do ((variable init step) …) (test expression …) command …
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 with a specified result value.
do expressions are evaluated as follows: The init expressions are evaluated (in some unspecified order), the variables are bound to fresh locations, the results of the init expressions are stored in the bindings of the variables, and then the iteration phase begins.
Each iteration begins by evaluating test, if the result is false, then the command expressions are evaluated in order for effect, the step expressions are evaluated in some unspecified order, the variables are bound to fresh locations, the results of the steps are stored in the bindings of the variables, and the next iteration begins.
If test evaluates to a true value, then the expressions are evaluated from left to right and the value of the last expression is returned as the value of the do expression. If no expressions are present, then the value of the do expression is unspecified in standard Scheme; in MIT and Inlab Scheme, the value of test is returned.
The region of the binding of a variable consists of the entire do expression except for the inits. 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 >