[R] How to convert a factor column into a numeric one?

[Robert A. LaBudde]

I have a data frame:

 > head(df)
   Time Temp Conc Repl    Log10
1    0  -20    H    1 6.406547
2    2  -20    H    1 5.738683
3    7  -20    H    1 5.796394
4   14  -20    H    1 4.413691
5    0    4    H    1 6.406547
7    7    4    H    1 5.705433
 > str(df)
'data.frame':   177 obs. of  5 variables:
  $ Time : Factor w/ 4 levels "0","2","7","14": 1 2 3 4 1 3 4 1 3 4 ...
  $ Temp : Factor w/ 4 levels "-20","4","25",..: 1 1 1 1 2 2 2 3 3 3 ...
  $ Conc : Factor w/ 3 levels "H","L","M": 1 1 1 1 1 1 1 1 1 1 ...
  $ Repl : Factor w/ 5 levels "1","2","3","4",..: 1 1 1 1 1 1 1 1 1 1 ...
  $ Log10: num  6.41 5.74 5.8 4.41 6.41 ...
 > levels(df$Temp)
[1] "-20" "4"   "25"  "45"
 > levels(df$Time)
[1] "0"  "2"  "7"  "14"

As you can see, "Time" and "Temp" are currently factors, not numeric.

I would like to change these columns into numerical ones.

df$Time<- as.numeric(df$Time)

doesn't work, as it changes to the factor level indices (1,2,3,4) 
instead of the values (0,2,7,14).

There must be a direct way of doing this in R.

I tried recode() in 'car':

 > df$Temp<- recode(df$Temp, '1=-20;2=25;3=4;4=45',as.factor.result=FALSE)
 > head(df)
   Time Temp Conc Repl     Freq
1    0  -20    H    1 6.406547
2    2  -20    H    1 5.738683
3    7  -20    H    1 5.796394
4   14  -20    H    1 4.413691
5    0   45    H    1 6.406547
7    7   45    H    1 5.705433

but note that the values for 'Temp' in rows 5 and 7 are 45 and not 4, 
as expected, although the result is numeric. The same happens if I 
use the order given by levels(df$Temp) instead of the sort order in 
the recode() 2nd argument.

Any hints?
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

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