[R] Changing some values within a variable

[Peter Alspach]

Tena koe Toni

Assume your data is in a data.frame called toniData (naming a data.frame data is not a good idea as data is a function in R - see ?data), and that Group is of class character (try str(toniData)) then:

toniData[toniData$Group %in% 'A','Group'] <- 'C'

will work.  But from your message below I suspect Group is of class factor.  So you either need to change its class:

toniData$Group <- as.character(toniData$Group)

or rename the level names.  For example:

set.seed(0)
toniG <- factor(LETTERS[sample(1:2, size=10, replace=TRUE)])
toniG
 [1] B A A B B A B B B B
Levels: A B
levels(toniG)
[1] "A" "B"
levels(toniG) <- c('C','B')
toniG
 [1] B C C B B C B B B B
Levels: C B

Regarding your second question, see sub() (or gsub()) - again paying attention to whether or not you are dealing with factor or character variables.

HTH ....

Peter Alspach

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Toni Pitcher
> Sent: Friday, 23 July 2010 9:44 a.m.
> To: r-help at r-project.org
> Subject: [R] Changing some values within a variable
> 
> Hi
> 
> I'm new to R and would like some help with a couple of problems. I
> suspect the solutions are quite simple.
> 
> I have a data.frame (data) with 40 variables and 5238 observations
> created from ~150 text files using read.table.
> 
> I would like to change some of the entries within two different
> columns.
> 
> Firstly, in the Group column I have groups A and B, I would like to
> select all As and change to C. I have tried the following;
> 
> data$Group = ifelse(data$Group == 'A', "C", data$Group)
> data$Group <- as.factor(data$Group)
> 
> This works to some extent, all As have been changes to Cs, but the Bs
> have also been changed, in this case to "2", instead of remaining as
> their original values. How do I get the Bs to stay the same?
> 
> 
> Second problems is similar;
> 
> I need to change some of the Subject IDs. The required format is 3
> numbers followed by 3 letters (999LLL). Some of the IDs have format
> 999LLL-LL, Thus I need to delete the -LL from the IDs.
> How do I generically specify "find 999LLL-LL and replace with 999LLL"?
> 
> 
> Many thanks in advance
> 
> Toni
> 
> 
> 
> 
> --
> Toni Pitcher PhD
> Dept. of Medicine
> University of Otago, Christchurch
> 
> Van der Veer Institute for Parkinson's and Brain Research
> 66 Stewart St
> Christchurch 8011
> New Zealand
> 
> Phone: +64 3 378 6075 (internal extn 86075)
> Email: toni.pitcher at otago.ac.nz
> 
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