Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2007
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: copying values of entries between matrices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77984] Re: copying values of entries between matrices
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 21 Jun 2007 05:32:39 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f5atd0$an9$1@smc.vnet.net>

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>

Assuming I have correctly understood what you wanted, one way of doing 
that is the following:

dOneCol =
  Reap[Do[If[j <= i, Sow[entMatr[[i, j]]]], {i, 18}, {j, 18}]][[2, 1]]

For instance,

entMatr = Array[Subscript[m, ##1] & , {18, 18}];
For[i = 1, i < 18 + 1, i++, For[j = i, j < 18 + 1, j++,
      entMatr[[j, i]] = i]]
MatrixForm[entMatr]
dOneCol =
  Reap[Do[If[j <= i, Sow[entMatr[[i, j]]]], {i, 18}, {j, 18}]][[
       2, 1]]
Length[dOneCol]
MatrixForm[dOneCol]

Regards,
Jean-Marc


  • Prev by Date: Re: Simple ODE with time-dep BC
  • Next by Date: Re: v5.2 preferred for stability over v6.0
  • Previous by thread: Re: copying values of entries between matrices
  • Next by thread: Re: copying values of entries between matrices