match {base}R Documentation

Value Matching

Description

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, incomparables = FALSE)

x %in% table

Arguments

x vector: the values to be matched.
table vector: the values to be matched against.
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 the nomatch value. Currently, FALSE is the only possible value, meaning that all values can be matched.

Details

%in% is currently defined as
"%in%" <- function(x, table) match(x, table, nomatch = 0) > 0

Factors 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 < list) before matching.

Matching for lists is potentially very slow and best avoided except in simple cases.

Value

In both cases, 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.

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.

is.element for an S-compatible equivalent of %in%.

Examples

## The intersection of two sets :
intersect <- function(x, y) y[match(x, y, nomatch = 0)]
intersect(1:10,7:20)

1:10 %in% c(1,3,5,9)
sstr <- c("c","ab","B","bba","c","@","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)

[Package base version 2.1.0 Index]