Using one data.frame to update another

Question

Given 2 data frames that are identical in terms of column names/datatypes, where some columns uniquely identify the rows, is there an efficient function/method for one data.

Answer 1

I produced a function that uses the method of indexing (see answer by John Colby above). Hopefully it can be useful for all such needs of updating one data frame with the values from another data frame.

update.df.with.df <- function(original, replacement, key, value) 
{
    ## PURPOSE: Update a data frame with the values in another data frame
    ## ----------------------------------------------------------------------
    ## ARGUMENT:
    ##   original: a data frame to update,
    ##   replacement: a data frame that has the updated values,
    ##   key: a character vector of variable names to form the unique key
    ##   value: a character vector of variable names to form the values that need to be updated
    ## ----------------------------------------------------------------------
    ## RETURN: The updated data frame from the old data frame "original". 
    ## ----------------------------------------------------------------------
    ## AUTHOR: Feiming Chen,  Date:  2 Dec 2015, 15:08

    n1 <- rownames(original) <- apply(original[, key, drop=F], 1, paste, collapse=".")
    n2 <- rownames(replacement) <- apply(replacement[, key, drop=F], 1, paste, collapse=".")

    n3 <- merge(data.frame(n=n1), data.frame(n=n2))[[1]] # make common keys
    n4 <- levels(n3)[n3]                # convert factor to character

    original[n4, value] <- replacement[n4, value] # update values on the common keys
    original
}
if (F) {                                # Unit Test 
    original <- data.frame(x=c(1, 2, 3), y=c(10, 20, 30))
    replacement <- data.frame(x=2, y=25)
    update.df.with.df(original, replacement, key="x", value="y") # data.frame(x=c(1, 2, 3), y=c(10, 25, 30))

    original <- data.frame(x=c(1, 2, 3), w=c("a", "b", "c"), y=c(10, 20, 30))
    replacement <- data.frame(x=2, w="b", y=25)
    update.df.with.df(original, replacement, key=c("x", "w"), value="y") # data.frame(x=c(1, 2, 3), w=c("a", "b", "c"), y=c(10, 25, 30))

    original = data.frame(Name = c("joe","john") , Id = c( 1 , 2) , Value1 = c(1.2,NA), Value2 = c(NA,9.2))
    replacement = data.frame(Name = c("john") , Id = 2 , Value1 = 2.2 , Value2 = 5.9)
    update.df.with.df(original, replacement, key="Id", value=c("Value1", "Value2"))
    ## goal = data.frame( Name = c("joe","john") , Id = c( 1 , 2) , Value1 = c(1.2,2.2), Value2 = c(NA,5.9) )
}