[R] symmetric matrix on both diagonals

[David Winsemius]

On Apr 21, 2012, at 1:13 AM, Petr Savicky wrote:

> On Fri, Apr 20, 2012 at 10:52:47AM -0400, David Winsemius wrote:
>>
>> On Apr 20, 2012, at 7:05 AM, Petr Savicky wrote:
>>
>>> On Fri, Apr 20, 2012 at 03:03:40AM -0700, juliane0212 wrote:
>>>>
>>>> I'm  having some problems computing a matrix being symmetric on  
>>>> both
>>>> diagonals.
>>>>
>>>> Does anyone know a way to get from this matrix
>>>>
>>>>
>>>> M <- matrix(c(1,0,0,0,2,7,0,0,3,4,0,0,6,0,0,0), ncol=4)
>>>>
>>>> to this one
>>>>
>>>>             M_final <- matrix(c(1,2,3,6,2,7,4,3,3,4,7,2,6,3,2,1),
>>>> ncol=4)
>>>
>>> Hi.
>>>
>>> Try the following.
>>>
>>> M[row(M) > col(M)] <- t(M)[row(M) > col(M)]
>>> n <- nrow(M)
>>> M[row(M) + col(M) > n + 1] <- M[n:1, n:1][row(M) + col(M) > n + 1]
>>> all(M == M_final)
>>>
>>> [1] TRUE
>>
>> How about?
>>
>>> M[3:4, ] <- rev(M[1:2,])
>>> M
>>     [,1] [,2] [,3] [,4]
>> [1,]    1    2    3    6
>> [2,]    2    7    4    3
>> [3,]    3    4    7    2
>> [4,]    6    3    2    1
>
> Hi.
>
> I am not sure, which matrix did you start from.


You are right. I misunderstood the requested task. (...and then used  
the wrong starting matrix.) Sorry for the noise.

-- 
David.


> If we start
> from the original matrix, then we get
>
>  M <- matrix(c(1,0,0,0,2,7,0,0,3,4,0,0,6,0,0,0), ncol=4)
>  M[3:4, ] <- rev(M[1:2,])
>  M
>
>       [,1] [,2] [,3] [,4]
>  [1,]    1    2    3    6
>  [2,]    0    7    4    0
>  [3,]    0    4    7    0
>  [4,]    6    3    2    1
>
> where the components 2 and 3 have two and not four copies.
>
> Petr.
>
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David Winsemius, MD
West Hartford, CT