r - for the length of each row in column Y duplicate row in column X -

(please feel free adjust title more fitting)

i have data.frame 2 columns, x , y of class list below

x <- list("a","b","c","d") y <- list("a",c("a", "b"),"c",c("a", "c", "d"))  df <- as.data.frame(cbind(x,y)) 

when column y has 2 or more entries or here characters, length of each row in y (number of characters) correspond number of identical rows in column x. easier put, each character in y must individual row in x.

# desired output  x y a b b b c c d d c d d 

im not sure how go this, pointers appreciated, thanks!

try

do.call(rbind, map(expand.grid, x, y)) ##   var1 var2 ## 1       ## 2    b    ## 3    b    b ## 4    c    c ## 5    d    ## 6    d    c ## 7    d    d