R語言之—字符串處理函數

nchar
取字符數量的函數
length與nchar不同，length是取向量的長度
# nchar表示字符串中的字符的個數
nchar("abcd")
[1] 4

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

字符替換

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

chartr(old="a", new="A", x="data")
[1] "dAtA"
paste和paste0

字符串粘合函數
paste在不指定分割符的情況下，默認分割符是空格
paste0在不指定分割符的情況下，默認分割符是空

# 默認以空格隔開
paste("Hello","world")
[1] "Hello world"

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

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

# 分別對向量的每一個元素進行連接
paste0("A", 1:6, sep = "")
[1] "A1" "A2" "A3" "A4" "A5" "A6"

# collapse參數：每一個元素操作之后，再把向量的每一個元素進行連接
paste0("A", 1:6, sep = "",collapse = "-")
[1] "A1-A2-A3-A4-A5-A6"
substr

字符串截取函數

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

字符串的分割函數，可以指定分割符，生成一個list

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

如果要對一個向量使用該函數，需要注意。

# 分割向量的每一個元素，并取分割后的第一個元素
unlist(lapply(X = c("abc", "bcd", "dfafadf"), FUN = function(x) {return(strsplit(x, split = "")[[1]][1])}))
[1] "a" "b" "d"
gsub和sub

字符串替換
gsub替換匹配到的全部
sub 替換匹配到的第一個

# 將b替換為B
gsub(pattern = "b", replacement = "B", x = "baby")
[1] "BaBy"

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

# 只替換第一個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函數返回的是索引值
grepl函數返回的是邏輯值

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

# 返回邏輯值
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

參數：
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. (沒有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. (不同來匹配的值)

match函數類似與 %in%，不同的是match返回的是索引，而%in%返回的是邏輯值。