- (do ((<variable> <init> <step>) ...) (<test> <expression> ...) <command> ...)syntax
Do is an iteration construct. It specifies a set of variables to be bound, how they are to be initialized at the start, and how they are to be updated on each iteration. When a termination condition is met, the loop exits after evaluating the <expression>s.
Do expressions are evaluated as follows: The <init> expressions are evaluated (in some unspecified order), the <variable>s are bound to fresh locations, the results of the <init> expressions are stored in the bindings of the <variable>s, and then the iteration phase begins.
Each iteration begins by evaluating <test>; if the result is false (see the section about booleans), then the <command> expressions are evaluated in order for effect, the <step> expressions are evaluated in some unspecified order, the <variable>s are bound to fresh locations, the results of the <step>s are stored in the bindings of the <variable>s, and the next iteration begins.
If <test> evaluates to a true value, then the <expression>s are evaluated from left to right and the value(s) of the last <expression> is(are) returned. If no <expression>s are present, then the value of the do expression is unspecified.
The region of the binding of a <variable> consists of the entire do expression except for the <init>s. It is an error for a <variable> to appear more than once in the list of do variables.
A <step> may be omitted, in which case the effect is the same as if (<variable> <init> <variable>) had been written instead of (<variable> <init>).
(do ((vec (make-vector 5)) (i 0 (+ i 1))) ((= i 5) vec) (vector-set! vec i i)) ===> #(0 1 2 3 4) (let ((x '(1 3 5 7 9))) (do ((x x (cdr x)) (sum 0 (+ sum (car x)))) ((null? x) sum))) ===> 25