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http://blog.csdn.net/u010255642
3）关于无效值
无效值或缺失值NA、NaN,主要用于应付某操作没完成，结果未知的情况：
> c(1:4,NA,2:3)->x
> x
[1] 1 2 3 4 NA 2 3
> is.na(x)
[1] FALSE FALSE FALSE FALSE TRUE FALSE FALSE
>
NAN应对数字计算
> 0/0
[1] NaN
>
> 0/0->y
> is.na(y)
[1] TRUE
> y
[1] NaN
>
4)字符串向量
字符串用单引号或双引号包围
> c("qq","bb")->z
> z
[1] "qq" "bb"
可在字符串中使用转义符\
\n 新行
\r 回车
\t tab
\b 退格
\a 鸣叫
\\ \
\' '
\" "
paste()函数接受任意数量的参数，然后将它们依次连接到字符串向量的元素中，
Sep指定连接时相隔的字符，默认为单个空格
> paste(1:12)
[1] "1" "2" "3" "4" "5" "6" "7" "8" "9" "10" "11" "12"
> paste("A", 1:6, sep = "")
[1] "A1" "A2" "A3" "A4" "A5" "A6"
> paste("A", 1:6)
[1] "A 1" "A 2" "A 3" "A 4" "A 5" "A 6"
> paste("A", 1:6, sep = "")
[1] "A1" "A2" "A3" "A4" "A5" "A6"
> paste(c("A","B"), 1:10, sep="")
[1] "A1" "B2" "A3" "B4" "A5" "B6" "A7" "B8" "A9" "B10"
> paste("今天是", date())
[1] "今天是 Sun Apr 21 14:18:38 2013"
>