Merge data frames whilst summing common columns in R

Question

My problem is very similar to the one posted here.

The difference is that they knew the columns that would be conflicting whereas I need a generic method that wont k

Answer 1

A data.table solution:

dt1 <- data.table(read.table(header=T, text="Date             Time    ColumnA    ColumnB
01/01/2013      08:00      10         30
01/01/2013      08:30      15         25
01/01/2013      09:00      20         20
02/01/2013      08:00      25         15
02/01/2013      08:30      30         10
02/01/2013      09:00      35         5"))

dt2 <- data.table(read.table(header=T, text="Date           ColumnA    ColumnB    ColumnC
01/01/2013      100        300         1
02/01/2013      200        400         2"))

setkey(dt1, "Date")
setkey(dt2, "Date")
# Note: The ColumnC assignment has to be come before the summing operations
# Else it gives out error (see below)
dt1[dt2, `:=`(ColumnC = i.ColumnC, ColumnA = ColumnA + i.ColumnA, 
                        ColumnB = ColumnB + i.ColumnB)]

#          Date  Time ColumnA ColumnB ColumnC
# 1: 01/01/2013 08:00     110     330       1
# 2: 01/01/2013 08:30     115     325       1
# 3: 01/01/2013 09:00     120     320       1
# 4: 02/01/2013 08:00     225     415       2
# 5: 02/01/2013 08:30     230     410       2
# 6: 02/01/2013 09:00     235     405       2

I'm not sure why placing ColumnC assignment on the right end throws this error. Perhaps MatthewDowle could explain the cause for this error.

dt1[dt2, `:=`(ColumnA = ColumnA + i.ColumnA, ColumnB = ColumnB + i.ColumnB, 
                        ColumnC = i.ColumnC)]

Error in `[.data.table`(dt1, dt2, `:=`(ColumnA = ColumnA + i.ColumnA,  : 
  Value of SET_STRING_ELT() must be a 'CHARSXP' not a 'NULL'

---

Update from v1.8.9 :

o Mixing adding new with updating existing columns into one :=() by group; i.e.,
DT[,:=(existingCol=...,newCol=...), by=...]
now works without error or segfault, #2778 and #2528. Many thanks to Arun for reporting both with reproducible examples. Tests added.