Dear Mark, A data frame is a list, and thus foo[5] returns a one-element list, while foo[[5]] returns a vector. One can also subscript a data frame as a matrix, so foo[1, 5] returns the element in the 1st row, 5th column, foo[,5] returns the 5th column as a vector, and foo[,c(3, 5)] returns a two-column data frame (and is equivalent to foo[c(3, 5)]), while foo[[c(3, 5)]] returns the element in column 3, row 5 (i.e., the 5th element of the 3rd list element).
I hope this helps, John -------------------------------- John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada L8S 4M4 905-525-9140x23604 http://socserv.mcmaster.ca/jfox > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > project.org] On Behalf Of Mark Farnell > Sent: April-06-08 11:15 PM > To: r-help@r-project.org > Subject: [R] getting numeric arrays from data frame > > Currently I invoke: > > foo <- read.table("foo.data", header=TRUE) > > to read a table into foo > > Then when I try to plot a histogram out of the 5th column of foo: > > hist(foo[5]) > > It fails and it says: > > Error in hist.default(foo[5]) : 'x' must be numeric > > Then I tried: > > >typeof(foo[5]) > [1] "List" > > So how can I get an numeric array out of one of the columns? > > Also suppose if the dataframe foo (above) has eight columns and I want > to make an x-y scatter plot on two of the columns, how can I extract > these two specific columns? > > Thanks > > Mark > > ______________________________________________ > 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.
