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MathGroup Archive 2007

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Re: apparently I don't know how to use Map (or Apply or MapThread)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77359] Re: apparently I don't know how to use Map (or Apply or MapThread)
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 7 Jun 2007 03:52:32 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f4676i$1n3$1@smc.vnet.net>

dbsearch04 at yahoo.com wrote:
> I started with this:
> 
> m = {{0.098, 0.951, 0.954}, {0.509, 0.914, 0.487}}
> 
> I tried to do this
> 
> Table[#[[i, i]], {i, Length[#]}] & /@ m
> 
> It does not work, but this works
> 
> Diag := Function[x, Table[x[[i, i]], {i, Length[x]}]]; Diag[m]
> 
> !@#$%^&*(
> 
> Can anyone tell me why? I also tried: Apply, MapThread,
> DiagonalMatrix, TakeMatrix, adding () around the pure function...
> 
> I know that this is a simple problem. Would Evaluate help?
> 
> TIA.
> 
> Regards..

Map goes one level to deep with respect to what you try to achieve. The 
following will work as expected.

In[1]:= m = {{0.098, 0.951, 0.954}, {0.509, 0.914, 0.487}};
Table[#1[[i, i]], {i, Length[#1]}] & @ m
Table[#1[[i, i]], {i, Length[#1]}] & [m]

Out[2]= {0.098, 0.914}

Out[3]= {0.098, 0.914}

Regards
Jean-Marc


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