来自内置常量的向量字母月份名称的序列

R 有许多内置常量。可以使用以下常量：

• LETTERS：罗马字母的 26 个大写字母
• letters：罗马字母的 26 个小写字母
• month.abb：英文月份名称的三个字母缩写
• month.name：一年中的几个月的英文名称
• pi：圆周长与直径的比值

从字母和月份常量，可以创建向量。

1） 字母序列：

> letters
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"

> LETTERS[7:9]
[1] "G" "H" "I"

> letters[c(1,5,3,2,4)]
[1] "a" "e" "c" "b" "d"

2） 月份缩写或月份名称的序列：

> month.abb
 [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"

> month.name[1:4]
[1] "January"  "February" "March"    "April"   

> month.abb[c(3,6,9,12)]
[1] "Mar" "Jun" "Sep" "Dec"