覆盖/附加到R data.frame中的多值列

Question

I have a data frame which contains a column in which I want to put multiple values (it can have a different amount of values per record).

I'm trying to append data to it using a for loop but it keeps giving the error:

Error in `[<-.data.frame`(`*tmp*`, 1, "piet", value = c(1, 2)) : 
replacement has 2 rows, data has 1

Is it impossible to overwrite/append to this column? Small example code:

a = data.frame(piet=numeric())
a[1,'piet'] = c(1)
a[1,'piet'] = c(1,2) # this throws the error

I realise there are better ways than using a for loop, but my data is spread out somewhat randomly over multiple .csv files (of which some have overlapping records, whose results I'm trying to combine here).

Answer 1

Ok this is really weird but it works using list(c()) (thanks @Frank) and a very specific coding format:

a = data.frame(piet=list()) #initialize data frame
a[1,'piet'] = '' # create first row
a$piet[c(T)] = list(c(1,2,3)) # overwrite with list
a$piet[c(T)] = list(c(4,5,6)) # overwrite with new list
a$piet[c(T)] = list(c(unlist(a[1,'piet']),8,9)) # append to existing list

final contents of a:

> a
           piet
1 4, 5, 6, 8, 9

Note that the a$piet[row selection] is mandatory as well as using list(c()) !