[R-sig-DB] Extracting a column of numbers from a data frame

[Douglas Bates]

On 2/20/06, Jason Horn <jhorn using bu.edu> wrote:
> When using RMySQL to retrieve data from a database, the result is
> always a data frame.  What is the best way to take get the numbers
> from a column of the data frame into a vector?  That is, I just want
> the column of numbers from the data frame and not the column label.
> I'm sure this has been done many times, but I'm new to this, so I
> thought I'd ask.

You use the "[[" subscripting operator or its short form for use with
names, the $ operator.

If your data frame is called "df" and you want the third column as a vector use

df[[3]]

If the name of the column is "foo" you can also use

df[["foo"]]

The expression

df$foo

is syntactic sugar for df[["foo"]]