How can I combine rows within the same data frame in R (based on duplicate values under a specific column)?

Question

Sample of 2 (made-up) example rows in df:

userid   facultyid  courseid schoolid
167       265        NA       1678  
167       71111      301      NA
         


        

Answer 1

# initialize a vector that will contain row numbers which should be erased
rows.to.erase <- c()

# loop over the rows, starting from top
for(i in 1:(nrow(dat)-1)) {
  if(dat$userid[i] == dat$userid[i+1]) {
    # loop over columns to recuperate data when a NA is present
    for(j in 2:4) {
      if(is.na(dat[i,j]))
        dat[i,j] <- dat[i+1,j]
    }
    rows.to.erase <- append(rows.to.erase, i+1)
  }
}

dat.clean <- dat[-rows.to.erase,]
dat.clean
#   userid facultyid courseid schoolid
# 1    167       265      301     1678