字符串操作一般分割、拼接、替換、提取等等
拆分
strsplit
strsplit默認輸出格式為列表
strsplit(x, split, fixed = FALSE, perl = FALSE, useBytes = FALSE)
- x：字符串向量，向量中的每個字符串元素都會被分割
- split：位置的字串向量，即在哪個字串處開始拆分；該參數默認是正則表達式匹配；若設置fixed= T則表示是用普通文本匹配或者正則表達式的精確匹配。用普通文本來匹配的運算速度要快些。
x <- c(as = "asfef", qu = "qwerty", "yuiop[", "b", "stuff.blah.yech")
strsplit(x, "e")
運行結果：
$`as` $qu [[3]] [[4]] [[5]]
[1] "asf" "f" [1] "qw" "rty" [1] "yuiop[" [1] "b" [1] "stuff.blah.y" "ch"
str_split
stringr包中的str_split函數與標准庫中的strsplit一樣
str_split(string, pattern, n = Inf, simplify = FALSE)
- string：字符串向量，向量中的每個字符串元素都會被分割
- pattern：分割位置的字符串向量，即在哪個字符串處開始
library(stringr)
fruits <- c( "apples and oranges and pears and bananas","pineapples and mangos and guavas")
str_split(fruits, " and ")
運行結果：
[[1]] [[2]]
[1] "apples" "oranges" "pears" "bananas" [1] "pineapples" "mangos" "guavas"
拼接
paste和paste0
paste和paste0之間的區別是拼接的字符之間是否帶有空格
paste (..., sep = " ", collapse = NULL)
paste0(..., collapse = NULL)
- ...：一個或者多個R對象，該對象需轉換為字符向量.如果是字符串，則所有字符串拼接在一起，如果是字符串向量，則匹配。具體看實例
- sep：分割字符串
paste0(1:12, c("st", "nd", "rd", rep("th", 9)))
# 結果
[1] "1st" "2nd" "3rd" "4th" "5th" "6th" "7th" "8th" "9th" "10th" "11th" "12th"
paste(1:12, c("st", "nd", "rd", rep("th", 9)))
# 結果
"1 st" "2 nd" "3 rd" "4 th" "5 th" "6 th" "7 th" "8 th" "9 th" "10 th" "11 th" "12 th"
paste(1:12, c("st", "nd"))
# 結果
[1] "1 st" "2 nd" "3 st" "4 nd" "5 st" "6 nd" "7 st" "8 nd" "9 st" "10 nd" "11 st" "12 nd"
paste0(1:12, c("st", "nd"))
# 結果
[1] "1st" "2nd" "3st" "4nd" "5st" "6nd" "7st" "8nd" "9st" "10nd" "11st" "12nd"
paste("I","love","you")
# 結果
[1] "I love you"
paste0("I","love","you")
# 結果
[1] "Iloveyou"
str_c
str_c(..., sep = "", collapse = NULL)
str_c和paste0函數一樣
str_c(1:12, c("st", "nd", "rd", rep("th", 9)))
# 結果
[1] "1st" "2nd" "3rd" "4th" "5th" "6th" "7th" "8th" "9th" "10th" "11th" "12th"
str_c(1:12, c("st", "nd"))
# 結果
[1] "1st" "2nd" "3st" "4nd" "5st" "6nd" "7st" "8nd" "9st" "10nd" "11st" "12nd"
str_c("I","love","you")
# 結果
[1] "Iloveyou"
替換
chartr
chartr(old, new, x)
- x：字符串向量
- old：需要被替換的字符/字符串，其長度不能長於new。也就是說只會更改下標上的字符，而不能更改下標。而且替換的時候，會old和new根據下標對應替換
- new：替換的字符/字符串
chartr(old = "a",new = "c",c("a123","a15","a23"))
# 結果
[1] "c123" "c15" "c23"
chartr(old = "a12345",new = "c6789101456",c("a123","a15","a23"))
# 結果
[1] "c678" "c61" "c78" # 拿a15說明，a在old中下標為1，便替換為new[1]。1在old中下標為2，所以替換為new[2]。5在old中下標為6，所以替換為new[6]，所以最后a15替換為c61。
chartr(old = "a1",new = "c4",c("a123","a15","a23"))
# 結果
[1] "c423" "c45" "c23"
sub
sub可以替換字符串，但是sub()函數不會對原字符串進行操作。所以需要創建一個變量來儲存該操作后的字符串。另外，sub函數只會替換匹配到的第一個
sub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
- pattern：包含正則表達式的字符串
- replacement：與pattern匹配的部分進行替換的值
- x：字符串向量或者轉化為字符的R對象
str <- "Now is the time "
sub(" +$", " 12:00", str) #正則表達式，即str尾部的空格替換為12:00
# 結果
"Now is the time 12:00"
# 此時我們只是調用了sub函數，卻沒有保存這個結果。而且該函數不會對原函數操作的。
print(str)
"Now is the time "
sub("Now","what",str)
# 結果
[1] "what is the time "
sub(pattern = "nd",replacement = "ND",c("andbndcnd","sndendfund"))
# 結果，字符串元素中有很多"nd"，但是只會替換第一個"nd"。
[1] "aNDbndcnd" "sNDendfund"
gsub
gsub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
gsub()函數和sub用法一樣，不過，gsub()函數可以替換所有匹配字符
gsub(pattern = "nd",replacement = "ND",c("andbndcnd","sndendfund"))
# 結果
[1] "aNDbNDcND" "sNDeNDfuND"
substr和substring
這兩個函數可以提取、替換字符串。而且是對原字符串進行操作
substr(x, start, stop) <- value
substring(text, first, last = 1000000L) <- value
- x, text：字符串向量
- start, first：整型，替換字符的起始下標
- stop,：整型，替換字符的結束下標
- last：字符串長度
- value：替換的字符，如果需要的話（與代替換向量長度不同)，自動循環補齊
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
substr(shopping_list,1,3) <- "AAA"
# 結果
[1] "AAAles x4" "AAA of flour" "AAA of sugar" "AAAk x2"
substr(shopping_list,1) <- "AAA"
# 結果
[1] "AAAles x4" "AAA of flour" "AAA of sugar" "AAAk x2"
substr(shopping_list,1,20) <- "yesterday once more"
# 結果
[1] "yesterday" "yesterday on" "yesterday on" "yesterd"
substring(shopping_list,1) <- "yesterday once more"
# 結果
[1] "yesterday" "yesterday on" "yesterday on" "yesterd"
str_replace和str_replace_all
第三方包中的str_replace和str_replace_all
str_replace(string, pattern, replacement) # 和sub一樣，只替換第一個匹配字符
str_replace_all(string, pattern, replacement) # 和gsub一樣，替換所有匹配字符
fruits <- c("one apple", "two pears", "three bananas")
str_replace(fruits, "[aeiou]", "-") #正則表達式，即對字符串中的小寫字母a或e或i或o或u，替換為-
# 結果
[1] "-ne apple" "tw- pears" "thr-e bananas"
str_replace_all(fruits, "[aeiou]", "-")
# 結果
[1] "-n- -ppl-" "tw- p--rs" "thr-- b-n-n-s"
str_sub
第三方包stringr
str_sub(string, start = 1L, end = -1L, omit_na = FALSE) <- value
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
str_sub(shopping_list,1,3) <- "AAA"
# 結果
[1] "AAAles x4" "AAA of flour" "AAA of sugar" "AAAk x2"
str_sub(shopping_list,1) <- "AAA"
# 結果
[1] "AAA" "AAA" "AAA" "AAA"
提取
substr 和substring
substr(x, start, stop)
substring(text, first, last = 1000000L)
substr("abcdef", 2, 4)
# 結果
"bcd"
substr("abcdef", 1:6, 1:6)
# 結果
"a","b","c","d","d","e"
str_extract 和str_extract_all
第三方包stringr
str_extract(string, pattern)
str_extract_all(string, pattern, simplify = FALSE)
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
str_extract(shopping_list, "[a-z]+")
# 結果
[1] "apples" "bag" "bag" "milk"
str_extract_all(shopping_list, "[a-z]+")
# 結果
[[1]] [[2]] [[3]] [[4]]
[1] "apples" "x" [1] "bag" "of" "flour" [1] "bag" "of" "sugar" [1] "milk" "x"
str_sub
第三方包stringr
str_sub(string, start = 1L, end = -1L)
str_sub(shopping_list,1,5)
# 結果
[1] "apple" "bag o" "bag o" "milk "
測定字符串長度
nchar
nchar(x, type = "chars", allowNA = FALSE, keepNA = NA) #以字符串為向量，返回向量元素--字符串的長度組成的向量
nzchar(x, keepNA = FALSE) #快速判定字符串向量元素是否為非空值
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
nchar(shopping_list)
# 結果
[1] 9 12 12 7
nzchar(shopping_list)
# 結果
[1] TRUE TRUE TRUE TRUE
str_count
str_count(string, pattern = "")
str_count不僅可以測定元素長度，還以測定某字符在字符串中的下標位置
str_count(shopping_list)
# 結果
[1] 9 12 12 7
str_count(shopping_list, "a")
# 結果，如果不包含則返回0
[1] 1 1 2 0
str_length
第三方包stringr
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
str_length(string)
str_length(shopping_list)
# 結果
[1] 9 12 12 7
字符串匹配
grep
grep(pattern, x, ignore.case = FALSE, perl = FALSE, value = FALSE,
fixed = FALSE, useBytes = FALSE, invert = FALSE)
- pattern: 包含一個正則表達式的字符串（或者，當fixed = True時，為字符串）
- x: 一個待匹配的字符串向量，或者是一個可強制轉換為字符串的R對象
- value:當value = False時，函數返回匹配值的下標。當value = True，函數返回匹配值
shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
grep("apple",shopping_list)
# 結果
[1] 1
grep("apple",shopping_list,value = T)
# 結果
[1] "apples x4"
grepl
grepl(pattern, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
grepl和grep的用法差不多，只是grepl返回的是邏輯變量TRUE或FALSE
grepl("apple",shopping_list)
# 結果
[1] TRUE FALSE FALSE FALSE
str_subset
str_subset(string, pattern, negate = FALSE)
- string: 待匹配的字符串向量
- pattern: 一個包含正則表達式的字符串
- negate: 當negate = False，函數返回匹配值。當negate = True，函數返回與pattern不匹配的字符串
fruit <- c("apple", "banana", "pear", "pinapple")
str_subset(fruit, "a") #匹配所有含有a的字符串
# 結果
[1] "apple" "banana" "pear" "pinapple"
str_subset(fruit, "^p", negate = TRUE) # 返回所有不以p開頭的字符串
# 結果
[1] "apple" "banana"
str_which
str_which(string, pattern, negate = FALSE)
str_which(fruit, "a")
# 結果
[1] 1 2 3 4
排序
str_sort
str_sort(x, decreasing = FALSE, na_last = TRUE, locale = "en",
numeric = FALSE, ...)
- x:待排序的字符串向量
- decreasing：布爾值，默認FALSE，表示從低到高排序。如果為TRUE，表示從高到低排序
- na_last：NA 應該排在什么位置，TRUE表示放在末端，FALSE表示放在開頭，NA向下排。
- numeric：如果為True，則按照數字排序而不是按照字符排序
x <- c("100a10", "100a5", "2b", "2a")
str_sort(x)
# 結果
[1] "100a10" "100a5" "2a" "2b"
str_sort(x, numeric = TRUE)
# 結果
[1] "2a" "2b" "100a5" "100a10"