On 07-05-2013, at 06:39, Mark Coletti <mcole...@gmail.com> wrote: > I have a matrix of data that has a corresponding vector of indices. I > would like to use those indices to extract specific matrix elements into a > new vector. In other words, I have an R X C matrix with a corresponding > vector of C elements that have numbers mapping into specific elements of > the matrix. I'd like to generate a new vector C long that contains those > elements. >
It's not clear what you want. The vector C contains row numbers or column numbers? > My first approach is to iteratively grind through each matrix column to > manually extract the element corresponding to that column as referenced by > the appropriate index, and then appending that to a vector. So you want to extract a single element from each column? Your matrix is Mat and the vector is called vec Mat[vec,] should do it. If this is not what you want you will have to be more specific. Berend > However, my R > instincts are that any time one has to craft a for loop to do something in > R that you are very likely Doing It Wrong; there's probably a very simple > one liner in R that will do all that that for loop would do. > > I know there must be a simple "R way" to do this, but I remain flummoxed on > how to do so. I've been feebly poking at the plyr package's maplyr() and > colwise() since I have a matrix and want to extract an array from it, and > this is inherently a column wise operation, so ... hmm. > > Should I just give up and use a for loop? > -- > mcole...@gmail.com > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
