Split Around First Predicate Match — split_around_by

Splits at the first point where a predicate function becomes TRUE while scanning the sequence.

Usage

split_around_by_predicate(t, predicate, monoid_name, accumulator = NULL)

Arguments

t: A flexseq (or subclass).
predicate: Function applied to accumulated monoid values.
monoid_name: Name of the monoid used for scanning.
accumulator: Optional starting accumulator value.

Value

A list with fields:

left: elements before the split point.
value: the matched element at the split point.
right: elements after the split point.

Details

This function generally requires the sequence be annotated with a measure_monoid(); see the examples and measure_monoid() for more information.

value is the matched leaf entry for the input structure:

flexseq: stored user element.
ordered_sequence: list(value, key).
priority_queue: list(value, priority).
interval_index: list(value, start, end).

left and right preserve subclass when the input is a subclass of flexseq.

Examples

x <- flexseq("a", "b", "c", "d")

# Each element e has measure 1; the accumulated measure for
# a set of elements is computed by the associative function `+`
# along with the identity value 0.
size_monoid <- measure_monoid(`+`, 0L, function(e) 1L)
x2 <- add_monoids(x, list(size = size_monoid))

# the first time the measure stored in the size monoid
# accumulates to greater than or equal to 3 is the 3rd
# element in sequence.
split_around_by_predicate(x2, function(v) v >= 3L, "size")
#> $left
#> Unnamed flexseq with 2 elements.
#> 
#> Elements:
#> 
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"
#> 
#> 
#> $value
#> [1] "c"
#> 
#> $right
#> Unnamed flexseq with 1 element.
#> 
#> Elements:
#> 
#> [[1]]
#> [1] "d"
#> 
#> 

# Split at the first element
split_around_by_predicate(x2, function(v) v >= 1L, "size")
#> $left
#> Unnamed flexseq with 0 elements.
#> 
#> $value
#> [1] "a"
#> 
#> $right
#> Unnamed flexseq with 3 elements.
#> 
#> Elements:
#> 
#> [[1]]
#> [1] "b"
#> 
#> [[2]]
#> [1] "c"
#> 
#> [[3]]
#> [1] "d"
#> 
#> 

# Split at the last element
split_around_by_predicate(x2, function(v) v >= 4L, "size")
#> $left
#> Unnamed flexseq with 3 elements.
#> 
#> Elements:
#> 
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"
#> 
#> [[3]]
#> [1] "c"
#> 
#> 
#> $value
#> [1] "d"
#> 
#> $right
#> Unnamed flexseq with 0 elements.
#>