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Re: Simply but handy question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124845] Re: Simply but handy question
  • From: David Reiss <dbreiss at gmail.com>
  • Date: Thu, 9 Feb 2012 05:38:42 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jgtist$6bb$1@smc.vnet.net>

I'm sure you will get many suggestions here.  Here is one possibility:

In[31]:= Y = {{a, b}, {0, c}}

Out[31]= {{a, b}, {0, c}}

In[32]:= testValue[0 | 0.] = 0

Out[32]= 0

In[33]:= testValue[_] = 1

Out[33]= 1

In[34]:= Map[testValue, Y, {2}]

Out[34]= {{1, 1}, {0, 1}}

But the key thing that you need to analyze with this and the list of
other ways of approaching this problem is which works the most
efficiently for the class of problems you want to apply it to.

--David


On Feb 8, 5:30 am, jediwhelan <jediwhe... at googlemail.com> wrote:
> Dear group members,
>
> I am new to mathematica (well, I'm not actually but I haven't used it
> in 5 years +).
>
> Is there an easy way to return a matrix X of 1's and 0's to test
> whether the entries in Y
> are different from zero.
>
> i.e.,
>
> if Y = {{a,b},{0,c}}
>
> Then X would return
>
> X = {{1,1},{0,1}}
>
> This would be handy for very large or complicated matrices where one
> would like to know if specific entries are zero?
>
> Many thanks,
> Paul




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