[R] Question about NULL matrix? Can I define a NULL matrix in R?

[Rolf Turner]

On 11/08/11 13:27, David Winsemius wrote:
>
> On Aug 10, 2011, at 9:22 PM, Rolf Turner wrote:
>
>> On 11/08/11 13:11, David Winsemius wrote:
>>>
>>> On Aug 10, 2011, at 8:23 PM, Andra Isan wrote:
>>>
>>>> Hi All,
>>>> I would like to create a matrix in R but I dont know the size of my 
>>>> matrix. I only know the size of the columns but not the size of the 
>>>> rows. So, is there any way to create a dynamic matrix of size NULL 
>>>> by n_cols? and then add to that matrix?
>>>> I know for a vector, I can do this: x= NULL but is there any way to 
>>>> do the same for a matrix as well?
>>>
>>> No. You cannot make an R matrix without knowing the number of rows. 
>>> By definition an R matix has two integer dimensions. Alternatives: 
>>> You can rbind to an existing matrix, or you can make a larger than 
>>> necessary matrix filled with NA's and then fill and later extract a 
>>> subset of the rows.
>>>
>>
>> Wrong-oh, David.  :-) Check this out:
>
> 0 is an integer.

Yes it is.   But you are being pedantic.  My suggestion does exactly 
what I am sure the OP wanted.
He used the phrase ``NULL by n_cols'' simply because of unclear 
thinking.  What he wanted was an
``empty matrix'', with no rows and a given number of columns.  Which is 
what I gave him.

It is interesting that the syntax works with matrices but not with data 
frames.  If you want an
empty data frame, with no rows and a given number of columns, you need 
to create a matrix,
in the manner indicated, and then use as.data.frame, which yields the 
desired result.

    cheers,

         Rolf