Merge arbitrary number of xts objects of different lengths by index

Question

I'd like to merge 3 xts objects of different lengths into one xts with 3 columns and indexed by the first xts object's dates.

a_xts <- xts(4:10, Sys.Date()+4:10)
b_xts <- xts(c(7:12), Sys.Date()+4:9)
c_xts <- xts(15:18, Sys.Date()+4:7)

I'd like to preserve a_xts index dates. The merged xts should look like:

           a_xts b_xts c_xts
2021-03-05     4     7    15
2021-03-06     5     8    16
2021-03-07     6     9    17
2021-03-08     7    10    18
2021-03-09     8    11    NA
2021-03-10     9    12    NA
2021-03-11    10    NA    NA

My solution is to merge each one by one making a new xts object for each addition:

ab_xts <- merge(a_xts,b_xts, all = c(TRUE,FALSE))
abc_xts <- merge(ab_xts, c_xts, all = c(TRUE,FALSE))

Is there a better way to do this without converting to .zoo and back to .xts objects?

Edit

when merging all xts objects, I'd like to keep only the rows that match the index dates for a_xts without adding any additional index dates. For example:

d_xts <- xts(c(7:18), Sys.Date()+4:15

d_xts has greater length than a_xts, while b_xts and c_xts are less. Continuing my solution by adding xts objects one by one gets me there but seems inefficient.

> abcd_xts <- merge(abc_xts, d_xts, all = c(TRUE,FALSE))
> abcd_xts
           a_xts b_xts c_xts d_xts
2021-03-05     4     7    15     7
2021-03-06     5     8    16     8
2021-03-07     6     9    17     9
2021-03-08     7    10    18    10
2021-03-09     8    11    NA    11
2021-03-10     9    12    NA    12
2021-03-11    10    NA    NA    13

Answer 1

We can use the following (although for the specific example in the question the [...] is not needed).

merge(a_xts, b_xts, c_xts)[time(a_xts), ]

giving:

           a_xts b_xts c_xts
2021-03-05     4     7    15
2021-03-06     5     8    16
2021-03-07     6     9    17
2021-03-08     7    10    18
2021-03-09     8    11    NA
2021-03-10     9    12    NA
2021-03-11    10    NA    NA

Answer 2

We can place it in a list and use merge with Reduce

out <- Reduce(merge, mget(ls(pattern = '_xts$')))
colnames(out) <- paste0(letters[1:3], "_xts")

Answer 3

I've discovered that merge() can left outer join on multiple xts objects with argument all = c(TRUE,FALSE). This matches only the rows from the first xts passed to merge().

> merge(a_xts,b_xts,c_xts,d_xts, all = c(TRUE,FALSE))

                   a_xts b_xts c_xts d_xts
        2021-03-05     4     7    15     7
        2021-03-06     5     8    16     8
        2021-03-07     6     9    17     9
        2021-03-08     7    10    18    10
        2021-03-09     8    11    NA    11
        2021-03-10     9    12    NA    12
        2021-03-11    10    NA    NA    13