which_first: Where does a logical expression first return 'TRUE'?

In hutilscpp: Miscellaneous Functions in C++

Where does a logical expression first return TRUE?

Description

A faster and safer version of which.max applied to simple-to-parse logical expressions.

Usage

which_first(
  expr,
  verbose = FALSE,
  reverse = FALSE,
  sexpr,
  eval_parent_n = 1L,
  suppressWarning = getOption("hutilscpp_suppressWarning", FALSE),
  use.which.max = FALSE
)

which_last(
  expr,
  verbose = FALSE,
  reverse = FALSE,
  suppressWarning = getOption("hutilscpp_suppressWarning", FALSE)
)

Arguments

expr	An expression, such as x == 2.
verbose	logical(1), default: FALSE If TRUE a message is emitted if expr could not be handled in the advertised way.
reverse	logical(1), default: FALSE Scan expr in reverse.
sexpr	Equivalent to substitute(expr). For internal use.
eval_parent_n	Passed to eval.parent, the environment in which expr is evaluated.
suppressWarning	Either a FALSE or TRUE, whether or not warnings should be suppressed. Also supports a string input which suppresses a warning if it matches as a regular expression.
use.which.max	If TRUE, which.max is dispatched immediately, even if expr would be amenable to separation. Useful when evaluating many small expr's when these are known in advance.

Details

If the expr is of the form LHS <operator> RHS and LHS is a single symbol, operator is one of ==, !=, >, >=, <, <=, %in%, or %between%, and RHS is numeric, then expr is not evaluated directly; instead, each element of LHS is compared individually.

If expr is not of the above form, then expr is evaluated and passed to which.max.

Using this function can be significantly faster than the alternatives when the computation of expr would be expensive, though the difference is only likely to be clear when length(x) is much larger than 10 million. But even for smaller vectors, it has the benefit of returning 0L if none of the values in expr are TRUE, unlike which.max.

Compared to Position for an appropriate choice of f the speed of which_first is not much faster when the expression is TRUE for some position. However, which_first is faster when all elements of expr are FALSE. Thus which_first has a smaller worst-case time than the alternatives for most x.

Missing values on the RHS are handled specially. which_first(x %between% c(NA, 1)) for example is equivalent to which_first(x <= 1), as in data.table::between.

Value

The same as which.max(expr) or which(expr)[1] but returns 0L when expr has no TRUE values.

Examples

N <- 1e5
# N <- 1e8  ## too slow for CRAN

# Two examples, from slowest to fastest,
# run with N = 1e8 elements

                                       # seconds
x <- rep_len(runif(1e4, 0, 6), N)
bench_system_time(x > 5)
bench_system_time(which(x > 5))        # 0.8
bench_system_time(which.max(x > 5))    # 0.3
bench_system_time(which_first(x > 5))  # 0.000

## Worst case: have to check all N elements
x <- double(N)
bench_system_time(x > 0)
bench_system_time(which(x > 0))        # 1.0
bench_system_time(which.max(x > 0))    # 0.4  but returns 1, not 0
bench_system_time(which_first(x > 0))  # 0.1

x <- as.character(x)
# bench_system_time(which(x == 5))     # 2.2
bench_system_time(which.max(x == 5))   # 1.6
bench_system_time(which_first(x == 5)) # 1.3

hutilscpp documentation built on April 4, 2025, 12:42 a.m.