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MathGroup Archive 2006

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Re: Newbie question about column sums of arrays

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68535] Re: Newbie question about column sums of arrays
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Wed, 9 Aug 2006 04:19:02 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 8/8/06 at 6:29 AM, gwgilc at wm.edu (George W. Gilchrist) wrote:

>I have spent several hours trying to find an answer to what must be
>an incredibly simple problem: how to sum the columns of an array
>containing a random mix of +1 and -1s. For example:

>In[7]:= AA1=Array[x, {4,4}]/. x:> (-1)^Random[Integer]

Array isn't doing what you expect or want. Array expects a function as its first argument, not a number. So, what you get for the first term will either be 1[1,1] or -1[1,1] which in Mathematica syntax is the function 1 evaluated with arguments 1,1 clearly something that doesn't make sense.

For generating a 2D random mix of 1 and -1 the simplest method is to use Table, i.e.,

Table[(-1)^Random[Integer],{4},{4}]

However, if you did want to use Array it is possible to get what you want. For example, this would work

Array[(-1)^Random[Integer] Times @@ Sign[{##}] &, {4, 4}]

But this clearly has Mathematica do more work to get the same result.


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