# [R] building random matrices from vectors of random parameters

Jeff Newmiller jdnewmil at dcn.davis.ca.us
Thu Sep 28 19:22:10 CEST 2017

```The use of aperm is unnecessary if you call array() properly.

ms <- array(c(rep(0, 5),so,sa*m,sa), c(5, 2, 2))
--
Sent from my phone. Please excuse my brevity.

On September 28, 2017 9:10:26 AM PDT, Evan Cooch <evan.cooch at gmail.com> wrote:
>Sure -- thanks -- only took me 3-4 attempts to get aperm to work (as
>opposed to really thinking hard about how it works ;-)
>
>On 9/28/2017 11:55 AM, Duncan Murdoch wrote:
>> On 28/09/2017 9:10 AM, Evan Cooch wrote:
>>> Thanks for both the mapply and array approaches! However, although
>>> intended to generate the same result, they don't:
>>>
>>> # mapply approach
>>>
>>> n = 3
>>> sa <- rnorm(n,0.8,0.1)
>>> so <- rnorm(n,0.5,0.1)
>>> m <- rnorm(n,1.2,0.1)
>>> mats = mapply(function(sa1, so1, m1)
>>> matrix(c(0,sa1*m1,so1,sa1),2,2,byrow=T), sa, so, m, SIMPLIFY =
>FALSE)
>>>
>>> print(mats)
>>>
>>> [[1]]
>>>            [,1]      [,2]
>>> [1,] 0.0000000 0.8643679
>>> [2,] 0.4731249 0.7750431
>>>
>>> [[2]]
>>>            [,1]      [,2]
>>> [1,] 0.0000000 0.8838286
>>> [2,] 0.5895258 0.7880983
>>>
>>> [[3]]
>>>            [,1]      [,2]
>>> [1,] 0.0000000 1.1491560
>>> [2,] 0.4947322 0.9744166
>>>
>>>
>>> Now, the array approach:
>>>
>>> # array approach
>>>
>>> ms <- array(c(rep(0, 3),sa*m,so,sa), c(3, 2, 2))
>>>
>>> for (i in 1:n) { print(ms[i,,])
>>>
>>>            [,1]      [,2]
>>> [1,] 0.0000000 0.4731249
>>> [2,] 0.8643679 0.7750431
>>>
>>>            [,1]      [,2]
>>> [1,] 0.0000000 0.5895258
>>> [2,] 0.8838286 0.7880983
>>>
>>>           [,1]      [,2]
>>> [1,] 0.000000 0.4947322
>>> [2,] 1.149156 0.9744166
>>>
>>>
>>> These matrices are the transpose of those returned by the mapply
>>> approach. To see if one approach or the other is 'confused', I
>simply
>>> rerun setting sd=0 for the parameters -- thus, every matrix will be
>>> the same. The correct matrix would be:
>>>
>>>       [,1] [,2]
>>> [1,]  0.0 0.96
>>> [2,]  0.5 0.80
>>>
>>>
>>> In fact, this is what is returned by the mapply approach, while the
>>> array approach returns the transpose. I gather the 'missing step' is
>
>>> to use aperm, but haven't figured out how to get that to work...yet.
>>>
>>>
>>> On 9/28/2017 5:11 AM, Duncan Murdoch wrote:
>>>> ms <- array(c(rep(0, 5),sa*m,so,sa), c(5, 2, 2))
>>>
>>
>>
>> Sorry about that -- I didn't notice the "byrow = T" in your original.
>>
>> Duncan Murdoch
>>
>
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help