R语言之——字符串处理函数

nchar

取字符数量的函数
length与nchar不同，length是取向量的长度

# nchar表示字符串中的字符的个数
nchar("abcd")
[1] 4

# length表示向量中元素的个数
length("abcd")
[1] 1
length(c("hello", "world"))
[1] 2

chartr

字符替换

chartr(old="a", new="c", x="a123")
[1] "c123"

chartr(old="a", new="A", x="data")
[1] "dAtA"

paste和paste0

字符串粘合函数
paste在不指定分割符的情况下，默认分割符是空格
paste0在不指定分割符的情况下，默认分割符是空

# 默认以空格隔开
paste("Hello","world")
[1] "Hello world"

# 没有空格
paste0("Hello","world")
[1] "Helloworld"

# 指定分割符
paste("abc", "efg", "hijk", sep = "-")
[1] "abc-efg-hijk"

# 分别对向量的每一个元素进行连接
paste0("A", 1:6, sep = "")
[1] "A1" "A2" "A3" "A4" "A5" "A6"

# collapse参数：每一个元素操作之后，再把向量的每一个元素进行连接
paste0("A", 1:6, sep = "",collapse = "-")
[1] "A1-A2-A3-A4-A5-A6"

substr

字符串截取函数

substr(x = "hello", start = 1, stop = 2)
[1] "he"

strsplit

字符串的分割函数，可以指定分割符，生成一个list

strsplit("abc", split = "")
[[1]]
[1] "a" "b" "c"

如果要对一个向量使用该函数，需要注意。

# 分割向量的每一个元素，并取分割后的第一个元素
unlist(lapply(X = c("abc", "bcd", "dfafadf"), FUN = function(x) {return(strsplit(x, split = "")[[1]][1])}))
[1] "a" "b" "d"

gsub和sub

字符串替换
gsub替换匹配到的全部
sub 替换匹配到的第一个

# 将b替换为B
gsub(pattern = "b", replacement = "B", x = "baby")
[1] "BaBy"

gsub(pattern = "b", replacement = "B", x = c("abcb", "boy", "baby"))
[1] "aBcB" "Boy"  "BaBy"

# 只替换第一个b
sub(pattern = "b", replacement = "B", x = "baby")
[1] "Baby"

sub(pattern = "b", replacement = "B", x = c("abcb", "baby"))
[1] "aBcb" "Baby"

grep和grepl

字符串匹配
grep函数返回的是索引值
grepl函数返回的是逻辑值

# 返回匹配到的元素的索引
grep(pattern = "boy", x = c("abcb", "boy", "baby"))
[1] 2

# 返回逻辑值
grepl(pattern = "boy", x = c("abcb", "boy", "baby"))
[1] FALSE  TRUE FALSE

match && pmatch &&charmatch

1、match
Usage
match(x, table, nomatch = NA_integer_, incomparables = NULL)
x %in% table

参数：
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. (被匹配的值)

nomatch: the value to be returned in the case when no match is found. Note that it is coerced to integer. (没有match上的返回的值)

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. (不同来匹配的值)

match函数类似与 %in%，不同的是match返回的是索引，而%in%返回的是逻辑值。