Re: [R] Matching a vector with a matrix row

From: Niels Richard Hansen <>
Date: Fri, 22 Apr 2011 10:30:08 -0400

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?


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.

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
> <> 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:	
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Received on Fri 22 Apr 2011 - 14:36:24 GMT

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