R: Merge of rows in same data table, concatenating certain columns

Question

I have my data table in R. I want to merge rows which have an identical customerID, and then concatenate the elements of other merged columns.

I want to

Answer 1

Maybe not the best solution but easy to understand:

df <- data.frame(author=LETTERS[1:5], title=LETTERS[1:5], id=c(1, 2, 1, 2, 3), stringsAsFactors=FALSE)

uniqueIds <- unique(df$id)

mergedDf <- df[1:length(uniqueIds),]

for (i in seq(along=uniqueIds)) {
    mergedDf[i, "id"] <- uniqueIds[i]
    mergedDf[i, "author"] <- paste(df[df$id == uniqueIds[i], "author"], collapse=",")
    mergedDf[i, "title"] <- paste(df[df$id == uniqueIds[i], "title"], collapse=",")
}

mergedDf
#  author title id
#1    A,C   A,C  1
#2    B,D   B,D  2
#3      E     E  3