Access zoo or xts index

Question

I am using zoo objects, buy my question also applies to xts objects. It looks to me like it is a one column vector with an index. In my case the index is the vector of dates and the one column vector my data. All is good except that I would like to access the dates (from the index).

For example I have the following result:

ObjZoo <- structure(c(10, 20), .Dim = c(2L, 1L), index = c(14788, 14789),
                    class = "zoo", .Dimnames = list(NULL, "Data"))
unclass(ObjZoo)
#      Data
# [1,]   10
# [2,]   20
# attr(,"index")
# [1] 14788 14789

I want to get 14789 in a variable or a vector, but I'm not sure how to access it.

Solution 1

From the help for ?zoo, there are two convenience methods to access the data in zoo objects:

• coredata() returns the data in the zoo object
• index() returns the index

For example:

x.Date <- as.Date("2003-02-01") + c(1, 3, 7, 9, 14) - 1
x <- zoo(rnorm(5), x.Date)
index(x)
[1] "2003-02-01" "2003-02-03" "2003-02-07" "2003-02-09" "2003-02-14"
coredata(x)
[1] -1.2487943  0.8911630  1.2713133 -0.1024638  0.2989194

Solution 2

In general when you see attr, this means that this data is an attribute of an object.

attributes function can be used to dump all attributes as a list, so you can access certain element with $:

attributes(ObjZoo)$index

attr gives you direct access to the attribute by its name:

attr(ObjZoo,"index")

In fact this is what index does:

> zoo:::index.zoo
function (x, ...) 
{
    attr(x, "index")
}
<environment: namespace:zoo>

Comments

[Matt Bannert]

Welcome! Here's a related thread emphasizing the additional relevance of mbq's answer even if Andrie's fits perfectly.

[Dirk Eddelbuettel]

I spend hours looking for that Really? Maybe you should have started with the excellent zoo vignettes.

[Andrie]

@DirkEddelbuettel, to be fair to the OP, thinking back to my early days with R, I also struggled to make sense of the help files. ts objects were a complete mystery (they still are, to some extent). Yes it's true that I found the answer to this question within 2 minutes of reading ?zoo, but still, I have sympathy with the struggles of the newby.