The answer was (thanks to Mark Leeds) to do with the use of a factor instead of a vector.
on [2017-08-05] at 08:57 Myles English writes: > I am having trouble understanding how the 'by' function works. Using > this bit of code: > > i <- data.frame(x=c(1,2,3), y=c(0,0,0), B=c("red","blue","blue")) > j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=c('red','blue','green')) The use of I() prevents conversion to a factor: i <- data.frame(x=c(1,2,3), y=c(0,0,0), B=I(c("red","blue","blue"))) j <- data.frame(x=c(1,2,3), y=c(1,1,1), B=I(c('red','blue','green'))) > plot(0, 0, type="n", xlim=c(0,4), ylim=c(0,1)) > by(i, i$B, function(s){ points(s$x, s$y, col=s$B) }) > by(j, j$B, function(s){ points(s$x, s$y, col=s$B) }) > > I would have expected the point at (1,1) to be coloured red. When > plotted, this row is indeed red: > >> i[1,] > x y B > 1 1 0 red > > however, this next point is green on the plot even though I would like > it to be red: > >> j[1,] > x y B > 1 1 1 red > > How can I achieve that? > > Myles ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.