R -- match,pmatch,charmatch

文章目录

match

?match

Description
match returns a vector of the positions of (first) matches of its first argument in its second.
第一个向量中的元素在第二个向量中的位置，如果第二个向量中有多个仅返回第一个match 元素的位置，第二个向量中不存在则返回NULL

%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.

操作符 %in% 是match的简易变形版本，返回结果为TRUE,FALSE

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.
如果没有match，应该返回什么值，默认是NULL

incomparables     a vector of values that cannot be matched. Any value in x matching a value in this vector is assigned the nomatch value. For historical reasons, FALSE is equivalent to NULL.
类似于排除第一个向量中的某些元素，当第一个向量中的某些元素match 该参数指定的向量时，则返回NULL。

Examples
# 取两个向量的交集
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",NA,"@","bla","a","Ba","%")
sstr[sstr %in% c(letters, LETTERS)]

pmatch

?pmatch

Description
pmatch seeks matches for the elements of its first argument among those of its second.

Usage
pmatch(x, table, nomatch = NA_integer_, duplicates.ok = FALSE)

Arguments
x    the values to be matched: converted to a character vector by as.character. Long vectors are supported.

table    the values to be matched against: converted to a character vector. Long vectors are not supported.

nomatch    the value to be returned at non-matching or multiply partially matching positions. Note that it is coerced to integer.

duplicates.ok    should elements be in table be used more than once?
这个参数控制该函数的行为，决定第二个向量中可以match几次，是遇到第一个match的就终止match还是寻找所有可以match的

Examples
pmatch("", "")                             # returns NA
pmatch("m",   c("mean", "median", "mode")) # returns NA    <--- 更具这一个示例，我觉得还是不要考虑使用这个函数了，通常我们会认为应该返回一个位置的，但返回了NA，容易给人造成误导。
pmatch("me",c("mean", "median", "mode"),duplicates.ok = TRUE) # returns NA
pmatch("med", c("mean", "median", "mode")) # returns 2

pmatch(c("", "ab", "ab"), c("abc", "ab"), dup = FALSE)
pmatch(c("", "ab", "ab"), c("abc", "ab"), dup = TRUE)
## compare
charmatch(c("", "ab", "ab"), c("abc", "ab"))

charmatch

?charmatch

Description
charmatch seeks matches for the elements of its first argument among those of its second.

Usage
charmatch(x, table, nomatch = NA_integer_)

Arguments
x    the values to be matched: converted to a character vector by as.character. Long vectors are supported.

table    the values to be matched against: converted to a character vector. Long vectors are not supported.

nomatch        the (integer) value to be returned at non-matching positions.

if multiple exact or multiple partial matches are found then 0 is returned and if no match is found then nomatch is returned.

Examples
charmatch("", "")                             # returns 1
charmatch("m",   c("mean", "median", "mode")) # returns 0
charmatch("med", c("mean", "median", "mode")) # returns 2