转化为大写字母
toupper(x=c("apple", "I like your style")) [1] "APPLE" "I LIKE YOUR STYLE"
转化为小写字母
tolower(x="APPLE") [1] "apple"
R语言字符串处理函数之——字符串的大小写转换
原文：http://blog.csdn.net/u011402596/article/details/44282363
