> In the other system, I would create a 2x3 matrix using
> 
> a = [1 2 3; 4 5 6]
> 
> resulting in
> 
> [1 2 3]
> [4 5 6]
> 
> and then assign any element of the matrix whose value is greater than 2 the value -1 using
> 
> a(a>2) = -1
> 
> resulting in
> 
> [ 1  2 -1]
> [-1 -1 -1]
> 
> I can do this in Mathematica by:
> 
> a = ReplacePart[a, Position[a, x_ /; x > 2] -> -1]
> 
> but is there a more elegant method?
> 
> Another way (in the other system) is to create a logical array:
> 
> logical = a>2
> 
> resulting in
> 
> [0 0 1]
> [1 1 1]
> 
> and I could then do
> 
> a(logical) = -1
> 
> again resulting in
> 
> [ 1  2 -1]
> [-1 -1 -1]
> 
> I have been able to approximate this in Mathematica as
> 
> logical = a /. x_ /; x > 2 -> True
> (* Note, unlike above, logical contains values of True and other integers. *)
> 
> ReplacePart[a, Position[logical, x_ /; x == True] -> -1]
> 
> Is there a more elegant method in Mathematica? (Of course, 'elegant' is a subjective quality; perhaps 'brevity' is a better word :-)
> 
> Many thanks in advance
> 
> Chris
> 
> 
> 
> 


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