R函数-letters()、month.name()等几个内置函数

LETTERS[]: 26个大写字母;

letters[]: 26个小写字母;

month.abb[]: 英文月份缩写名称(三个字母);

month.name[]: 一年的十二个月的英文名字;

pi[]: 圆周率.

例：

> LETTERS[10]
[1] "J"
> LETTERS[1:30]
[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" NA  NA  NA  NA
> letters[10]
[1] "j"
> letters[1:30]
[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" NA  NA  NA  NA
> month.abb[1:12]
[1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"
> month.name[1:12]
[1] "January"   "February"  "March"     "April"     "May"       "June"      "July"      "August"    "September" "October"   "November"  "December"
> pi[]
[1] 3.141593