Hi Carol, carol white wrote: > Hi, > I > want to multiply a vector (1st argument) by another vector or a matrix > (2nd argument) depends if a column or a row of a matrix or a subset of > a matrix is taken. The first argument presents a coefficient vector (of > 1 or more elements) that will be multiplied by all elements of the 2nd > argument. Therefore, the final product (multiplication of the 2 > arguments) will be a vector of the length of the number of rows of the > 2nd argument. Are you trying here to describe what's commonly called "matrix multiplication" but in the particular case where the first operand is an horizontal vector i.e. a 1xN matrix? Then the second operand must be an NxP matrix. When you say the 2nd operand can be a vector or a matrix, you're just saying that P can be any value, not just 1, is that right? Then, according to the general rules of matrix multiplication, the final product will be a 1xP matrix i.e. a vector of length the number of cols of the 2nd argument. > > I wanted to use %*% for all cases, either taking > directly the 2nd argument or its tansposal but it doesn't work for all > cases. Do I have to test the type of 2nd argument and write a different > script lines or can I use a script line for all cases? > > for example, I will have the following different cases: > > a1 = c(1) > a2 = c(2) > res = a1*a2 = 2 > c(1)%*%c(2) > > [,1] > [1,] 2 > > a1 = c(1) > a2 = c(1,2) > res = a1*a2 = [3] > sum(c(1)*c(1,2)) to get 3 > here, %*% is not of help only if you transpose a2, so that it becomes 1x2 matrix, otherwise it's treated as a 2x1 matrix and hence is not compatible with a1. The man page for %*% (?`%*%`) doesn't explicitly state it but the way a vector is promoted to a row or column matrix depends on which side of %*% it is. > > a1 = c(1,2) > a2 = c(2,1) > res = 4 > c(1,2)%*%c(2,1) > so a1%*%a2 would work > > a1 = c(1,2) > a2 = rbind(c(1,2), c(2,1)) > res = [5 4] > a1%*%a2 > [,1] [,2] > [1,] 5 4 > > a1 = c(1,2,3) > a2 = rbind(c(1,2,3), c(2,1,3)) a1 will be promoted to a 1x3 matrix but a2 is a 2x3 matrix so they are not conformable. > a1%*%t(a2) > [,1] [,2] > [1,] 14 13 yes, or use cbind() instead of rbind() in the first place. H. > > etc > > > > > [[alternative HTML version deleted]] > > > > -------------------------------------------------------------------- ---- > > _______________________________________________ > Bioconductor mailing list > Bioconductor at stat.math.ethz.ch > https://stat.ethz.ch/mailman/listinfo/bioconductor > Search the archives: http://news.gmane.org/gmane.science.biology.informatics.conductor -- Hervé Pagès Program in Computational Biology Division of Public Health Sciences Fred Hutchinson Cancer Research Center 1100 Fairview Ave. N, M2-B876 P.O. Box 19024 Seattle, WA 98109-1024 E-mail: hpages at fhcrc.org Phone: (206) 667-5791 Fax: (206) 667-1319