real numeric variable transforms into factor:

On Apr 17, 2009, at 2:52 PM, Aldi Kraja wrote:

Hi
Test made in: R in windows Vista OS, R version 2.8.1
From FAQ:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-do-I-convert-factors-to-numeric_003f 

"It may happen that when reading numeric data into R (usually, when
reading in a file), they come in as factors. If |f| is such a factor
object, you can use as.numeric(as.character(f)) to get the numbers 
back."

1: Why it may happen? Why R transforms x1 from real numeric with decimal
values into factor???
2: Doesn't it look strange to get "internal numbers" when one applies
as.numeric(x$x1)?
3. What are the internal numbers mentioned in the FAQ?
Why is needed to write:
as.numeric(as.character(x$x1)) to get finally the right numbers I read
with read.table?
Are the missing values shown as dot to force R (or the programmer who
wrote the function read.table) to consider x1 as factor?

Is it possible who is maintaining the read.table function to improve it
to recognize numbers with decimal places as numeric and not as factors
and dots as missing values which transform into NA?

The data file saved as text:
test.txt
ob,x1,y1
1,1.1,1/1
2,2.1,1/2
3,3.2,2/2
4,.,0/0
5,4.5,1/1
6,5.1,0/0
7,6.3,1/1
8,.,1/2
reading it from d directory:
x<-read.table(file="d:\\test\\test.txt",header=T,sep=',')

x

 ob  x1  y1
1  1 1.1 1/1
2  2 2.1 1/2
3  3 3.2 2/2
4  4   . 0/0
5  5 4.5 1/1
6  6 5.1 0/0
7  7 6.3 1/1
8  8   . 1/2

as.numeric(x$x1)

[1] 2 3 4 1 5 6 7 1

as.numeric(as.character(x$x1))

[1] 1.1 2.1 3.2  NA 4.5 5.1 6.3  NA
Warning message:
NAs introduced by coercion

Thanks,

Aldi

Looks like you are taking data from SAS perhaps, where the missing 
value indicator is '.'.  In R, where type.convert() is used to 
determine the data types for incoming text data, you get:

type.convert(".")

[1] .
Levels: .

That is, a factor.

What you want to do is to set the 'na.strings' argument to 
read.table() to '.' rather than the default 'NA', so that the periods 
are interpreted as missing values and set to NA during import. Thus:

# Create from your data in the clipboard (on OSX)
DF <- read.table(pipe("pbpaste"), header = TRUE, sep = ",", na.strings 
= ".")

DF

  ob  x1  y1
1  1 1.1 1/1
2  2 2.1 1/2
3  3 3.2 2/2
4  4  NA 0/0
5  5 4.5 1/1
6  6 5.1 0/0
7  7 6.3 1/1
8  8  NA 1/2

str(DF)

'data.frame':    8 obs. of  3 variables:
 $ ob: int  1 2 3 4 5 6 7 8
 $ x1: num  1.1 2.1 3.2 NA 4.5 5.1 6.3 NA
 $ y1: Factor w/ 4 levels "0/0","1/1","1/2",..: 2 3 4 1 2 1 2 3

This is now because:

type.convert(".", na.strings = ".")

[1] NA


See ?read.table and ?type.convert for more information.

HTH,

Marc Schwartz