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.pa...@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
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