[R语言] stringr包 字符处理《R for data s

《R for Data Science》第十四章 strings 啃书知识点积累
参考链接：R for Data Science

涉及部分：stringr非正则相关内容 + stringi
正则匹配会单独划分一个专题做笔记

参考书籍

1. 《R for data science》
2. 《R数据科学》

String basics

" or ': There is no difference in behaviour.

• 字符串中有引号的处理
  使用反义\或者不同的引号组合

double_quote <- "\"" # or '"'
single_quote <- '\'' # or "'"

• 转义用print依然会呈现，可以用writelines显示转义后的文字效果

x <- "hello\\world"
x
# [1] "hello\\world"

writeLines(x)
# hello\world

# 另一个例子
x <- "hello\tworld"
x
# [1] "hello\tworld"

writeLines(x)
# hello world

• utf-8类型非英文字符的处理

x <- "\u00b5"
x
#> [1] "µ"

- String length

• 关于stringr包的函数

These have more intuitive names, and all start with str_.

str_length(c("a", "R for data science", NA))
# [1]  1 18 NA

- Combining strings

str_c("x", "y")
#> [1] "xy"
str_c("x", "y", "z")
#> [1] "xyz"

# 符合向量化运算
str_c("prefix-", c("a", "b", "c"), "-suffix")
# [1] "prefix-a-suffix" "prefix-b-suffix" "prefix-c-suffix"

# 也可以使用paste0
paste0('x','y')
# [1] "xy"

# sep参数控制连接的分隔符，默认为空
str_c("x", "y", sep = ", ")
#> [1] "x, y"

• 处理带有NA的向量

NA有contagious的特性

x <- c("abc", NA)

str_c("|-", x, "-|")
# [1] "|-abc-|" NA   

# 可以使用str_replace_na()保留NA形式输出
str_c("|-", str_replace_na(x), "-|")
# [1] "|-abc-|" "|-NA-|" 

• 在str_c中添加条件判断

name <- "Xi Chen"
time_of_day <- "morning"
birthday <- FALSE

str_c(
  "Good ", time_of_day, " ", name,
  if (birthday) " and HAPPY BIRTHDAY",
  "."
)
# [1] "Good morning Xi Chen."

birthday <- TRUE
str_c(
  "Good ", time_of_day, " ", name,
  if (birthday) " and HAPPY BIRTHDAY",
  "."
)
# [1] "Good morning Xi Chen and HAPPY BIRTHDAY."

• 将含多个字符串的向量合并成一个字符串

str_c(c("x", "y", "z"), collapse = ", ")
# [1] "x, y, z"
str_c(c("x", "y", "z"), collapse = "")
# [1] "xyz"

- Subsetting strings

str_sub: 第一个参数是向量，第二个参数是提取的起始位置，第三个为终末位置

x <- c("Apple", "Banana", "Pear")
str_sub(x, 1, 3)
#> [1] "App" "Ban" "Pea"

str_sub(x, -3, -1)
#> [1] "ple" "ana" "ear"

• 终末位置超出量程不会error，会尽可能返回多的元素

str_sub("a", 1, 5)
# [1] "a"
str_sub(c('xichen','Oct','hi'), 1, 3)
# [1] "xic" "Oct" "hi" 

• 结合str_to_lower可以将首字母替换为小写

x <- c("APple", "BaNana", "PeaR");x
# [1] "APple"  "BaNana" "PeaR" 

str_to_lower(x)
# [1] "apple"  "banana" "pear"  

# 这个赋值很关键，会通过子集替换全局
str_sub(x, 1, 1) <- str_to_lower(str_sub(x, 1, 1));x
# [1] "aPple"  "baNana" "peaR"  

- Locales

这个参数我貌似用不上

1. str_to_lower()
2. str_to_upper()
3. str_to_title()

• locale可以指定语言环境

str_to_upper(c("i", "ı"))
#> [1] "I" "I"
str_to_upper(c("i", "ı"), locale = "tr")
#> [1] "İ" "I"

• 关于排序中locale的作用

The base R order() and sort() functions sort strings using the current locale.

str_sort可以依据locale的语言环境进行排序

x <- c("apple", "eggplant", "banana")

str_sort(x, locale = "en")  # English
#> [1] "apple"    "banana"   "eggplant"

str_sort(x, locale = "haw") # Hawaiian
#> [1] "apple"    "eggplant" "banana"

- 其他函数

(1) str_wrap

This is a wrapper around stringi::stri_wrap() which implements the Knuth-Plass paragraph wrapping algorithm.
str_wrap(string, width = 80, indent = 0, exdent = 0)

str_wrap帮助文档中有word函数，先来学习一下

sentences <- c("Jane saw a cat", "Jane sat down")
word(sentences, 1)
# [1] "Jane" "Jane"
word(sentences, 2)
# [1] "saw" "sat"
word(sentences, -1)
# [1] "cat"  "down"
word(sentences, 2, -1)
# [1] "saw a cat" "sat down" 

word(sentences[1], 1:3, -1)
# [1] "Jane saw a cat" "saw a cat"      "a cat"
word(sentences[1], 1, 1:4)
# [1] "Jane"           "Jane saw"       "Jane saw a"     "Jane saw a cat"

str <- 'abc.def..123.4568.999'
word(str, 6, sep = fixed('.'))
# [1] "999"
word(str, 2, sep = fixed('..'))
# [1] "123.4568.999"

# str_wrap控制文字段落的显示方式
# 有空再研究一下cat函数
library(stringr)
?str_wrap
thanks_path <- file.path(R.home("doc"), "THANKS")
thanks <- str_c(readLines(thanks_path), collapse = "\n")
thanks <- word(thanks, 1,1, fixed("\n\n"))
cat(str_wrap(thanks), "\n")
cat(str_wrap(thanks, width = 40), "\n")
cat(str_wrap(thanks, width = 60, indent = 2), "\n")
cat(str_wrap(thanks, width = 60, exdent = 2), "\n")
cat(str_wrap(thanks, width = 0, exdent = 2), "\n")

(2) str_trim和str_pad

# str_trim可以去除字符串两边的空白
str_trim(" XiChen ", side = 'both') # 默认是双边去除
# [1] "XiChen"
str_trim(" XiChen ", side = "left")
# [1] "XiChen "
str_trim(" XiChen ", side = "right")
# [1] " XiChen"

# str_pad可以有str_trim相反的功能，且功能更丰富
str_pad('XiChen', 8, 'both') # pad默认是空格，可以指定/side默认是left
# [1] " XiChen "
str_pad('XiChen', 8, 'left', pad = '?') 
# [1] "??XiChen"
str_pad("a", 2, pad = c("-", "_", " "))
# [1] "-a" "_a" " a"

- Exercises

(1) paste paste0 str_c处理一般向量和含NA向量的异同

paste("Xi", "Chen")
# [1] "Xi Chen"
paste0("Xi", "Chen")
# [1] "XiChen"

# 注意添加连接符对paste0的影响：仅体现在末尾
paste("Xi", "Chen",sep=',')
# [1] "Xi,Chen"
paste0("Xi", "Chen",sep=',')
# [1] "XiChen,"

# paste等价于str_c(var,sep=' ')
# paste0等价于str_c(var)
str_c("Xi", "Chen")
# [1] "XiChen"
str_c("Xi", "Chen",sep=' ')
# [1] "Xi Chen"

针对NA的异同

# paste paste0都可以直接将NA拼接入字符串
paste("Xi", "Chen", NA)
# [1] "Xi Chen NA"
paste0("Xi", "Chen", NA)
# [1] "XiChenNA"

str_c("Xi", "Chen", NA)
# NA
str_c("Xi", "Chen", str_replace_na(NA))
# [1] "XiChenNA"

x <- c("Xi", "Chen", NA)
str_c(x,collapse = '')
# NA
str_c(str_replace_na(x), collapse = '')
# [1] "XiChenNA"

(2) str_c中sep和collapse的区别
sep对多个向量的拼合起作用
collap对单个向量内部拼接起作用

x <- c('Xi','Chen')
str_c(x)
# [1] "Xi"   "Chen"
str_c(x, sep = ',')
# [1] "Xi"   "Chen"
str_c(x, collapse = ',')
# [1] "Xi,Chen"

str_c('Xi','Chen')
# [1] "XiChen"
str_c('Xi','Chen', sep = ',')
# [1] "Xi,Chen"
str_c('Xi','Chen', collapse = ',')
# [1] "XiChen"

# 这个例子更能说明seq和collapse的作用范围和区别
str_c('x',c('y','z'))
# [1] "xy" "xz"
str_c('x',c('y','z'),sep = ',')
# [1] "x,y" "x,z"
str_c('x',c('y','z'),collapse = ',')
# [1] "xy,xz"

stringi

• stringr is built on top of the stringi package.
• stringi has 244 functions to stringr’s 49.
• The main difference is the prefix: str_ vs. stri_.

- Exercises

(1) stringi几种特殊用法

library(stringi)
# 计算单词数
x <- c('Xi Chen','Happy birthday to you')
stri_count_words(x)
# [1] 2 4


# 判断查找重复字符串
x <- c('Xi Chen', 'XiChen', 'apple', 'Xi Chen','apple')
stri_duplicated(x)
# [1] FALSE FALSE FALSE  TRUE  TRUE

stri_duplicated_any(x)
# [1] 4
# 返回第一个重复元素的下标


# 产生随机字符串
stri_rand_strings(4, 3) # 4个含3字符的字符串
# [1] "Ebd" "JDg" "3Kf" "8k6"

# 现有字符串随机重排
stri_rand_shuffle(c('Xi Chen','Happy birthday to you'))
# [1] "nX eChi"               "byHur oiyd hap ptoyta"

# 乱数假文
stri_rand_lipsum(4) # 数字表示产生几段假文

(2) stri_sort涉及语言环境的设置
我很少用估计

stri_sort(c("hladny", "chladny"), locale="pl_PL")
# [1] "chladny" "hladny" 
stri_sort(c("hladny", "chladny"), locale="sk_SK")
# [1] "hladny"  "chladny"