first: Return First or Last n Elements of A Data Object

In joshuaulrich/xts: eXtensible Time Series

Return First or Last n Elements of A Data Object

Description

Generic functions to return the first or last elements or rows of a vector or two-dimensional data object.

Usage

first(x, ...)

## Default S3 method:
first(x, n = 1, keep = FALSE, ...)

## S3 method for class 'xts'
first(x, n = 1, keep = FALSE, ...)

last(x, ...)

## Default S3 method:
last(x, n = 1, keep = FALSE, ...)

## S3 method for class 'xts'
last(x, n = 1, keep = FALSE, ...)

Arguments

x	An object.
...	Arguments passed to other methods.
n	Number of observations to return.
keep	Should removed values be kept as an attribute on the result?

Details

A more advanced subsetting is available for zoo objects with indexes inheriting from POSIXt or Date classes.

Quickly and easily extract the first or last n observations of an object. When n is a number, these functions are similar to head() and tail(), but only return the first or last observation by default.

n can be a character string if x is an xts object or coerceable to xts. It must be of the form ‘n period’, where 'n' is a numeric value (1 if not provided) describing the number of periods to return. Valid periods are: secs, seconds, mins, minutes, hours, days, weeks, months, quarters, and years.

The 'period' portion can be any frequency greater than or equal to the frequency of the object's time index. For example, first(x, "2 months") will return the first 2 months of data even if x is hourly frequency. Attempts to set 'period' to a frequency less than the object's frequency will throw an error.

n may be positive or negative, whether it's a number or character string. When n is positive, the functions return the obvious result. For example, first(x, "1 month") returns the first month's data. When n is negative, all data except first month's is returned.

Requesting more data than is in x will throw a warning and simply return x.

Value

A subset of elements/rows of the original data.

Author(s)

Jeffrey A. Ryan

Examples

first(1:100)
last(1:100)

data(LakeHuron)
first(LakeHuron,10)
last(LakeHuron)

x <- xts(1:100, Sys.Date()+1:100)
first(x, 10)
first(x, '1 day')
first(x, '4 days')
first(x, 'month')
last(x, '2 months')
last(x, '6 weeks')

joshuaulrich/xts documentation built on Nov. 17, 2024, 6:37 a.m.