一些函数不知道什么意思要查，看数值例子，做笔记，知道函数的功能，函数和返回值。
网页上查找关键词，巧用查找（ctrl+F）
R语言 sub与gsub函数的区别
> text <- c("we are the world", "we are the children")
> sub("w", "W", text)
[1] "We are the world" "We are the children"
> sub("W","w",text)
[1] "we are the world" "we are the children"
> gsub("W","w",text)
[1] "we are the world" "we are the children"
> gsub("w","W",text)
[1] "We are the World" "We are the children"
> sub(" ", "", "abc def ghi")
[1] "abcdef ghi"
> ## [1] "abcdef ghi"
> gsub(" ", "", "abc def ghi")
[1] "abcdefghi"
> ## [1] "abcdefghi"
从上面的输出结果可以看出，sub（）和gsub（）的区别在于，前者只替换第一次匹配的字符串，而后者会替换掉所有匹配的字符串。
R子集subset
> x<-c(6,1,2,3,NA,12)
> x[x>5] #x[5]是未知的，因此其值是否大于5也是未知的
[1] 6 NA 12
> subset(x,x>5) #subset直接会把NA移除
[1] 6 12
> subset(airquality, Temp > 80, select = c(Ozone, Temp))
Ozone Temp
29 45 81
35 NA 84
36 NA 85
38 29 82
39 NA 87
40 71 90
...
> subset(airquality, Day == 1, select = -Temp)
Ozone Solar.R Wind Month Day
1 41 190 7.4 5 1
32 NA 286 8.6 6 1
62 135 269 4.1 7 1
93 39 83 6.9 8 1
124 96 167 6.9 9 1
...
> subset(airquality, select = Ozone:Wind)
Ozone Solar.R Wind
1 41 190 7.4
2 36 118 8.0
3 12 149 12.6
4 18 313 11.5
5 NA NA 14.3
就是把x中所有不是NA的值赋予x，比如说x=c(1,2,NA,4)，那么运行这个程序
x<-x[!is.na(x)]
以后，x=c(1,2,4)