Hi, On Fri, Nov 5, 2010 at 10:56 AM, Stephen Liu <sati...@yahoo.com> wrote: > Hi Steve, > >> It's not clear what you're having problems understanding. By >> setting the "dim" attribute of your (1d) vector, you are changing >> itsdimenensions. > > I'm following An Introduction to R to learn R > > On > > 5.1 Arrays > http://cran.r-project.org/doc/manuals/R-intro.html#Vectors-and-assignment > > > It mentions:- > ... > For example if the dimension vector for an array, say a, is c(3,4,2) then > there > are 3 * 4 * 2 = 24 entries in a and the data vector holds them in the order > a[1,1,1], a[2,1,1], ..., a[2,4,2], a[3,4,2]. > > > I don't understand "on .... =24 entries in a and the data vector holds them in > the order a[1,1,1], a[2,1,1], ..., a[2,4,2], a[3,4,2]." the order a[1,1,1], > a[2,1,1], ..., a[2,4,2], a[3,4,2]? What does it mean "the order a[1,1,1], > a[2,1,1], ..., a[2,4,2], a[3,4,2]"?
Let's just stick with a 2d matrix -- it's easier to think about. I'm not sure if you are coming from a different programming language or not, so perhaps this isn't helpful if you don't, but you might imagine holding data for a 2d matrix in an 'array of arrays' structure. R doesn't do this. It holds the data for a 1d, 2d, 3d, ... 10d array in a 1d vector. The data is stored in "column major" format, so the rows of a 2d matrix are filled first. If I have a 2d matrix like this: 1 2 3 4 5 6 7 8 R holds this in a 1d vector/array that looks like this: 1, 5, 2, 6, 3, 7, 4, 8 This idea follows through to higher dimensions. Hope that helps, -steve -- Steve Lianoglou Graduate Student: Computational Systems Biology | Memorial Sloan-Kettering Cancer Center | Weill Medical College of Cornell University Contact Info: http://cbio.mskcc.org/~lianos/contact ______________________________________________ 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.