xts (version 0.10-0)

[.xts: Extract Subsets of xts Objects

Description

Details on efficient subsetting of xts objects for maximum performance and compatibility.

Usage

# S3 method for xts
[(x, i, j, drop = FALSE, which.i=FALSE, ...)

Arguments

x

xts object

i

the rows to extract. Numeric, timeBased or ISO-8601 style (see details)

j

the columns to extract, numeric or by name

drop

should dimension be dropped, if possible. See NOTE.

which.i

return the ‘i’ values used for subsetting. No subset will be performed.

…

additional arguments (unused)

Value

An extraction of the original xts object. If which.i is TRUE, the corresponding integer ‘i’ values used to subset will be returned.

Details

One of the primary motivations, and key points of differentiation of the time series class xts, is the ability to subset rows by specifying ISO-8601 compatible range strings. This allows for natural range-based time queries without requiring prior knowledge of the underlying time object used in construction.

When a raw character vector is used for the i subset argument, it is processed as if it was ISO-8601 compliant. This means that it is parsed from left to right, according to the following specification:

CCYYMMDD HH:MM:SS.ss+

A full description will be expanded from a left-specified truncated one.

Additionally, one may specify range-based queries by simply supplying two time descriptions seperated by a forward slash:

CCYYMMDD HH:MM:SS.ss+/CCYYMMDD HH:MM:SS.ss

The algorithm to parse the above is .parseISO8601 from the xts package.

ISO-style subsetting, given a range type query, makes use of a custom binary search mechanism that allows for very fast subsetting as no linear search though the index is required. ISO-style character vectors may be longer than length one, allowing for multiple non-contiguous ranges to be selected in one subsetting call.

If a character vector representing time is used in place of numeric values, ISO-style queries, or timeBased objects, the above parsing will be carried out on each element of the i-vector. This overhead can be very costly. If the character approach is used when no ISO range querying is needed, it is recommended to wrap the ‘i’ character vector with the I() function call, to allow for more efficient internal processing. Alternately converting character vectors to POSIXct objects will provide the most performance efficiency.

As xts uses POSIXct time representations of all user-level index classes internally, the fastest timeBased subsetting will always be from POSIXct objects, regardless of the indexClass of the original object. All non-POSIXct time classes are converted to character first to preserve consistent TZ behavior.

References

ISO 8601: Date elements and interchange formats - Information interchange - Representation of dates and time https://www.iso.org

See Also

xts, .parseISO8601,

Examples

# NOT RUN {
x <- xts(1:3, Sys.Date()+1:3)
xx <- cbind(x,x)

# drop=FALSE for xts, differs from zoo and matrix
z <- as.zoo(xx)
z/z[,1]

m <- as.matrix(xx)
m/m[,1]

# this will fail with non-conformable arrays (both retain dim)
tryCatch(
  xx/x[,1], 
  error=function(e) print("need to set drop=TRUE")
)

# correct way
xx/xx[,1,drop=TRUE]

# or less efficiently
xx/drop(xx[,1])
# likewise
xx/coredata(xx)[,1]


x <- xts(1:1000, as.Date("2000-01-01")+1:1000)
y <- xts(1:1000, as.POSIXct(format(as.Date("2000-01-01")+1:1000)))

x.subset <- index(x)[1:20]
x[x.subset] # by original index type
system.time(x[x.subset]) 
x[as.character(x.subset)] # by character string. Beware!
system.time(x[as.character(x.subset)]) # slow!
system.time(x[I(as.character(x.subset))]) # wrapped with I(), faster!

x['200001'] # January 2000
x['1999/2000'] # All of 2000 (note there is no need to use the exact start)
x['1999/200001'] # January 2000 

x['2000/200005'] # 2000-01 to 2000-05
x['2000/2000-04-01'] # through April 01, 2000
y['2000/2000-04-01'] # through April 01, 2000 (using POSIXct series)
# }