[R] Matching a vector with a matrix row

[Niels Richard Hansen]

Joshua and Luis

Neither of you is exactly solving the problem as stated, see
below. Luis, could you clarify if you want rows that are _equal_
to a vector or rows with entries _contained_ in a vector?

If

m <- matrix(c("A", "B", "C", "B", "A", "A"), 3, 2)
LHS <- c("A", "B")

then LHS equals the first row only, while

apply(m, 1, function(x) all(x %in% LHS))
[1]  TRUE  TRUE FALSE

finds the rows with entries contained in LHS and

which(m %in% LHS)
[1] 1 2 4 5 6

finds all entries in m that equals an entry in LHS. While
you can turn the latter into the former, this will have some
computational costs too. The R-code

apply(m, 1, function(x) all(x == LHS))
[1]  TRUE FALSE FALSE

finds the rows that are equal to LHS.

- Niels

On 22/04/11 00.18, Joshua Wiley wrote:
> Hi Felipe,
>
> Since matrices are just a vector with dimensions, you could easily use
> something like this (which at least on my system, is slightly faster):
>
> results<- which(Matrix %in% LHS)
>
> I'm not sure this is the fastest technique thought.  It will return a
> vector of the positions in "Matrix" that match "LHS".  You can easily
> convert to row numbers if you want since all columns have the same
> number of rows.
>
> HTH,
>
> Josh
>
> On Thu, Apr 21, 2011 at 8:56 PM, Luis Felipe Parra
> <felipe.parra at quantil.com.co>  wrote:
>> Hello I am trying to compare a vector with a Matrix's rows.The vector has
>> the same length as the number of columns of the matrix, and I would like to
>> find the row numbers where the matrix's row us the same as the given vector.
>> What I am doing at the moment is using apply as follows:
>>
>> apply(Matrix,1,function(x)all(x%in%LHS))
>>
>> but this isn't too fast actually. I would like  to know if any body knows an
>> efficient (fast) way of doing this? The matrix contains stings (not
>> numbers).
>>
>> Thank you
>>
>> Felipe Parra
>>
>>         [[alternative HTML version deleted]]
>>
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>
>
>

-- 
Niels Richard Hansen                     Web:   www.math.ku.dk/~richard	
Associate Professor                      Email: Niels.R.Hansen at math.ku.dk
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