如何基于另一行的内容在dataframe中创建新行

Given

鉴于

index = c(1,2,3,4,5)
codes = c("c1","c1,c2","","c3,c1","c2")
df=data.frame(index,codes)
df
  index codes
1     1    c1
2     2 c1,c2
3     3      
4     4 c3,c1
5     5    c2
index = c(1,2,3,4,5)
codes = c("c