match
Value Matching
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.
Usage
match(x, table, nomatch = NA_integer_, incomparables = NULL)
x %in% table
Arguments
 x
 vector or
NULL
: the values to be matched. Long vectors are supported.  table
 vector or
NULL
: the values to be matched against. Long vectors are not supported.  nomatch
 the value to be returned in the case when no match is
found. Note that it is coerced to
integer
.  incomparables
 a vector of values that cannot be matched. Any
value in
x
matching a value in this vector is assigned thenomatch
value. For historical reasons,FALSE
is equivalent toNULL
.
Details
%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"
.
Value

A vector of the same length as
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
.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
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 Scompatible equivalent of %in%
.
Examples
library(base)
## 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, 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)
Community examples
`%in%` stops you having to do lots of `==` comparisons. Instead of writing something clunky like: ```{r} x < sample(letters, 1) if(x == "a"  x == "e"  x == "i"  x == "o"  x == "u") { message("x is a vowel") } else { message("x is a consonant") } ``` …you can do: ```{r} if(x %in% c("a", "e", "i", "o", "u")) { message("x is a vowel") } else { message("x is a consonant") } ``` `%in%` accepts vectors for both the left and righthand sides. The output has the same length as the lefthand side. ```{r} letters %in% c("a", "e", "i", "o", "u") c("a", "e", "i", "o", "u") %in% letters ``` `%in%` also works with numbers. ```{r} seq.int(0.25, 5, 0.25) %in% 1:5 ``` `NA` is just another value to be matched. ```{r} NA %in% 1 1 %in% NA NA %in% NA ``` `match()` gives you the positions of the matches rather than just `TRUE` or `FALSE`. (Think [`grep()`]()https://www.rdocumentation.org/packages/base/topics/grep) vs. `grepl()`.) ```{r} match(c("a", "e", "i", "o", "u"), letters) match(letters, c("a", "e", "i", "o", "u")) ``` You can change the value that is returned when there isn't a match. Zero is a common choice. ```{r} match(letters, c("a", "e", "i", "o", "u"), nomatch = 0) ``` You can force some values to always fail to match by specifying them as incomparable. (It is usually easier change the table argument though.) ```{r} match(letters, c("a", "e", "i", "o", "u"), incomparables = "a") ``` The set manipulation functions [`intersect()`](https://www.rdocumentation.org/packages/base/topics/sets) and `setdiff()` are based upon match. Here are some simplified implementations. ```{r} intersect1 < function(x, y) { y[match(x, y, nomatch = 0)] } setdiff1 < function(x, y) { x[match(x, y, nomatch = 0L) == 0L] } odds < c(1, 3, 5, 7, 9) primes < c(2, 3, 5, 7, 11) intersect1(odds, primes) setdiff1(odds, primes) setdiff1(primes, odds) ```