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

```

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