The R Book

Selecting Variables on the Basis of their Attributes

In this example, we want to extract all of the columns from nums (above) that are numeric. Use sapply to obtain a vector of logical values:

sapply(nums,is.numeric)

 name   date  response  treatment  dates
FALSE  FALSE      TRUE      FALSE   TRUE

Now use this object to form the column subscripts to extract the two numeric variables:

nums[,sapply(nums,is.numeric)]

     response       dates
1  0.05963704  2003-08-25
2  1.46555993  2003-05-21
3  1.59406539  2003-10-12
4  2.09505949  2003-12-02

Note that dates is numeric but date was not (it is a factor, having been converted from a character string by the read.table function).