Question

(do ((n 0 (1+ n))
     (cur 0 next)
     (next 1 (+ cur next)))
    ((= 10 n) cur)))

This is an example from a Lisp textbook about the keyword do.

The do basic template is:

(do (variable-definitions*)
    (end-test-form result-form*)
 statement*)

But, for this example, it’s not clear to me which part is which. And also, what do the middle 2 lines do?

Answer 1

Your good indentation clearly shows which part is which:

(do ((n 0 (1+ n))
    ^(cur 0 next)
    |(next 1 (+ cur next)))
    |
    +-- first argument of do

    ((= 10 n) cur)))
    ^
    |
    +-- start of second argument of do

Look, they line up nicely, and the inner material is indented:

   ((n 0 (1+ n))
    (cur 0 next)
    (next 1 (+ cur next)))
    ^
    |
    +- inner material of argument: three forms which are
       indented by 1 character and aligned together.

Your do doesn’t have a third argument there: there is no body of statements (empty loop).