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\author{xts Deveopment Team%
 \footnote{Contact author: Joshua M. Ulrich \email{josh.m.ulrich@gmail.com}}
 \footnote{Thanks to Alberto Giannetti and Michael R. Weylandt for their many contributions.}
}

\title{\bf xts FAQ}

%\Keywords{irregular time series, time index, daily data, weekly data, returns}

%\Abstract{
%  This is a collection of frequently asked questions (FAQ) about the
%  \pkg{xts} package together with their answers.
%}


\begin{document}

\SweaveOpts{concordance=TRUE, engine=R, eps=FALSE}
%\SweaveOpts{engine=R, eps=FALSE}

<<preliminaries, echo=FALSE, results=hide>>=
library("xts")
Sys.setenv(TZ="GMT")
@

\makeatletter
\makeatother
\maketitle

\tableofcontents                                                                     
                                                                     
\q{What is \pkg{xts}?}
%
\pkg{xts} is an \pkg{R} package offering a number of functionalities to work on
time-indexed data. \pkg{xts} extends \pkg{\pkg{zoo}}, another popular package
for time-series analysis.
% should point to the zoo FAQ here (or at some early point)

\q{Why should I use \pkg{xts} rather than \pkg{zoo} or another time-series
package?}
%
The main benefit of \pkg{xts} is its seamless compatibility with other packages
using different time-series classes (\pkg{timeSeries}, \pkg{zoo}, ...). In
addition, \pkg{xts} allows the user to add custom attributes to any object. See
the main \pkg{xts} vignette for more information.

\q{How do I install \pkg{xts}?}
%
\pkg{xts} depends on \pkg{zoo} and suggests some other packages. You should be
able to install \pkg{xts} and all the other required components by simply
calling \code{install.packages('pkg')} from the \pkg{R} prompt.

\q{I have multiple .csv time-series files that I need to load in a single
\pkg{xts} object. What is the most efficient way to import the files?}
%
If the files have the same format, load them with \code{read.zoo} and
then call \code{rbind} to join the series together; finally, call \code{as.xts}
on the result. Using a combination of \code{lapply} and \code{do.call} can
accomplish this with very little code:
<<eval=FALSE>>=
filenames <- c("a.csv", "b.csv", "c.csv")
sample.xts <- as.xts(do.call("rbind", lapply(filenames, read.zoo)))
@

\q{Why is \pkg{xts} implemented as a matrix rather than a data frame?}
%
\pkg{xts} uses a matrix rather than data.frame because:
\begin{enumerate}
  \item \pkg{xts} is a subclass of \pkg{zoo}, and that's how \pkg{zoo} objects
    are structured; and
  \item matrix objects have much better performance than data.frames.
\end{enumerate}

\q{How can I simplify the syntax when referring to \pkg{xts} object column names?}
%
\code{with} allows you to use the colmn names while avoiding the full square
brackets syntax. For example:
<<eval=FALSE>>=
lm(sample.xts[, "Res"] ~ sample.xts[, "ThisVar"] + sample.xts[, "ThatVar"])
@
can be converted to
<<eval=FALSE>>=
with(sample.xts, lm(Res ~ ThisVar + ThatVar))
@

\q{How can I replace the zeros in an \pkg{xts} object with the last non-zero value
in the series?}
%
Convert the zeros to \code{NA} and then use \code{na.locf}:
<<>>=
sample.xts <- xts(c(1:3, 0, 0, 0), as.POSIXct("1970-01-01")+0:5)
sample.xts[sample.xts==0] <- NA
cbind(orig=sample.xts, locf=na.locf(sample.xts))
@

\q{How do I create an \pkg{xts} index with millisecond precision?}
%
Milliseconds in \pkg{xts} indexes are stored as decimal values. This example
builds an index spaced by 100 milliseconds, starting at the current system time:
<<>>=
data(sample_matrix)
sample.xts <- xts(1:10, seq(as.POSIXct("1970-01-01"), by=0.1, length=10))
@

\q{I have a millisecond-resolution index, but the milliseconds aren't
displayed. What went wrong?}
%
Set the \code{digits.secs} option to some sub-second precision. Continuing from
the previous example, if you are interested in milliseconds:
<<>>=
options(digits.secs=3)
head(sample.xts)
@

\q{I set \code{digits.sec=3}, but \pkg{R} doesn't show the values correctly.}
%
Sub-second values are stored with approximately microsecond precision. Setting
the precision to only 3 decimal hides the full index value in microseconds and
might be tricky to interpret depending how the machine rounds the millisecond
(3rd) digit. Set the \code{digits.secs} option to a value higher than 3 or
convert the date-time to numeric and use \code{print}'s \code{digits} argument,
or \code{sprintf} to display the full value. For example:
<<>>=
dt <- as.POSIXct("2012-03-20 09:02:50.001")
print(as.numeric(dt), digits=20)
sprintf("%20.10f", dt)
@

\q{I am using \code{apply} to run a custom function on my \pkg{xts} object.
Why does the returned matrix have different dimensions than the original one?}
%
When working on rows, \code{apply} returns a transposed version of the
original matrix. Simply call \code{t} on the returned matrix to restore the
original dimensions:
<<eval=FALSE>>=
sample.xts.2 <- xts(t(apply(sample.xts, 1, myfun)), index(sample.xts))
@

\q{I have an \pkg{xts} object with varying numbers of observations per day (e.g.,
one day might contain 10 observations, while another day contains 20 observations).
How can I apply a function to all observations for each day?}
%
You can use \code{apply.daily}, or \code{period.apply} more generally:
<<>>=
sample.xts <- xts(1:50, seq(as.POSIXct("1970-01-01"),
  as.POSIXct("1970-01-03")-1, length=50))
apply.daily(sample.xts, colMeans)
period.apply(sample.xts, endpoints(sample.xts, "days"), colMeans)
period.apply(sample.xts, endpoints(sample.xts, "hours", 6), colMeans)
@

\q{How can I process daily data for a specific time subset?}
%
First use time-of-day subsetting to extract the time range you want to work on (note
the leading \code{"T"} and leading zeros are required for each time in the range:
\code{"T06:00"}), then use \code{apply.daily} to apply your function to the subset:
<<eval=FALSE>>=
apply.daily(sample.xts['T06:00/T17:00',], colMeans)
@

\q{How can I analyze my irregular data in regular blocks, adding observations
for each regular block if one doesn't exist in the origianl time-series object?}
%
Use \code{align.time} to round-up the indexes to the periods you are interested
in, then call \code{period.apply} to apply your function. Finally, merge the
result with an empty xts object that contains all the regular index values
you want:
<<>>=
sample.xts <- xts(1:6, as.POSIXct(c("2009-09-22 07:43:30",
  "2009-10-01 03:50:30", "2009-10-01 08:45:00", "2009-10-01 09:48:15",
  "2009-11-11 10:30:30", "2009-11-11 11:12:45")))
# align index into regular (e.g. 3-hour) blocks
aligned.xts <- align.time(sample.xts, n=60*60*3)
# apply your function to each block
count <- period.apply(aligned.xts, endpoints(aligned.xts, "hours", 3), length)
# create an empty xts object with the desired regular index
empty.xts <- xts(, seq(start(aligned.xts), end(aligned.xts), by="3 hours"))
# merge the counts with the empty object
head(out1 <- merge(empty.xts, count))
# or fill with zeros
head(out2 <- merge(empty.xts, count, fill=0))
@

\q{Why do I get a \pkg{zoo} object when I call \code{transform} on my
\pkg{xts} object?}
%
There's no \pkg{xts} method for \code{transform}, so the \pkg{zoo} method is
dispatched. The \pkg{zoo} method explicitly creates a new \pkg{zoo} object. To
convert the transformed object back to an \pkg{xts} object wrap the
\code{transform} call in \code{as.xts}:
<<eval=FALSE>>=
sample.xts <- as.xts(transform(sample.xts, ABC=1))
@

You might also have to reset the index timezone:
<<eval=FALSE>>=
tzone(sample.xts) <- Sys.getenv("TZ")
@

\q{Why can't I use the \code{\&} operator in \pkg{xts} objects when querying
dates?}
%
\code{"2011-09-21"} is not a logical vector and cannot be coerced to a logical
vector. See \code{?"\&"} for details.

\pkg{xts}' ISO-8601 style subsetting is nice, but there's nothing we can do to
change the behavior of \code{.Primitive("\&")}. You can do something like this
though:
<<eval=FALSE>>=
sample.xts[sample.xts$Symbol == "AAPL" & index(sample.xts) == as.POSIXct("2011-09-21"),]
@
or:
<<eval=FALSE>>=
sample.xts[sample.xts$Symbol == "AAPL"]['2011-09-21']
@

\q{How do I subset an \pkg{xts} object to only include weekdays (excluding
Saturday and Sundays)?}
%
Use \code{.indexwday} to only include Mon-Fri days:
<<>>=
data(sample_matrix)
sample.xts <- as.xts(sample_matrix)
wday.xts <- sample.xts[.indexwday(sample.xts) %in% 1:5]
head(wday.xts)
@

\q{I need to quickly convert a data.frame that contains the time-stamps in one
of the columns. Using \code{as.xts(Data)} returns an error. How do I build my
\pkg{xts} object?}
%
The \code{as.xts} function assumes the date-time index is contained in the
\code{rownames} of the object to be converted. If this is not the case, you
need to use the \code{xts} constructor, which requires two arguments: a
vector or a matrix carrying data and a vector of type \code{Date},
\code{POSIXct}, \code{chron}, \ldots, supplying the time index information.
If you are certain the time-stamps are in a specific column, you can use:
<<>>=
Data <- data.frame(timestamp=as.Date("1970-01-01"), obs=21)
sample.xts <- xts(Data[,-1], order.by=Data[,1])
@
If you aren't certain, you need to explicitly reference the column name that
contains the time-stamps:
<<>>=
Data <- data.frame(obs=21, timestamp=as.Date("1970-01-01"))
sample.xts <- xts(Data[,!grepl("timestamp",colnames(Data))],
  order.by=Data$timestamp)
@

\q{I have two time-series with different frequency. I want to combine the data
into a single \pkg{xts} object, but the times are not exactly aligned. I want
to have one row in the result for each ten minute period, with the time index
showing the beginning of the time period.}
%
\code{align.time} creates evenly spaced time-series from a set of indexes,
\code{merge} ensure two time-series are combined in a single \pkg{xts} object
with all original columns and indexes preserved. The new object has one entry
for each timestamp from both series and missing values are replaced with
\code{NA}.
<<eval=FALSE>>=
x1 <- align.time(xts(Data1$obs, Data1$timestamp), n=600)
x2 <- align.time(xts(Data2$obs, Data2$timestamp), n=600)
merge(x1, x2)
@

\q{Why do I get a warning when running the code below?}
<<>>=
data(sample_matrix)
sample.xts <- as.xts(sample_matrix)
sample.xts["2007-01"]$Close <- sample.xts["2007-01"]$Close + 1
#Warning message:
#In NextMethod(.Generic) :
#  number of items to replace is not a multiple of replacement length
@
%
This code creates two calls to the subset-replacement function
\code{xts:::`[<-.xts`}.  The first call replaces the value of \code{Close}
in a temporary copy of the first row of the object on the left-hand-side of
the assignment, which works fine. The second call tries to replace
the first \emph{element} of the object on the left-hand-side of the
assignment with the modified temporary copy of the first row.  This is
the problem.

For the command to work, there needs to be a comma in the first
subset call on the left-hand-side:
<<eval=FALSE>>=
sample.xts["2007-01",]$Close <- sample.xts["2007-01"]$Close + 1
@

This isn't encouraged, because the code isn't clear. Simply remember to
subset by column first, then row, if you insist on making two calls to
the subset-replacement function. A cleaner and faster solution is below.
It's only one function call and it avoids the \code{\$} function (which
is marginally slower on xts objects).
<<eval=FALSE>>=
sample.xts["2007-01","Close"] <- sample.xts["2007-01","Close"] + 1
@

%%% What is the fastest way to subset an xts object?

\end{document}
