Common LISP Hints: Iteration

## 18. Iteration

The simplest iteration construct in LISP is `loop`: a `loop` construct repeatedly executes its body until it hits a `return` special form. For example,

> `

>     > (setq a 4)
>     4
>     > (loop
>        (setq a (+ a 1))
>        (when (> a 7) (return a)))
>     8
>     > (loop
>        (setq a (- a 1))
>        (when (< a 3) (return)))
>     NIL

> `

The next simplest is `dolist`: `dolist` binds a variable to the elements of a list in order and stops when it hits the end of the list.

> `

>     > (dolist (x '(a b c)) (print x))
>     A
>     B
>     C
>     NIL

> `

`Dolist` always returns `nil`. Note that the value of `x` in the above example was never `nil`: the`NIL` below the C was the value that `dolist` returned, printed by the read-eval-print loop.

The most complicated iteration primitive is called `do`. A `do` statement looks like this:

> `

>     > (do ((x 1 (+ x 1))
>            (y 1 (* y 2)))
>           ((> x 5) y)
>         (print y)
>         (print 'working)
>       )
>     1
>     WORKING
>     2
>     WORKING
>     4
>     WORKING
>     8
>     WORKING
>     16
>     WORKING
>     32

> `

The first part of a `do` specifies what variables to bind, what their initial values are, and how to update them. The second part specifies a termination condition and a return value. The last part is the body. A do form binds its variables to their initial values like a `let`, then checks the termination condition. As long as the condition is false, it executes the body repeatedly; when the condition becomes true, it returns the value of the return-value form.

The `do`* form is to `do` as `let`* is to `let`.