how to use merge() to update a table in R

Question

I\'m trying to figure out how to use merge() to update a database.

Here is an example. Take for example the data frame foo

Answer 1

I think the most simple way is to "mark" the value which need to be update prior to the merge.

bar$update <- TRUE
foo <- merge(foo, bar, by='index', all=T, suffixes=c("",".update"))
foo[!is.na(foo$update),]$value <- foo[!is.na(foo$update),]$value.update
foo$value.update <- NULL
foo$update <- NULL

It would be faster using 'data.table'

library(data.table)
foo <- as.data.table(foo)
bar <- as.data.table(bar)
bar[, update:=TRUE]
foo <- merge(foo, bar, by='index', all=T, suffixes=c("",".update"))
foo[!is.na(update),value:=value.update]
foo[, c("value.update","update"):=NULL]
foo

   index value
1:     a   100
2:     b   101
3:     c   200
4:     d   201