Common LISP Hints: Sorting

## 22. Sorting

LISP provides two primitives for sorting: `sort` and `stable-sort`.

> `

>     > (sort '(2 1 5 4 6) #'<)
>     (1 2 4 5 6)
>     > (sort '(2 1 5 4 6) #'>)
>     (6 5 4 2 1)

> `

The first argument to `sort` is a list; the second is a comparison function. The `sort` function does not guarantee stability: if there are two elements `a` and `b` such that `(and (not (< a b)) (not (< b a)))`, sort may arrange them in either order. The `stable-sort` function is exactly like `sort`, except that it guarantees that two equivalent elements appear in the sorted list in the same order that they appeared in the original list.

Be careful: `sort` is allowed to destroy its argument, so if the original sequence is important to you, make a copy with the `copy-list` or `copy`-seq/ function.