[R] Retrieving Factors with Levels Ordered

[David Winsemius]

On Jan 1, 2011, at 12:41 AM, H. T. Reynolds wrote:

> Hello (and Happy New Year),
>
> When I create a factor with labels in the order I want, write the  
> data as a text file,

Why? What is the reason for this process.

> and then retrieve them, the factor levels are no longer in the  
> proper order.

Two further ideas to those offered by Turlach and Bolker:
You can name them with leading digits that ascend in the desired order  
or
I have seen described ( but not found a fully worked example despite  
what I thought was an adequate search)  the use of an as() method  
which in this instance might  apply as.factor with your own level  
specification while reading with colClasses.


>
> Here is what I do (I tried many variations):
>
> # educ is a numeric vector with 1,001 observations.
> # There is one NA
>
> # Use educ to create a factor
>
> feducord <- factor(educ, labels = c('Elem', 'Mid', 'HS',
> +            'Bus', 'Some', 'Col', 'Post'), ordered = T)
>
> levels(feducord)
> [1] "Elem" "Mid"  "HS"   "Bus"  "Some" "Col"  "Post"
>
> table(feducord)
> feducord
> Elem  Mid   HS  Bus Some  Col Post
> 30   90  303  108  236  144   89
>
> # The above is what I want. The frequencies agree with
> # the codebook
>
> # Make a data frame and save it. (I want a text file.)
>
> testdf <- data.frame(feducord)
> str(testdf)
> 'data.frame':   1001 obs. of  1 variable:
> $ feducord: Ord.factor w/ 7 levels "Elem"<"Mid"<"HS"<..:
>     5 6 5 7 3 4 3 3 3 5 ...
> write.table(testdf, file = 'Junkarea/test.txt')
>
> # So far, so good.
>
> rm(testdf, feducord)
>
> # Go away.
> # Come back later to retrieve the data.
>
> testdf <- read.table(file = 'Junkarea/test.txt')
>
> # But levels are no longer ordered
>
> str(testdf)
> 'data.frame':   1001 obs. of  1 variable:
> $ feducord: Factor w/ 7 levels "Bus","Col","Elem",..:
>     7 2 7 6 4 1 4 4 4 7
>
> table(testdf$feducord)
> Bus  Col Elem   HS  Mid Post Some
> 108  144   30  303   90   89  236
>
> # The frequencies are correct, but the ordering is wrong.
>
> Clearly I am missing something obvious, but I can't see it. If I  
> save "feducord" and load it, the order of the levels is as it should  
> be. But I don't know why writing to a test file should change  
> anything. Any help would be greatly appreciated.
>
> (You're right, I don't have anything better to do on New Year's eve.)
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT