Question

I’m looking for a general solution for updating one large data frame with the contents of a second similar data frame. I have dozens of datasets, each with thousands of rows and upwards of 10,000 columns. An “update” dataset will overlap its corresponding “base” dataset by anywhere from a few percent to perhaps 50 percent, rowwise. The datasets have a “key” column and there will be only one row per each unique key value in any given dataset.

The basic rule is: if a non-NA value exists in the update dataset for a given cell, replace the same cell in the base dataset with that value. (The “same cell” means same value of the “key” column and colname.)

Note the update dataset will likely contain new rows (“inserts”) which I can handle with an rbind.

So given the base data frame “df1”, where column “K” is the unique key column, and “P1” .. “P3” represent the 10,000 columns, whose names will vary from one pair of datasets to the next:

  K P1 P2 P3
1 A  1  1  1
2 B  1  1  1
3 C  1  1  1

…and the update data frame “df2”:

  K P1 P2 P3
1 B  2 NA  2
2 C NA  2  2
3 D  2  2  2

The result I need is as follows, where the 1’s for “B” and “C” were overwritten by the 2’s but not overwritten by the NA’s:

  K P1 P2 P3
1 A  1  1  1
2 B  2  1  2
3 C  1  2  2
4 D  2  2  2

This doesn’t seem to be a merge candidate as merge gives me either duplicate rows (with respect to the “key” column) or duplicate columns (e.g. P1.x, P1.y), which I have to iterate over to collapse somehow.

I have tried pre-allocating a matrix with the dimensions of the final rows/columns, and populating it with the contents of df1, then iterating over the overlapping rows of df2, but I cannot get better than 20 cells per second performance, requiring hours to complete (compared to minutes for the equivalent DATA step UPDATE functionality in SAS).

I’m sure I’m missing something, but can’t find a comparable example.

I see ddply usage that looks close, but not a general solution. The data.table package didn’t seem to help as it’s not obvious to me that this is a join problem, at least not generally over so many columns.

Also a solution that focuses only on the intersecting rows is adequate as I can identify the others and rbind them in.

Here is some code to fabricate the data frames above:

cat("K,P1,P2,P3", "A,1,1,1", "B,1,1,1", "C,1,1,1", file="f1.dat", sep="\n");
cat("K,P1,P2,P3", "B,2,,2", "C,,2,2", "D,2,2,2", file="f2.dat", sep="\n");
df1 <- read.table("f1.dat", sep=",", header=TRUE, stringsAsFactors=FALSE);
df2 <- read.table("f2.dat", sep=",", header=TRUE, stringsAsFactors=FALSE);

Thanks

Answer 1

This loops by column, setting dt1 by reference and (hopefully) should be quick.

dt1 = as.data.table(df1)
dt2 = as.data.table(df2)
if (!identical(names(dt1),names(dt2)))
    stop("Assumed for now. Can relax later if needed.")
w = chmatch(dt2$K, dt1$K)
for (i in 2:ncol(dt2)) {
    nna = !is.na(dt2[[i]])
    set(dt1,w[nna],i,dt2[[i]][nna])
}
dt1 = rbind(dt1,dt2[is.na(w)])
dt1
     K P1 P2 P3
[1,] A  1  1  1
[2,] B  2  1  2
[3,] C  1  2  2
[4,] D  2  2  2