[R] Basic Q on coercing factors in data frames to numeric

[Roger D. Peng]

I believe this is a FAQ.  See 
http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-do-I-convert-factors-to-numeric_003f

-roger

Robert Brown FM CEFAS wrote:
> 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
> 
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-- 
Roger D. Peng
http://www.biostat.jhsph.edu/~rpeng/