is_constant: Is a vector constant?

In hutilscpp: Miscellaneous Functions in C++

Is a vector constant?

Description

Efficiently decide whether an atomic vector is constant; that is, contains only one value.

Equivalent to

data.table::uniqueN(x) == 1L

or

forecast::is.constant(x)

Usage

is_constant(x, nThread = getOption("hutilscpp.nThread", 1L))

isntConstant(x)

Arguments

x	An atomic vector. Only logical, integer, double, and character vectors are supported. Others may work but have not been tested.
nThread	integer(1) Number of threads to use in is_constant.

Value

Whether or not the vector x is constant:

is_constant

TRUE or FALSE. Missing values are considered to be the same as each other, so a vector entirely composed of missing values is considered constant. Note that is_constant(c(NA_real_, NaN)) is TRUE.

isntConstant

If constant, 0L; otherwise, the first integer position at which x has a different value to the first.

This has the virtue of !isntConstant(x) == is_constant(x).

Multithreaded is_constant(x, nThread) should only be used if x is expected to be true. It will be faster when x is constant but much slower otherwise.

Empty vectors are constant, as are length-one vectors.

Examples

library(hutilscpp)
library(data.table)
N <- 1e9L
N <- 1e6  # to avoid long-running examples on CRAN

## Good-cases
nonconst <- c(integer(1e5), 13L, integer(N))
bench_system_time(uniqueN(nonconst) == 1L)
#> process    real
#> 15.734s  2.893s
bench_system_time(is_constant(nonconst))
#> process    real
#>   0.000   0.000
bench_system_time(isntConstant(nonconst))
#> process    real
#>   0.000   0.000

## Worst-cases
consti <- rep(13L, N)
bench_system_time(uniqueN(consti) == 1L)
#> process    real
#>  5.734s  1.202s
bench_system_time(is_constant(consti))
#>   process      real
#> 437.500ms 437.398ms
bench_system_time(isntConstant(consti))
#>   process      real
#> 437.500ms 434.109ms

nonconsti <- c(consti, -1L)
bench_system_time(uniqueN(nonconsti) == 1L)
#> process    real
#> 17.812s  3.348s
bench_system_time(is_constant(nonconsti))
#>   process      real
#> 437.500ms 431.104ms
bench_system_time(isntConstant(consti))
#>   process      real
#> 484.375ms 487.588ms

constc <- rep("a", N)
bench_system_time(uniqueN(constc) == 1L)
#> process    real
#> 11.141s  3.580s
bench_system_time(is_constant(constc))
#> process    real
#>  4.109s  4.098s

nonconstc <- c(constc, "x")
bench_system_time(uniqueN(nonconstc) == 1L)
#> process    real
#> 22.656s  5.629s
bench_system_time(is_constant(nonconstc))
#> process    real
#>  5.906s  5.907s

hutilscpp documentation built on Oct. 11, 2023, 9:06 a.m.