加载包间接赋值报错的解决办法

from Load R package from character string - Stack Overflow

例如，我想这样：

load.this <- "ggplot"
library(load.this) 

会报错'package' must be of length 1。
解决方法是

load.this <- "ggplot"
library(load.this, character.only=TRUE) 
# 如果你要返回是否加载成功
library(load.this, character.only=TRUE, logical.return = T)