Question

Possible Duplicate:
R – How to convert a factor to an integer\numeric in R without a loss of information

The following fact about the as.numeric() function has been brought to my attention

> blah<-c("4","8","10","15")
> blah
[1] "4"  "8"  "10" "15"
> blah.new<-as.factor(blah)
> blah.new
[1] 4  8  10 15
Levels: 10 15 4 8
> blah.new1<-as.numeric(blah.new)
> blah.new1
[1] 3 4 1 2

When I convert a factor with levels 4, 8, 10, and 15 to a quantitative variable using as.numeric(), every number is converted to a ranking, and the original values are lost.

How do I take the vector ‘blah.new’ that has levels 10,15, 4, and 8, and convert it to the numeric values 10, 15, 4, and 8?

(This issue has arisen because of a dataset where a quantitative variable is read by read.table() to be a factor)

Thank you!!!!

*****Update: FIGURED IT OUT******

blah.new1<-as.numeric(as.character(blah.new))

However, I am wondering where in documentation for as.numeric() does it say that this function converts arguments into a list of rankings?

Answer 1

First, factor consists of indices and levels. This fact is very very important when you are struggling with factor.

For example,

> z <- factor(letters[c(3, 2, 3, 4)])

# human-friendly display, but internal structure is invisible
> z
[1] c b c d
Levels: b c d

# internal structure of factor
> unclass(z)
[1] 2 1 2 3
attr(,"levels")
[1] "b" "c" "d"

here, z has 4 elements.
The index is 2, 1, 2, 3 in that order.
The level is associated with each index: 1 -> b, 2 -> c, 3 -> d.

Then, as.numeric converts simply the index part of factor into numeric.
as.character handles the index and levels, and generates character vector expressed by its level.

?as.numeric says that Factors are handled by the default method.