hutilscpp (version 0.1.0)

which_first: 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)

Arguments

expr

An expression, such as x == 2.

verbose

(logical, default: FALSE) If TRUE a message is emitted if expr could not be handled in the advertised way.

Value

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

Details

If the expr is of the form LHS <operator> RHS and LHS is a single symbol, operator is one of ==, !=, >, >=, <, <=, or %in%. and RHS is a single numeric value, 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.

Examples

# NOT RUN {
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

# }