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Re: Using implicit information about row indices


On Jul 31, 2006, at 3:45 AM, Diamond, Mark wrote:

> I am wanting to fill a square matrix M (say, r by r)  using  
> information from
> a list, L.
>  The list L is also of length r, but is not a square matrix. It  
> contains
> lists of ordered pairs {col, num}
> It typically looks something like this; I have arranged it into  
> rows so that
> it is more obvious
> {
>   {{1, a}, {2, b}},
>
>   {{1, c}, {2, d}, {3, e}, {4, f}},
>
>  {{2, g}, {3, h}, {4, i}},
>
>  {{3, k}, {4, l}, {5,m}},
>
>  {{4, n}, {5, p}}
>
> }
>
> Now what I am trying to do is, for example, set M[[1,1]]=a, M[[1,2]] 
> =b,
> M[[2,1]]=c, M[[2.2]]=d, M[[2,3]]=e and so forth, and leave (or set)  
> all the
> other entries in M to zero. The row information for M is implicit  
> in the
> structure of the list L; additionally, the column indices as they  
> appear
> within sublist of L are guraranteed to be sequential. It seems with  
> all this
> info, there should be an easy way to fill M correctly, but I have  
> struggled
> without success to do anything other than an iterative process with  
> Do[]. I
> would appreciate any guidance you might have.

M=Normal[SparseArray[Flatten[MapIndexed[({#2[[1]], #1[[1]]} -> #1 
[[2]]) &, L, {2}], 1]]]

Regards,

Ssezi

PS - The usual caveats about using variables with capital letters apply.


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