学习《R for Data Science》(8)——Stringr

String length 字符长度

str_length(c("a", "R for data science", NA))

## [1]  1 18 NA

Combining strings 字符合并

str_c("x", "y")

## [1] "xy"

str_c("x", "y", "z")

## [1] "xyz"

str_c("x", "y", sep = ", ")

## [1] "x, y"

将所要合并的向量作为第一个参数传入，需要用到collapse参数

str_c(1:10,collapse = "")

## [1] "12345678910"

str_c是向量化的，对于短向量会自动循环补齐

str_c("prefix-", c("a", "b", "c"), "-suffix")

## [1] "prefix-a-suffix" "prefix-b-suffix" "prefix-c-suffix"

Subsetting strings 字符子集

str_sub取字符串的闭区间（即包括开始和结束）

x <- c("Apple", "Banana", "Pear")
str_sub(x, 1, 3)

## [1] "App" "Ban" "Pea"

负数，则从后往前数，但仍是左在前，右在后

str_sub(x,-3,-1)

## [1] "ple" "ana" "ear"

字符串过短时，str_sub也不会崩溃

str_sub("a", 1, 5)

## [1] "a"

str_sub可以原地修改

str_sub(x, 1, 1) <- str_to_lower(str_sub(x, 1, 1))
x

## [1] "apple"  "banana" "pear"

Locales 大小写转换

• str_to_lower()
• str_to_upper()
• str_to_title()

basic

x <- c("apple", "banana", "pear")
str_view(x, "an")

.可以匹配任意字符（除了空行）

str_view(x, ".a.")

如果要匹配. 前面还需再加个\.，但为了正则表达式，还需要再加个\.

str_view(c("abc", "a.c", "bef"), "a\\.c")

如果要匹配\，需要四个斜杠

x <- "a\\b"
str_view(x, "\\\\")

Anchors 锚

从开头或结尾开始匹配

• \^ 从开头开始匹配
• \$ 从结尾开始匹配

x <- c("apple", "banana", "pear")
str_view(x, "^a")

str_view(x, "a$")

如果要严格匹配某个字符串，可以同时使用\^和\$

x <- c("apple pie", "apple", "apple cake")
str_view(x, "apple")

str_view(x, "^apple$")

Character classes and alternatives

除了\.外其他的匹配

• \d: matches any digit.
• \s: matches any whitespace (e.g. space, tab, newline).

如果要创建正则表达式，需要有双斜杠