[R] coercing factors to matrix() --> num/char ? --

[John Thaden]

Peter Dalgaard:
>> Hm. I'm not sure there really is much logic in this... Factors are
>> sometimes character-like, sometimes integer-like.

Z. Todd Taylor:
>I'd be interested to hear a discussion of when they need to
>behave like integers.

Yesterday, using S+ 4.5 (Win), for instance:

#Exp is factor containing experiment numbers
>levels(mydata$Exp)
 [1] "0"  "1"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"  "10" "11" "12"
 [14] "13" "14" "15" "16" "17" "18" "19"

>lm(y ~ x1 * x2, data=mydata, subset = Exp == 3 | Exp == 5,
+  weights = 5.5 - Exp)
Error in Ops.factor(5.5, Exp): "-" not meaningful for factors
Dumped

>lm(y ~ x1 * x2, data=mydata, subset = Exp == 3 | Exp == 5,
+  weights = 5.5 - as.integer(Exp) )
Error in lm.wfit(X, Y, weights, method): negative weights not allowed
Dumped
# . . . because as.integer(Exp) gives the codes as integers, not the
values, and
# the code for Exp 5 is 6.

> lm(y ~ x1 * x2, data=mydata, subset = Exp == 3 | Exp == 5,
+   weights = 5.5 - as.integer(as.character(Exp)))

Call: 
lm(formula = y ~ x1 * x2, data = mydata,
#. . . Remainder of regression report truncated.

Z.T.T.:
>I can see the need to access the integers for
>"lower level" programming, for example, but when else?

ALL of my programming is "lower level"!  ;-)
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