In the J programming language, self-classify is the informal name for the mondadic case of =. It (unsurprisingly) marks each element by the equivalence class to which it belongs. It is closely tied to the concept of the nub of its argument (found by using ~.), which is the vector found by eliminating duplicates. An instructive example (user interaction indented three spaces, as in the interpreter):
```   ]v=: 3 4 5 3 4 6 5 7 3
3 4 5 3 4 6 5 7 3
~. v
3 4 5 6 7
= v
1 0 0 1 0 0 0 0 1
0 1 0 0 1 0 0 0 0
0 0 1 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 1 0
```

We see that the result of the nub, is, in fact, the unique elements of v in order of first appearance.

The result of self-classify is a boolean two-dimensional array (matrix or table) in which each column indicates which member of the nub the element at a given index matches. In fact, an alternative formulation for = v is ~. v =/ v, literally ``the equals table for v's nub and itself.''

A tacit definition of the self-classify above, using a hook and the passive adverb:

```   sc =: =/~ ~.
sc v
1 0 0 1 0 0 0 0 1
0 1 0 0 1 0 0 0 0
0 0 1 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 1 0
```