Question

Is there a function like table that outputs the result in a ragged array?

I have a dataframe and I’d like to tabulate the occurrences of different items in a ragged array. I know that I can use table to output the data in an array. However the data is sparse. The xtab function can output in a sparse matrix, but it is limited to two dimensions.

I want a function to take a data frame like this:

Purchases  WeeksSinceFirstPurcahse
1          10.1
1          4.2
1          4.2
2          2.4
3          1
2          2.4

And output counts like this:

Purchases  DaysSinceFirstPurcahse  Count
1          10.1                    1
1          4.2                     2
2          2.5                     2
3          1                       1

Answer 1

You can use the plyr package and specialy the count function.
Using your data, we can do something like this :

require(plyr) 
toydat
  Purchases WeeksSinceFirstPurcahse
1         1                    10.1
2         1                     4.2
3         1                     4.2
4         2                     2.4
5         3                     1.0
6         2                     2.4

str(toydat)
'data.frame':   6 obs. of  2 variables:
 $ Purchases              : int  1 1 1 2 3 2
 $ WeeksSinceFirstPurcahse: num  10.1 4.2 4.2 2.4 1 2.4

ddply(toydat, .(Purchases), function(x) count(x, "WeeksSinceFirstPurcahse"))
  Purchases WeeksSinceFirstPurcahse freq
1         1                     4.2    2
2         1                    10.1    1
3         2                     2.4    2
4         3                     1.0    1

Edit to add Count Function Example:

count(toydat,.(Purchases,WeeksSinceFirstPurchase))
  Purchases WeeksSinceFirstPurchase freq
1         1                     4.2    2
2         1                    10.1    1
3         2                     2.4    2
4         3                     1.0    1