How can R loop over data frames?

Question

Suppose there are many data frames that need the same operation performed on them. For example:

prefix <- c("Mrs.","Mrs.","Mr","Dr.","Mrs.","Mr.","Mrs.","Ms","Ms","Mr")
measure <- rnorm(10)
df1 <- data.frame(prefix,measure)
df1$gender[df1$prefix=="Mrs."] <- "F"

Would create an indicator variable called gender when the value in the adjacent row was "Mrs.". A general way to loop over string variables in R was adapted from here with the function as.name() added to remove the quotes from "i":

dflist <- c("df1","df2","df3","df4","df5")

for (i in dflist) { 
  as.name(i)$gender[as.name(i)$prefix=="Ms."] <- "F"
  }

Unfortunately this doesn't work. Any suggestions?

Answer 1

The single instance example would not really create an indicator in the usual sense since the non-"F" values would be <NA> and those would not work well within R functions. Both arithmetic operations and logical operations will return . Try this instead:

  df1$gender <- ifelse(prefix %in% c("Mrs.", "Ms") , "probably F",
                ifelse( prefix=="Dr.", "possibly F",  # as is my wife.
                                       "probably not F"))

Then follow @HongDoi's advice to use lists. And do not forget to a) return a full dataframe-object , and b) assign the result to an object name (both of which were illustrated but often forgotten by R-newbs.)

Answer 2

Put all your data frames into a list, and then loop/lapply over them. It'll be much easier on you in the long run.

dfList <- list(df1=df1, df2=df2, ....)

dfList <- lapply(dfList, function(df) {
    df$gender[df$prefix == "Mrs."] <- "F"
    df
})

dfList$df1