Using R for Introductory Statistics : John Verzani

Multivariate data 113Accessing a data frame using [,] notationWhen we use a spreadsheet, we refer to the cell entries by their column names and rowsnumber. Data-frame entries can be referred to by their column names (or numbers) and/ortheir row names (or numbers).Entries of a data vector are accessed with the [] notation. This allows us to specify theentries we want by their indices or names. If df is the data frame, the basic notation isdf[row, column]There are two positions to put values, though we may leave them blank on purpose. Inparticular, the value of row can be a single number (to access that row), a vector ofnumbers (to access those rows), a single name or vector of names (to match the rownames), a logical vector of the appropriate length, or left blank to match all the rows.Similarly for the value of column. As with data vectors, rows and columns beginnumbering at 1.In this example, we create a simple data frame with two variables, each with threeentries, and then we add row names. Afterward, we illustrate several styles of access.> df=data.frame(x=1:3,y=4:6) # add in column names> rownames(df)=c("row 1","row 2","row 3") # add rownames> dfx yrow 1 1 4row 2 2 5row 3 3 6> df[3,2] # row=3,col=2[1] 6> df["row 3","y"] # by name[1] 6> df[1:3,1] # rows 1, 2 and 3;column 1[1] 1 2 3> df[1:2,1:2] # rows 1 and 2, columns1 and 2x yrow 1 1 4row 2 2 5> df[,1] # all rows, column 1,returns vector[1] 123> df [1,] # row 1, all columnsx yrow 1 1 4> df[c(T,F,T),] # rows 1 and 3 (T=TRUE)x yrow 1 1 4row 3 3 6■ Example 4.4: Using data-frame notation to simplify tasks