[R] dput vs unclass to see what a factor really is composed of

[Duncan Murdoch]

On 31/07/2008 5:21 PM, Jacob Wegelin wrote:
> I used read.dta() to read in a Stata 9 dataset to R. The "Sex01" variable
> takes on two values in Stata: 0 and 1, and it is labeled "M" and "F"
> respectively, analogous to an R factor. Thus, read.dta reads it in as a
> factor.
> 
> Now, I wanted to see what this variable *really* is, in R. For instance,
> sometimes R converts a 0/1 variable into a 1/2 variable when it considers it
> a factor. The "dput" function often enables me to see what a variable really
> is, but in this case it tells me that the components of the factor are "1L"
> and "2L", that is, "one uppercase ell" and "two uppercase ell."  What does
> the uppercase ell mean?
> 
> In this case, "unclass" seems to enable me to see what the variable really
> consists of. But what do "1L" and "2L" mean?

Those are integer constants.  Compare:

 > typeof(1)
[1] "double"
 > typeof(1L)
[1] "integer"

A digit 1 is taken to be "double" (or "numeric"), i.e. a 64 bit double 
precision floating point.  A constant 1L is "integer", i.e. a 32 bit 
signed integer.  Factors only contain integer values, so we use integers 
to store them, and dput() writes them out that way so that they wouldn't 
get converted to floating point when you read them in.

Duncan Murdoch

> 
>> summary(DAT$Sex01[1:150])
>   M   F
> 137  13
>> dput(DAT$Sex01[1:150])
> structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L,
> 1L, 2L, 2L, 1L, 1L, 1L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L,
> 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L,
> 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("M", "F"), class = "factor")
> 
>> unclass(junk)
>   [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
> 1 2 1 1 2 2 1 1 1 1 1 2 2 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
> 1 1 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1
> 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 1 1 1 1 1
> [149] 1 1
> attr(,"levels")
> [1] "M" "F"
> 
>> str(junk)
>  Factor w/ 2 levels "M","F": 1 1 1 1 1 1 1 1 1 1 ...
> 
> 
> Jake
> 
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> 
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