Split data based on grouping column - r

Question

I'm trying to work out how, in Azure ML (and therefore R solutions are acceptable), to randomly split data based on a column, such that all records with any given value in that column wind up in one side of the split or another. For example:
+------------+------+--------------------+------+
| Student ID | pass | some_other_feature | week |
+------------+------+--------------------+------+
| 1234 | 1 | Foo | 1 |
| 5678 | 0 | Bar | 1 |
| 9101112 | 1 | Quack | 1 |
| 13141516 | 1 | Meep | 1 |
| 1234 | 0 | Boop | 2 |
| 5678 | 0 | Baa | 2 |
| 9101112 | 0 | Bleat | 2 |
| 13141516 | 1 | Maaaa | 2 |
| 1234 | 0 | Foo | 3 |
| 5678 | 0 | Bar | 3 |
| 9101112 | 1 | Quack | 3 |
| 13141516 | 1 | Meep | 3 |
| 1234 | 1 | Boop | 4 |
| 5678 | 1 | Baa | 4 |
| 9101112 | 0 | Bleat | 4 |
| 13141516 | 1 | Maaaa | 4 |
+------------+------+--------------------+------+
Acceptable output from that if I chose, say, a 50/50 split and to be grouped based on the Student ID column would be two new datasets:
+------------+------+--------------------+------+
| Student ID | pass | some_other_feature | week |
+------------+------+--------------------+------+
| 1234 | 1 | Foo | 1 |
| 1234 | 0 | Boop | 2 |
| 1234 | 0 | Foo | 3 |
| 1234 | 1 | Boop | 4 |
| 9101112 | 1 | Quack | 1 |
| 9101112 | 0 | Bleat | 2 |
| 9101112 | 1 | Quack | 3 |
| 9101112 | 0 | Bleat | 4 |
+------------+------+--------------------+------+
and
+------------+------+--------------------+------+
| Student ID | pass | some_other_feature | week |
+------------+------+--------------------+------+
| 5678 | 0 | Bar | 1 |
| 5678 | 0 | Baa | 2 |
| 5678 | 0 | Bar | 3 |
| 5678 | 1 | Baa | 4 |
| 13141516 | 1 | Meep | 1 |
| 13141516 | 1 | Maaaa | 2 |
| 13141516 | 1 | Meep | 3 |
| 13141516 | 1 | Maaaa | 4 |
+------------+------+--------------------+------+
Now, from what I can tell this is basically the opposite of stratified split, where it would get a random sample with every student represented on both sides.
I would prefer an Azure ML function that did this, but I think that's unlikely so is there an R function or library that gives this kind of functionality? All I could find was questions about stratification which obviously don't help me much.

Answer 1

You can use te following command:
data.fold <- mutate(df, fold = sample(rep_len(1:2, n_distinct(Student ID)))[Student ID])
It returns the original dataframe with an new column that indicates the fold that the student is in. If you want more folds, just adjust the '1:2' part.
I've tried the 'sample unique' way but it did not always work for me in the past.