First/last item of an object

Returns the first/last item of a vector or list, or the first/last row of a data.frame or data.table. The main difference to head/tail is that the default for n is 1 rather than 6.

Usage

first(x, n=1L, ...)
last(x, n=1L, ...)

Arguments

x

A vector, list, data.frame or data.table. Otherwise the S3 method of xts::first is deployed.

n

A numeric vector length 1. How many items to select.

...

Not applicable for data.table first/last. Any arguments here are passed through to xts's first/last.

Note

For zero-length vectors, first(x) and last(x) mimic head(x, 1) and tail(x, 1) by returning an empty vector instead of NA. However, unlike head()/tail() and base R subsetting (e.g., x[1]), they do not preserve attributes like names.

Value

If no other arguments are supplied it depends on the type of x. The first/last item of a vector or list. The first/last row of a data.frame or data.table. For other types, or if any argument is supplied in addition to x (such as n, or keep in xts) regardless of x's type, then xts::first/ xts::last is called if xts has been loaded, otherwise utils::head/utils::tail.

See also

NROW, head, tail

Examples

first(1:5) # [1] 1
#> [1] 1
x = data.table(x=1:5, y=6:10)
first(x) # same as head(x, 1)
#>        x     y
#>    <int> <int>
#> 1:     1     6

last(1:5) # [1] 5
#> [1] 5
x = data.table(x=1:5, y=6:10)
last(x) # same as tail(x, 1)
#>        x     y
#>    <int> <int>
#> 1:     5    10