combine two zoo time series

Question

I have 2 sucesive ZOO time series (the date of one begins after the other finishes), they have the following form (but much longer and not only NA values):

a:

1979-01-01 1979-01-02 1979-01-03 1979-01-04 1979-01-05 1979-01-06 1979-01-07 1979-01-08 1979-01-09 
    NA         NA         NA         NA         NA         NA         NA         NA         NA 

b:

1988-08-15 1988-08-16 1988-08-17 1988-08-18 1988-08-19 1988-08-20 1988-08-21 1988-08-22 1988-08-23 1988-08-24 1988-08-25 
    NA         NA         NA         NA         NA         NA         NA         NA         NA         NA         NA 

all I want to do is combine them in one time serie as a ZOO object, it seems to be a basic task but I am doing something wrong. I use the function "merge":

combined <- merge(a, b)

but the result is something in the form:

             a   b
1980-03-10  NA   NA
1980-03-11  NA   NA
1980-03-12  NA   NA
1980-03-13  NA   NA
1980-03-14  NA   NA
1980-03-15  NA   NA
1980-03-16  NA   NA
.
.

which is not a time series, and the lengths dont fit:

> length(a)
[1] 10957
> length(b)
[1] 2557
> length(combined)
[1] 27028

how can I just combine them into one time series with the form of the original ones?

Answer 1

Assuming the series shown reproducibly in the Note at the end, the result of merging the two series has 20 times and 2 columns (one for each series). The individual series have lengths 9 and 11 elements and the merged series is a zoo object with 9 + 11 = 20 rows (since there are no intersecting times) and 2 columns (one for each input) and length 40 (= 20 * 2). Note that the length of a multivariate series is the number of elements in it, not the number of time points.

length(z1)
## [1] 9
length(z2)
## [1] 11

m <- merge(z1, z2)

class(m)
## [1] "zoo"

dim(m)
## [1] 20  2

nrow(m)
## [1] 20

length(index(m))
## [1] 20

length(m)
## [1] 40

If what you wanted is to string them out one after another then use c :

length(c(z1, z2))
## [1] 20

The above are consistent with how merge , c and length work in base R.

Note:

library(zoo)

z1 <- zoo(rep(NA, 9), as.Date(c("1979-01-01", "1979-01-02", "1979-01-03", 
"1979-01-04", "1979-01-05", "1979-01-06", "1979-01-07", "1979-01-08", 
 "1979-01-09")))

z2 <- zoo(rep(NA, 11), as.Date(c("1988-08-15", "1988-08-16", "1988-08-17",
"1988-08-18", "1988-08-19", "1988-08-20", "1988-08-21", "1988-08-22", 
"1988-08-23", "1988-08-24", "1988-08-25")))