[R] Changing entries of column of type "factor"/Adding a new level to a factor

[David Winsemius]

On Aug 27, 2012, at 3:09 AM, Fridolin wrote:

> What is a smart way to change an entry inside a column of a  
> dataframe or
> matrix which is of type "factor"?
>
> Here is my script incl. input data:
>> #set working directory:
>> setwd("K:/R")
>>
>> #read in data:
>> input<-read.table("Exampleinput.txt", sep="\t", header=TRUE)
>>
>> #check data:
>> input
>   Ind      M1      M2      M3
> 1    1   96/98 120/120     0/0
> 2    2 102/108 120/124 305/305
> 3    3  96/108 120/120     0/0
> 4    4     0/0 116/120 300/305
> 5    5  96/108 120/130 300/305
> 6    6   98/98 116/120 300/305
> 7    7  98/108 120/120 305/305
> 8    8  98/108 120/120 305/305
> 9    9  98/102 120/124 300/300
> 10  10 108/108 120/120 305/305
>> str(input)
> 'data.frame':	10 obs. of  4 variables:
> $ Ind: int  1 2 3 4 5 6 7 8 9 10
> $ M1 : Factor w/ 8 levels "0/0","102/108",..: 5 2 4 1 4 8 7 7 6 3
> $ M2 : Factor w/ 4 levels "116/120","120/120",..: 2 3 2 1 4 1 2 2 3 2
> $ M3 : Factor w/ 4 levels "0/0","300/300",..: 1 4 1 3 3 3 4 4 2 4
>>
>> #replace 0/0 by 999/999:
>> for (r in 1:10)
> +   for (c in 2:4)
> +     if (input[r,c]=="0/0") input[r,c]<-"999/999"
> Warnmeldungen:
> 1: In `[<-.factor`(`*tmp*`, iseq, value = "999/999") :
>  invalid factor level, NAs generated
> 2: In `[<-.factor`(`*tmp*`, iseq, value = "999/999") :
>  invalid factor level, NAs generated
> 3: In `[<-.factor`(`*tmp*`, iseq, value = "999/999") :
>  invalid factor level, NAs generated
>> input
>   Ind      M1      M2      M3
> 1    1   96/98 120/120    <NA>
> 2    2 102/108 120/124 305/305
> 3    3  96/108 120/120    <NA>
> 4    4    <NA> 116/120 300/305
> 5    5  96/108 120/130 300/305
> 6    6   98/98 116/120 300/305
> 7    7  98/108 120/120 305/305
> 8    8  98/108 120/120 305/305
> 9    9  98/102 120/124 300/300
> 10  10 108/108 120/120 305/305
>
>
> I want to replace all "0/0" by "999/999". My code should work for  
> columns of
> type "character" and "integer". But to make it work for a "factor"- 
> column I
> would need to add the new level of "999/999" at first, I guess. How  
> do I add
> a new level?

?levels

levels(input$M1) <- c(levels(input$M1), "999/999")

-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT