[R] transform a df with a condition

[Duncan Murdoch]

On 16/01/2011 9:13 AM, Patrick Hausmann wrote:
 > Dear all,
 >
 > for each A == 3 in 'df' I would like to change the variables B and K.
 > My result should be the whole df and not the subset (A==3)...
 >
 > df<- data.frame(A = c(1,1,3,2,2,3,3),
 >                    B = c(2,1,1,2,7,8,7),
 >                    K = c("a.1", "d.2", "f.3",
 >                          "a.1", "k.4", "f.9", "f.5"))
 >
 > x1<- within(df[df$A ==3, ], {

This is the problem:  you've subsetted the dataset here, and lost all 
the other lines.  You want something like this:

   x3 <- within(df, {
                B <- ifelse(A == 3, 5, B)
                K <- ifelse(A == 3, gsub("f", "m", K), as.character(K)) }
         )

You need the "as.character" as by default K will be a factor, not a 
character vector, and ifelse will take the factor level instead of the 
label.

If the condition was more complicated than A == 3, you might want to 
calculate it first and use the calculated value each time:


   x3 <- within(df, {
                changes <- A == 3
                B <- ifelse(changes, 5, B)
                K <- ifelse(changes, gsub("f", "m", K), as.character(K))
                rm(changes)
            }
         )

 >          B1<- 5
 >          K1<- gsub("f","m", K)
 >          })
 >
 > x2<- transform(df[df$A==3, ], B1 = 5, K1 = gsub("f","m", K))
 >
 > Thanks for any help!
 > Patrick
 >
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