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