4.2 字符串翻译

tolower 将字符串或字符串向量中含有的大写字母全都转化为小写， toupper 函数正好与之相反.

tolower(c("HELLO", "Hello, R", "hello"))

## [1] "hello"    "hello, r" "hello"

toupper(c("HELLO", "Hello, R", "hello"))

## [1] "HELLO"    "HELLO, R" "HELLO"