[R] re placing <NA> in character column

[David Winsemius]

On Oct 28, 2009, at 6:22 PM, Mark W. Miller wrote:

>
>
>
>
> I am guessing that your <NA> is not really an NA_character,  but
> rather a factor with a level of "<NA>".
>
> See if str(s1) confirms my suspicions.
>
>> df1 <- read.table(textConnection(" Firstname  Lastname Age
> + 1    Bob       Smith   20
> + 2   John       Clark   NA
> + 3   Andy       <NA>    40"), header=T)
>> levels(df1$Lastname)
> [1] "<NA>"  "Clark" "Smith"
>> levels(df1$Lastname)[1] <- "qqqq"
>> levels(df1$Lastname)
> [1] "qqqq"  "Clark" "Smith"
>> df1
>   Firstname Lastname Age
> 1       Bob    Smith  20
> 2      John    Clark  NA
> 3      Andy     qqqq  40
>
> --  
>
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
>
>
>
>
>
> Dr. Winsemius, I just tried your suggestion of typing str(s1).   
> Below is the
> code and the comments that R returned:
>
>> channel <- odbcConnectExcel('u:/test.xls')
>> sqlTables(channel)
>
>  TABLE_CAT TABLE_SCHEM TABLE_NAME   TABLE_TYPE REMARKS
> 1  u:\\test        <NA>    Sheet1$ SYSTEM TABLE    <NA>
> 2  u:\\test        <NA>    Sheet2$ SYSTEM TABLE    <NA>
> 3  u:\\test        <NA>    Sheet3$ SYSTEM TABLE    <NA>
>
>>
>> s1 <- sqlFetch(channel, "Sheet1")
>>
>> odbcClose(channel)
>>
>> s1
>
>  Firstname Lastname Age
> 1       Bob    Smith  20
> 2      John    Clark  NA
> 3      Andy     <NA>  40
>
>
>> str(s1)
>
> 'data.frame':   3 obs. of  3 variables:
> $ Firstname: Factor w/ 3 levels "Andy","Bob","John": 2 3 1
> $ Lastname : Factor w/ 2 levels "Clark","Smith": 2 1 NA

My guess was wrong.

> $ Age      : num  20 NA 40
>
>> levels(s1$Lastname)
>
> [1] "Clark" "Smith"
>
>>
>
>
>
> When I used the code:
>
> levels(s1$Lastname)[1] <- "qqqq"
>
> R replaced Clark with qqqq and kept <NA> I guess because <NA> is not
> considered a level here:
>
>
>> levels(s1$Lastname)[1] <- "qqqq"

When I did that on the dataset I already had examined "df1". I knew  
that <NA> was a level. In your case is was not a named level. So you  
should not have expected the same outcome as with my efforts.

I am unable to explain why your methods did not work with that  
structure:

This seems to work:

 > df1$Lastname <- ifelse(is.na(df1$Lastname), "qqqq", df1$Lastname)
 > df1
   Firstname Lastname Age
1       Bob        3  20
2      John        2  NA
3      Andy     qqqq  40



>> s1
>  Firstname Lastname Age
> 1       Bob    Smith  20
> 2      John     qqqq  NA
> 3      Andy     <NA>  40
>>
>
>
>
>
> -- 


David Winsemius, MD
Heritage Laboratories
West Hartford, CT