
match
returns a vector of the positions of (first) matches of
its first argument in its second. %in%
is a more intuitive interface as a binary operator,
which returns a logical vector indicating if there is a match or not
for its left operand.
match(x, table, nomatch = NA_integer_, incomparables = NULL)
x %in% table
NULL
: the values to be matched.
Long vectors are supported.NULL
: the values to be matched against.
Long vectors are not supported.integer
.x
matching a value in this vector is assigned the
nomatch
value. For historical reasons, FALSE
is
equivalent to NULL
.x
.match
: An integer vector giving the position in table
of
the first match if there is a match, otherwise nomatch
.If x[i]
is found to equal table[j]
then the value
returned in the i
-th position of the return value is j
,
for the smallest possible j
. If no match is found, the value
is nomatch
.%in%
: A logical vector, indicating if a match was located for
each element of x
: thus the values are TRUE
or
FALSE
and never NA
.
%in%
is currently defined as
"%in%" <- function(x, table) match(x, table, nomatch = 0) > 0
Factors, raw vectors and lists are converted to character vectors, and
then x
and table
are coerced to a common type (the later
of the two types in R's ordering, logical < integer < numeric <
complex < character) before matching. If incomparables
has
positive length it is coerced to the common type.
Matching for lists is potentially very slow and best avoided except in simple cases.
Exactly what matches what is to some extent a matter of definition.
For all types, NA
matches NA
and no other value.
For real and complex values, NaN
values are regarded
as matching any other NaN
value, but not matching NA
.
That %in%
never returns NA
makes it particularly
useful in if
conditions.
Character strings will be compared as byte sequences if any input is
marked as "bytes"
(see Encoding
).
pmatch
and charmatch
for (partial)
string matching, match.arg
, etc for function argument
matching.
findInterval
similarly returns a vector of positions, but
finds numbers within intervals, rather than exact matches. is.element
for an S-compatible equivalent of %in%
.
## The intersection of two sets can be defined via match():
## Simple version:
## intersect <- function(x, y) y[match(x, y, nomatch = 0)]
intersect # the R function in base is slightly more careful
intersect(1:10, 7:20)
1:10 %in% c(1,3,5,9)
sstr <- c("c","ab","B","bba","c",NA,"@","bla","a","Ba","%")
sstr[sstr %in% c(letters, LETTERS)]
"%w/o%" <- function(x, y) x[!x %in% y] #-- x without y
(1:10) %w/o% c(3,7,12)
## Note that setdiff() is very similar and typically makes more sense:
c(1:6,7:2) %w/o% c(3,7,12) # -> keeps duplicates
setdiff(c(1:6,7:2), c(3,7,12)) # -> unique values
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