Question

I have repeated-measures data.

I need to create a loop that will incrementally count each observation, within a participant, and label it.

I am new to writing loops. My logic was to say, for each item in the list of unique ids, count each row in that, and apply some function to that row.

Could someone point our what I am doing wrong?

data$Ob <- 0

for (i in unique(data$id)) {
  count <- 1
  for (u in data[data$id == i,]) {
      data[data$id ==u,]$Ob <- count
      count <- count + 1
      print(count)
    }
}

Thanks!
Justin

Answer 1

# Generate some dummy data
data <- data.frame(Ob=0, id=sample(4,20,TRUE))

# Go through every id value
for(i in unique(data$id)){
    # Label observations
    data$Ob[data$id == i] = 1:sum(data$id == i)
}

Be aware though that for loops are notoriously slow in R. In this simple case they work fine, but should you have millions and millions of rows in your data frame you’d better do something purely vectorized.