Operators that extract parts of a Container. The behavior is similar to
base R lists and includes convenient extensions for interactive work.
Usage
# S3 method for class 'Container'
x[i, ..., .default = NULL]
# S3 method for class 'Container'
x[[i]]Arguments
- x
A
Containerfrom which to extract elements.- i, ...
Indices specifying elements to extract. Indices may be numeric, character, logical,
NULL, or empty. Logical vectors are recycled as needed. Negative integers drop by position. Negative character indices drop by name. Range expressions such asa:b,1:c, ord:2are supported for convenience and are resolved in the calling environment.- .default
A value used to fill missing items when extracting. If given, unknown names and out-of-bounds positive indices are kept and filled with this value.
Details
The [ operator selects one or more elements and returns a Container.
Order is preserved and duplicates are kept. Logical indices recycle to the
container length with a warning when lengths do not match. NA in logical
indices is treated as FALSE with a warning. Positive and negative numeric
indices cannot be mixed in a single call and will raise an error.
Out-of-bounds negative indices are ignored. Character indices match names.
Unknown names are ignored unless .default is supplied, in which case they
are kept and filled. Comma-separated indices and list(...) are accepted and
behave like a single combined index.
The [[ operator selects a single element and returns the value or NULL
if the element is not present.
Warning
Range expressions such as x[a:b] are intended for interactive use, where
the resulting indices can be easily inspected and corrected by the user.
They are convenient for quick exploration and data analysis, but not
intended for programming, where explicit indices should be used instead
to avoid unexpected results.
Examples
co <- container(a = 1, b = 2, c = 3, d = 4)
# Numeric
co[c(1, 4)] # [a = 1, d = 4]
#> [a = 1, d = 4]
co[1, 4] # same (comma-sugar)
#> [a = 1, d = 4]
co[1, 1] # duplicates kept -> [a = 1, a = 1]
#> [a = 1, a = 1]
co[0:5] # unknowns ignored -> [a = 1, b = 2, c = 3, d = 4]
#> [a = 1, b = 2, c = 3, d = 4]
co[5] # [] (unknown positive index)
#> []
# Negative numeric
co[-c(1:2)] # [c = 3, d = 4]
#> [c = 3, d = 4]
co[-1, -4] # [b = 2, c = 3]
#> [b = 2, c = 3]
try(co[-1, 3]) # error: cannot mix positive & negative
#> Error : cannot mix positive and negative indices.
co[-5] # out-of-bounds negatives ignored -> full container
#> [a = 1, b = 2, c = 3, d = 4]
# Character
co[c("a", "d")] # [a = 1, d = 4]
#> [a = 1, d = 4]
co["a", "d"] # same
#> [a = 1, d = 4]
co[letters[1:5]] # unknown names dropped -> [a = 1, b = 2, c = 3, d = 4]
#> [a = 1, b = 2, c = 3, d = 4]
co["x"] # []
#> []
# Negative character (drop by name)
co[-c("a", "d")] # [b = 2, c = 3]
#> [b = 2, c = 3]
co[-"a", -"d"] # [b = 2, c = 3]
#> [b = 2, c = 3]
# Logical
co[c(TRUE, FALSE, TRUE, FALSE)] # [a = 1, c = 3]
#> [a = 1, c = 3]
co[TRUE, FALSE] # [a = 1, c = 3] (recycled)
#> [a = 1, c = 3]
co[c(TRUE, NA)] # [a = 1, c = 3] (NA -> FALSE, warning)
#> Warning: Logical index contains NA; treating NA as FALSE.
#> [a = 1, c = 3]
# Mixed numeric and character
co[list(1, "d")] # [a = 1, d = 4]
#> [a = 1, d = 4]
co[1, "d"] # same
#> [a = 1, d = 4]
# Alphanumeric ranges (NSE)
co[a:b] # [a = 1, b = 2]
#> [a = 1, b = 2]
co[a:b, d:c] # [a = 1, b = 2, d = 4, c = 3]
#> [a = 1, b = 2, d = 4, c = 3]
co[1:c] # [a = 1, b = 2, c = 3]
#> [a = 1, b = 2, c = 3]
co[d:2] # [d = 4, c = 3, b = 2]
#> [d = 4, c = 3, b = 2]
co[-(a:c)] # [d = 4]
#> [d = 4]
# Default-filling of missing items
co[1:5, 0, .default = 0] # [a = 1, b = 2, c = 3, d = 4, 0]
#> [a = 1, b = 2, c = 3, d = 4, 0]
co["a", "b", "z", .default = 0] # [a = 1, b = 2, z = 0]
#> [a = 1, b = 2, z = 0]
co[1:2, "z", .default = 3:4] # [a = 1, b = 2, z = (3L 4L)]
#> [a = 1, b = 2, z = (3L 4L)]
co = container(a = 1, b = 2)
co[[1]]
#> [1] 1
co[["a"]]
#> [1] 1
co[["x"]]
#> NULL