Iterator that filters elements not satisfying a predicate function

i_keep(iterable, predicate) constructs an iterator that filters elements from iterable returning only those for which the predicate is TRUE.

Usage

i_keep(iterable, predicate, ...)

i_drop(iterable, predicate, ...)

Arguments

iterable

an iterable object.

predicate

a function that determines whether an element is TRUE or FALSE. The function is assumed to take only one argument.

...

passed along to iteror constructor.

Value

iterator object

Details

Originally called 'ifilter' from package itertools. Renamed because the order of arguments has changed to put the iterable in the first argument, the better to be used with the |> operator.

See also

i_drop i_keepwhile i_dropwhile

Examples

# Filters out odd numbers and retains only even numbers
is_even <- function(x) {
  x %% 2 == 0
}
it <- i_keep(1:10, is_even)
as.list(it)
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 4
#> 
#> [[3]]
#> [1] 6
#> 
#> [[4]]
#> [1] 8
#> 
#> [[5]]
#> [1] 10
#> 

# Similar idea here but anonymous function is used to retain only odd
# numbers
it2 <- i_drop(1:10, function(x) x %% 2 == 0)
nextOr(it2, NA) # 1
#> [1] 1
nextOr(it2, NA) # 3
#> [1] 3
nextOr(it2, NA) # 5
#> [1] 5
nextOr(it2, NA) # 7
#> [1] 7
nextOr(it2, NA) # 9
#> [1] 9

is_vowel <- function(x) {
  x %in% c('a', 'e', 'i', 'o', 'u')
}
it3 <- i_keep(letters, is_vowel)
as.list(it3)
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "e"
#> 
#> [[3]]
#> [1] "i"
#> 
#> [[4]]
#> [1] "o"
#> 
#> [[5]]
#> [1] "u"
#> 
# Filters out even numbers and retains only odd numbers
is_even <- function(x) {
  x %% 2 == 0
}
it <- i_drop(1:10, is_even)
as.list(it)
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 5
#> 
#> [[4]]
#> [1] 7
#> 
#> [[5]]
#> [1] 9
#> 

# Similar idea here but anonymous function is used to filter out odd
# numbers
it2 <- i_drop(1:10, function(x) x %% 2 == 1)
as.list(it2)
#> [[1]]
#> [1] 2
#> 
#> [[2]]
#> [1] 4
#> 
#> [[3]]
#> [1] 6
#> 
#> [[4]]
#> [1] 8
#> 
#> [[5]]
#> [1] 10
#> 

is_vowel <- function(x) {
  x %in% c('a', 'e', 'i', 'o', 'u')
}
it3 <- i_drop(letters, is_vowel)
nextOr(it3, NA) # b
#> [1] "b"
nextOr(it3, NA) # c
#> [1] "c"
nextOr(it3, NA) # d
#> [1] "d"
nextOr(it3, NA) # f
#> [1] "f"
nextOr(it3, NA) # g
#> [1] "g"
# nextOr(it, NA) continues through the rest of the consonants