R 给data.frame（dataframe）添加一列

 x<-data.frame(apple=c(1,4,2,3),pear=c(4,8,5,2))
 x
#  apple pear
# 1     1    4
# 2     4    8
# 3     2    5
# 4     3    2
x$banana<-c(9,5,6,2)
x
#   apple pear banana
# 1     1    4      9
# 2     4    8      5
# 3     2    5      6
# 4     3    2      2
 x<-data.frame(apple=c(1,4,2,3),pear=c(4,8,