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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: [mg77344] Re: [mg77290] apparently I don't know how to use Map (or Apply or MapThread)
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Thu, 7 Jun 2007 03:44:38 -0400 (EDT)
  • References: <200706061117.HAA00225@smc.vnet.net>

On Jun 6, 2007, at 7:17 AM, 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...

The arguments to the pure function in the first case are {0.098,  
0.951, 0.954} and {0.509, 0.914, 0.487} since Map applies its first  
argument to each element at level 1 by default.  The second form you  
are using applies the function to the zeroth level of its argument  
which is m.  An equivalent expression is:

Table[#[[i, i]], {i, Length[#]}] & /@ m

You can also do:

Tr[m,List]

Regards,

Ssezi


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