25 Recipes for Getting Started with R by Paul Teetor

Author:Paul Teetor
Language: eng
Format: epub, pdf
Tags: Data Visualization
ISBN: 9781449303235
Publisher: O'Reilly Media, Inc.

suburbs[[1]]

This returns one column.

suburbs[1]

This returns a data frame, and the data frame contains exactly one column. This is a special case of dfrm[c(n1,n2, ..., nk)]. We don’t need the c(...) construct because there is only one n.

The point here is that “one column” is different from “a data frame that contains one column.” The first expression returns a column, so it’s a vector or a factor. The second expression returns a data frame, which is different.

R lets you use matrix notation to select columns, as shown in the Solution. But an odd quirk can bite you: you might get a column or you might get a data frame, depending which many subscripts you use. In the simple case of one index you get a column, like this:

> suburbs[,1] [1] "Chicago" "Kenosha" "Aurora" "Elgin" [5] "Gary" "Joliet" "Naperville" "Arlington Heights" [9] "Bolingbrook" "Cicero" "Evanston" "Hammond" [13] "Palatine" "Schaumburg" "Skokie" "Waukegan"

But using the same matrix-style syntax with multiple indexes returns a data frame:

> suburbs[,c(1,4)] city pop 1 Chicago 2853114 2 Kenosha 90352 3 Aurora 171782 4 Elgin 94487 5 Gary 102746 6 Joliet 106221 7 Naperville 147779 8 Arlington Heights 76031 9 Bolingbrook 70834 10 Cicero 72616 11 Evanston 74239 12 Hammond 83048 13 Palatine 67232 14 Schaumburg 75386 15 Skokie 63348 16 Waukegan 91452

This creates a problem. Suppose you see this expression in some old R script:

dfrm[,vec]

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