R-根据行匹配，使用来自另一个数据框的值填充一个数据框

Question

I'm trying to replace values in myDF1 from myDF2, where rows match for column "studyno" but the solutions I have found so far don't seem to be giving me the desired output.

Below are the data.frames:

myDF1 <- structure(list(studyno = c("J1000/9", "J1000/9", "J1000/9", "J1000/9", 
"J1000/9", "J1000/9"), date = structure(c(17123, 17127, 17135, 
17144, 17148, 17155), class = "Date"), pf_mcl = c(NA_integer_, 
NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_
), year = c(2016, 2016, 2016, 2016, 2016, 2016)), .Names = c("studyno", 
"date", "pf_mcl", "year"), row.names = c(NA, 6L), class = "data.frame")

myDF2 <- structure(list(studyno = c("J740/4", "J1000/9", "J895/7", "J931/6", 
"J609/1", "J941/3"), pf_mcl = c(0L, 0L, 0L, 0L, 0L, 0L)), .Names = c("studyno", 
"pf_mcl"), row.names = c(NA, 6L), class = "data.frame")

One solution I tried that seemed to work is shown below, however, I find that whatever values were in myDF1 before have been removed.

myDF1$pf_mcl <- myDF2$pf_mcl[match(myDF1$studyno, myDF2$studyno)]

Answer 1

# Merge myDF1 & myDF2 by the "studyno", keeping all the rows in myDF1
agg_df = merge(myDF1, myDF2, "studyno", all.x=TRUE)
# Populate pf_mcl in the merged dataframe by using pf_mcl in myDF2 if it is available. Otherwise, use pf_mcl from myDF1
# is missing in myDF1
agg_df$pf_mcl = ifelse(is.na(agg_df$pf_mcl.y), agg_df$pf_mcl.x, agg_df$pf_mcl.y)
myDF1 = agg_df[, names(myDF1)]