Time Index Objects
The function ti is used to create time index objects, which are
useful for date calculations and as indexes for tis (time
indexed series).
as.ti and asTi coerce an object to a time index, the
difference being that as.ti calls the constructor ti,
while asTi simply forces the class of its argument to be "ti"
without any checking as to whether or not it makes sense to do so.
is.ti tests whether an object is a time index.
couldBeTi tests whether or not x is numeric and has all
elements within the range expected for a ti time index with the
given tif. If tif is NULL (the default), the test
is whether or not x could be a ti of any
frequency. If so, it can be safely coerced to class ti by
as.ti.
ti(x, ...) ## S3 method for class 'Date' ti(x, ...) ## Default S3 method: ti(x, tif = NULL, freq = NULL, ...) ## S3 method for class 'jul' ti(x, tif = NULL, freq = NULL, hour = 0, minute = 0, second = 0, ...) ## S3 method for class 'ssDate' ti(x, ...) ## S3 method for class 'ti' ti(x, tif = NULL, freq = NULL, ...) ## S3 method for class 'tis' ti(x, ...) ## S3 method for class 'yearmon' ti(x, ...) ## S3 method for class 'yearqtr' ti(x, ...) as.ti(x, ...) asTi(x) is.ti(x) couldBeTi(x, tif = NULL)
x |
object to be tested ( |
hour |
used if and only if |
minute |
used if and only if |
second |
used if and only if |
... |
other args to be passed to the method called by the generic function. |
tif |
a ti Frequency, given as either a numerical code or a string.
|
freq |
some |
A ti has a tif (ti Frequency) and a period. The period
represents the number of periods elapsed since the base period for that
frequency. Adding or subtracting an integer to a ti gives
another ti. Provided their corresponding element have matching
tifs, the comparison operators <, >, <=, >=, == all
work, and subtracting one ti from another gives the number of
periods between them. See the examples section below.
The ti class implements methods for a number of generic
functions, including "[", as.Date, as.POSIXct,
as.POSIXlt, c, cycle, edit, format,
frequency, jul, max, min, print,
rep, seq, tif, tifName, time,
ymd.
ti is a generic function with specialized methods to handle
jul, Date, ti. tis, yearmon and
yearqtr objects.
The default method (ti.default) deals with character x by
calling as.Date on it. Otherwise, it proceeds as follows:
If x is numeric, a check is made to see if x could be
a ti object that has somehow lost it's class attribute. Failing that,
isYmd is used to see if it could be yyyymmdd date, then
isTime is called to see if x could be a decimal time (a
number between 1799 and 2200). If x is of length 2, an attempt
to interpret it as a c(year, period) pair is made. Finally,
if all else fails, as.Date(x) is called to attempt to create a
Date object that can then be used to construct a ti.
is.ti and couldBeTi return TRUE or FALSE.
as.ti returns a ti object.
asTi returns its argument with the class attribute set to "ti".
ti constructs a ti object like x, except for two
special cases:
1. If x is a tis series, the return value is a vector
time index with elements corresponding to the observation periods of
x.
2. If x is a numeric object of length 2 interpretable as
c(year, period), the return value is a single ti.
The as.Date(x) call is not wrapped in a try-block, so it may be at
the top of the stack when ti fails.
The return value from asTi is not guaranteed to be a valid
ti object. For example, asTi("a") will not throw an
error, and it will return the string "a" with a class attribute "ti",
but that's not a valid time index.
z <- ti(19971231, "monthly") ## monthly ti for Dec 97 is.ti(z) ## TRUE is.ti(unclass(z)) ## FALSE couldBeTi(unclass(z)) ## TRUE ymd(z + 4) ## 19980430 z - ti(c(1997,6), freq = 12) ## monthly ti for June 1997 ti(z, tif = "wmonday") ## week ending Monday June 30, 1997
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