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MathGroup Archive 2007

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Re: copying values of entries between matrices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg78013] Re: [mg77952] copying values of entries between matrices
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Thu, 21 Jun 2007 05:47:31 -0400 (EDT)
  • Organization: LANL
  • References: <200706200937.FAA10111@smc.vnet.net>
  • Reply-to: cfo at lanl.gov

If you have v. 6 of Mathematica, the function "diagonal" should help you out; 
to get a list of the elements you want, try

Diagonal[s, #]&/@Range[-(Length@s) + 1, 0]

Of course, if you want these elements on the same level in your dOneCol, 

Flatten@Diagonal[s, #]&/@Range[-(Length@s) + 1, 0]



If you have an earlier version, you could try something like the following 
(which I haven't optimized for different types of lists or anything, but 
should work for matrix arguments):

takeLowerLeft[mat_]:=Take[mat, {2, Length@mat}, {1, Length@mat - 1}]

snagDiags[mat_] := mat[[#, #]] & /@ Range[Length[mat]]

snagDiags /@ NestList[ takeLowerLeft,  dOneCol, Length[dOneCol] -1 ]

Flatten@%



    Hope that helps!

                     C.O.


On Wednesday 20 June 2007 03:37:37 text.tex wrote:
> Hi,
> here is my problem:
> I have a 18x18 matrix called entMatr.
> I want to copy the values of the elements of the lower left half of
> entMatr into a 171x1 matrix called dOneCol.
> I had tried a 3-nested loop for copying the values from entMatr to
> dOneCol; the current version where I tried to replace the loop over
> the rows of dOneCol with a counter q produces a result, however the
> result is not what I want...
>
> I'd greatly appreciate any pointers how to get this working!
> Thanks,
> Claus
>
> This is what I tried:
>
> <pre>
>  (* initialize entMatr *)
> entMatr = Table[Subscript[m, i, j], {i, 18}, {j, 18}];
>
>  (* fill the lower left half of entMatr with some values *)
> For[i = 1, i < 18 + 1, i++,
>  For[j = i, j < 18 + 1, j++,
>   entMatr[[j, i]] = i
>   ]
>  ]
>
>  (* initialize dOneCol *)
> dOneCol = Table[Subscript[n, x], {x, 171}];
>
>  (* attempt to copy the values from entMatr to dOneCol *)
> q = 0;
> For[k = 1, k < 18 + 1, k++,
>  For[p = k, p < 18 + 1, p++,
>   dOneCol[[q]] = entMatr[[p,k]]
>     q++
>   ]
>  ]
> dOneCol
> </pre>



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