You need some caution with Brian's solution as it depends on the matrix being in the same order as the factors in the data frame.
a <- data.frame(cbind(x=c('a','a','a','b','c'), y=c('a','b','c','d','e'),z=c(1,2,3,4,5))) mm <- matrix(0,5,5) colnames(mm) <- c('a','b','c','d','e') rownames(mm) <- c('a','b','c','d','e') pp <- mm[5:1, 5:1] mm[cbind(a$x, a$y)] <- a$z # desired result pp[cbind(a$x, a$y)] <- a$z # not desired result It would be nice if the following worked: mm[cbind(as.character(a$x), as.character(a$y))] <- a$z Patrick Burns [EMAIL PROTECTED] +44 (0)20 8525 0696 http://www.burns-stat.com (home of S Poetry and "A Guide for the Unwilling S User") Prof Brian Ripley wrote: >On Thu, 28 Jun 2007, Andrej Kastrin wrote: > > > >>Dear all, >> >>I have a data frame >>a <- data.frame(cbind(x=c('a','a','a','b','c'), >>y=c('a','b','c','d','e'),z=c(1,2,3,4,5))) >> >> >>>a >>> >>> >> x y z >>1 a a 1 >>2 a b 2 >>3 a c 3 >>4 b d 4 >>5 c e 5 >> >>and a matrix >>mm <- matrix(0,5,5) >>colnames(mm) <- c('a','b','c','d','e') >>rownames(mm) <- c('a','b','c','d','e') >> >> >>>mm >>> >>> >> a b c d e >>a 0 0 0 0 0 >>b 0 0 0 0 0 >>c 0 0 0 0 0 >>d 0 0 0 0 0 >>e 0 0 0 0 0 >> >>How to populate matrix in a way that first column of data frame 'a' >>correspond to rownames(mm), second column to colnames(mm) and the third >>column is the element of matrix 'mm'? >> >> > >mm[cbind(a$x, a$y)] <- a$z > >Please read about the forms of indexing matrices in 'An Introduction to >R'. > > > ______________________________________________ R-help@stat.math.ethz.ch 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.