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R 字符处理基础函数

1、nchar(x)：返回字符串或者字符串向量x的长度。

> nchar("I love you!")
[1] 11
> nchar(c("I", "love", "you", "!"))
[1] 1 4 3 1

2、grep(pattern,x)：返回 pattern 在字符串向量 x 中的位置。

> grep("y", "I love you!")
[1] 1

> a <- c("I", "love", "you", "!")
> grep("y", a)
[1] 3

> grep("k", a)
integer(0)

3、paste(...，sep=" ")：连接字符串，分隔符为 sep (默认值为空格）。

> paste("I", "love", "you", "!")
[1] "I love you !"

> a <- c("I", "love", "you", "!")
> a
[1] "I"    "love" "you"  "!"   

> paste(a, 1:4)
[1] "I 1"    "love 2" "you 3"  "! 4"   
> paste(a, 1:4, sep="-")
[1] "I-1"    "love-2" "you-3"  "!-4"   

> paste("Today is","Sat Jan 11 2020")
[1] "Today is Sat Jan 11 2020"

4、paste0(...，sep=" ")：以空字符串连接字符。

> paste0("I", "love", "you", "!")
[1] "Iloveyou!"
 
> a <- c("I", "love", "you", "!")
> a
[1] "I"    "love" "you"  "!"   
> paste0(a, 1:4)
[1] "I1"    "love2" "you3"  "!4"   
> paste0(a, 1:4, sep="--")
[1] "I1--"    "love2--" "you3--"  "!4--"   
 
> b <- c("甲","乙","丙","丁","戊","己","庚","辛","壬","癸")
> d <- c("子","丑","寅","卯","辰","巳","午","未","申","酉","戌","亥")
> paste0(b, d)
 [1] "甲子" "乙丑" "丙寅" "丁卯" "戊辰" "己巳" "庚午"
 [8] "辛未" "壬申" "癸酉" "甲戌" "乙亥"

5、sprintf(...)：按照一定格式把若干的组件组合成字符串。

> a <- 11
> sprintf("The square of %d is %d", a, a^2)
[1] "The square of 11 is 121"

> sprintf("The square root of %d is %d", a^2, (a^2)^0.5)
[1] "The square root of 121 is 11"

相似于 Python 中的打印格式化字符串

示例：

a = 11

print('The square of %d is %d' % (a, a**2))
print('The square root of {} is {}'.format(a**2, a))

The square of 11 is 121
The square root of 121 is 11

6、substr(x,start,stop)：截取字符串x中start到stop范围的字串。

excel 中的 mid(), python 中的切片

示例：

> a <- paste0(letters[1:7], collapse="")
> a
[1] "abcdefg"

> substr(a, 1, 3)
[1] "abc"

> substr(a, 1, 3) <- "aaa"
> a
[1] "aaadefg"

> b <- c("1a","2bb", "3ccc", "4dddd" )
> substr(b, 1, 2)
[1] "1a" "2b" "3c" "4d"

7、strsplit(x,split)：根据split将x拆分成若干字串，返回这些字串组成的列表。

python 中的 s.split(split)

示例：

> a <-paste(letters[1:7], collapse="_")
> a
[1] "a_b_c_d_e_f_g"
> strsplit(a, "_")
[[1]]
[1] "a" "b" "c" "d" "e" "f" "g"
 
> b <- paste0(letters[1:7], 1:7, collapse="_")
> b
[1] "a1_b2_c3_d4_e5_f6_g7"
> strsplit(b, "_")
[[1]]
[1] "a1" "b2" "c3" "d4" "e5" "f6" "g7"
 
> d <- paste0(c(2020, 01, 10), collapse="/")
> d
[1] "2020/1/10"
> strsplit(d, "/")
[[1]]
[1] "2020" "1"    "10"  

> #  将列表转换为字符串向量
> unlist(strsplit(d, "/"))
[1] "2020" "1"    "10"

8、regexpr(pattern,x)：在字符串 x 中寻找 pattern，返回与pattern匹配的第一个子字符串的起始字符位置。

> a <- "I love you!"
> regexpr("y", a)
[1] 8
attr(,"match.length")
[1] 1
attr(,"index.type")
[1] "chars"
attr(,"useBytes")
[1] TRUE

“y” 在 a 的第八个位置开始，并且长度为1。

9、gregexpr(pattern,x)：查找x中的所有与pattern匹配的字串开始位置及长度。

> a <- "I love you!"
> b <- "You love me!"
> paste(a, b)
[1] "I love you! You love me!"
> gregexpr("v", paste(a, b))
[[1]]
[1]  5 19
attr(,"match.length")
[1] 1 1
attr(,"index.type")
[1] "chars"
attr(,"useBytes")
[1] TRUE

"v" 在 paste(a, b) 中出现了两次。