Basic Q on coercing factors in data frames to numeric

Hi there,

I'm running R 2.0.0 on Windows 95.  I'm trying to coerce a column of factors within a data frame to numeric.  This is not a problem with a vector, but I can't find a way to index a column within a data frame to achieve this. All the examples from 'An introduction to R', 'S-plus 6 programmers guide', etc, use simple vectors.  I'm sure I'm missing something obvious but I can't find a way to change a single column in data frame. Ive tried several approaches: here is one

summary(test)

      age        yrclass       weight              year     
 1      :10   1992   :10   Min.   :0.005333   Min.   :1993  
 2      :10   1989   : 9   1st Qu.:0.221790   1st Qu.:1995  
 3      :10   1990   : 9   Median :0.413411   Median :1997  
 4      :10   1991   : 9   Mean   :0.420119   Mean   :1997  
 5      :10   1988   : 8   3rd Qu.:0.559819   3rd Qu.:2000  
 6      :10   1993   : 8   Max.   :1.189000   Max.   :2002  
 (Other):82   (Other):89                                    

contents(test)

Data frame:test 142 observations and 4 variables    Maximum # NAs:0

        Levels Storage
age         23 integer
yrclass     28 integer
weight          double
year            double

+--------+---------------------------------------------------------------------+
|Variable|Levels                                                               |
+--------+---------------------------------------------------------------------+
| age    |0,1,10,11,12,13,14,15,16,18,19,2,20,21,24,25,3,4,5,6,7,8,9           |
+--------+---------------------------------------------------------------------+
| yrclass|1969,1974,1975,1978,1979,1980,1981,1982,1983,1984,1985,1986,1987,1988|
|        |1989,1990,1991,1992,1993,1994,1995,1996,1997,1998,1999,2000,2001,2002|
+--------+---------------------------------------------------------------------+

is(test$yrclass,"factor")

[1] TRUE

is(test$yrclass,"numeric")

[1] FALSE

as(test[,2],"numeric")

  [1]  1  1  2  3  3  4  5  6  6  6  7  7  8  8  8  8  9  9  9  9 10 10 10 10 10
 [26] 10 10 11 11 11 11 11 12 12 12 12 12 12 12 13 13 13 13 13 13 13 14 14 14 14
 [51] 14 14 14 14 15 15 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 17 17 17
 [76] 17 17 17 17 17 17 18 18 18 18 18 18 18 18 18 18 19 19 19 19 19 19 19 19 20
[101] 20 20 20 20 20 20 20 21 21 21 21 21 21 21 21 22 22 22 22 22 22 22 23 23 23
[126] 23 23 23 24 24 24 24 24 25 25 25 25 26 26 27 27 28

is(test$yrclass,"numeric")

[1] FALSE


Regards,

Robert Brown