最近还在适应美帝的生后，并且因为还没有开始新的课题，所以晚上时间会比较充裕一些，一直在看R for data science这本书，前面一直没有记笔记，后面觉得还是需要养成记笔记的习惯。所以从今天开始，即使每天少看一些，也要记下来自己的读书笔记。今天就从字符串处理stringr包开始，也是第一部分。
需要先加载包
library(tidyverse)
library(stringr)
1 string lenght
如何查看一个字符串中字符的长度
str_length(c("a", "R for data science", NA))
[1] 1 18 NA
2 合并字符串
str_c("x", "y")
[1] "xy"
str_c("x", "y", "z")
[1] "xyz"
同样可以使用sep参数来控制合并时中间分隔符。
str_c("x", "y", sep = ",")
[1] "x,y"
和很多其他函数一样，NA值是一个问题，比如下面的例子，这时候可以使用str_replace_na()函数来处理：
x <- c("abc", NA)
str_c("|-", x, "-|")
[1] "|-abc-|" NA
str_c("|-", str_replace_na(x), "-|")
[1] "|-abc-|" "|-NA-|"
同样从上面的例子也可以看到，str_c是一个vector函数，他会将长度较短的对象循环，使其与长度最长的对象长度一样。
str_c("prefix-", c("a", "b", "c"), "-suffix")
[1] "prefix-a-suffix" "prefix-b-suffix" "prefix-c-suffix"
下面要简单介绍一下在str_c中非常有用的两个参数，sep和collapse。其中collapse非常形象，坍塌，也就是将一个向量中的多个对象合并为一个字符。可以看下面的例子：
str_c(c("x", "y", "z"), collapse = ", ")
[1] "x, y, z"
str_c(c("x", "y", "z"), sep = ", ")
[1] "x" "y" "z"
3 提取字字符
有时候我们需要一个字符串中的某部分，这时候可以使用str_sub()。
x <- c("Apple", "Banana", "Pear")
str_sub(string = x, start = 1, end = 3)
[1] "App" "Ban" "Pea"
该函数一共有三个参数，start和end分别是指从哪开始和结束提取字符串。 还可以反过来提取字符串，也就是将参数设置为负数。
str_sub(x, -3, -1)
[1] "ple" "ana" "ear"
4 区域设置
对于某些规定，不同国家或者地区不太一样，比如排序，默认英语里面的排序是按照英语字母顺序，但是不同地方可能不一样，比如下面的例子：
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"
感觉这个功能不会太常用。了解一下就好了。
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