Using custom function with ddply

Question

For some reason I cannot use a custom function with ddply. It returns exactly the same data frame.

Basically, I want not to count the number of duplicates of id, but actually create a variable that says if it is the first, second, or third instance of that repetition of id. Wrote a function for that , create_guide, which works; but does not work with the id groups.

df<-data.frame(id=c(1,1,2,2,3,4))

create_guide <- function(dt) {

  guide <- rep(0,times=nrow(dt))

  for (i in 1:nrow(dt)) {
    guide[i] <- length(dt[1:i,1])
  }

  a <- cbind(guide,dt)

}

bi <- plyr::ddply(df,.(id),fun=create_guide)

What is happening? Thank you

Your function is using the variable i in the first expression without having defined it previously. What i is it using? — Konrad Rudolph
– Konrad Rudolph, Commented Mar 13, 2018 at 15:57
@KonradRudolph it should be a 0. it's just to store the results from the loop. thanks. does not solve the problem though... — user9486481
– user9486481, Commented Mar 13, 2018 at 16:00

Konrad Rudolph · Accepted Answer · 2018-03-13 16:11:34Z

1

You misspelled the argument name: it’s .fun, not fun. You can also omit it:

bi <- ddply(df, .(id), .fun = create_guide)
# or
bi <- ddply(df, .(id), create_guide)

Furthermore, your function can be drastically simplified, since your loop body is merely a convoluted way of assigning consecutive numbers:

create_guide = function(dt) {
    cbind(guide = seq_len(nrow(dt)), dt)
}

(Incidentally, it took me a substantial amount of time to simplify the function down to this single line because I couldn’t understand what it was doing — that’s how complex the code was.)

answered Mar 13, 2018 at 16:11

Konrad Rudolph

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