Replace values in selected columns by passing column name of data.frame into apply() or plyr function

Question

Suppose I have a date.frame like:

df <- data.frame(a=1:5, b=sample(1:5, 5, replace=TRUE), c=5:1)
df
  a b c
1 1 4 5
2 2 3 4
3 3 5 3
4 4 2 2
5 5 1 1
         


        

Answer 1

df <- data.frame(a=1:5, b=sample(1:5, 5, replace=TRUE), c=5:1)
df
var <- c("b","c")
df[,var] <- sapply(df[,var],function(x) ifelse(x==5,NA,x))
df

I find the ifelse notation easier to understand here, but most Rers would probably use indexing instead.