R: Count objects in column-list

Let me define a data frame with one column id formed by a vector of integer

df <- data.frame(id = c(1,2,2,3,3))

and a column objects which instead is list of character vectors. Let''s create the column with the following function

randomObjects <- function(argument) {
  numberObjects <- sample(c(1,2,3,4), 1)
  vector <- character()
  for (i in 1:numberObjects) {
    vector <- c(vector, sample(c("apple","pear","banana"), 1))
  }
  return(vector)

} 

which is then called with lapply

set.seed(28100)
df$objects <- lapply(df$id, randomObjects)

The resulting data frame is

df
#   id                 objects
# 1  1            apple, apple
# 2  2     apple, banana, pear
# 3  2                  banana
# 4  3    banana, pear, banana
# 5  3 pear, pear, apple, pear

Now I want to count the number of objects corresponding to each id with a data frame like this

summary <- data.frame(id = c(1, 2, 3),
                      apples = c(2, 1, 1), 
                      bananas = c(0, 2, 2),
                      pears = c(0, 1, 4))

summary
#   id apples bananas pears
# 1  1      2       0     0

# 2  2      1       2     1
# 3  3      1       2     4

How should I collapse the information of df into a more compact data frame such as summary without using a for loop?

Answer

library(plyr)

ddply(df, .(id), function(d, lev) {
  x <- factor(unlist(d$objects), levels = lev)

  t(as.matrix(table(x)))
}, lev = unique(unlist(df$objects)))
#  id apple banana pear
#1  1     2      0    0
#2  2     1      2    1
#3  3     1      2    4