‘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 section see 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
“Named ‘let’” is a variant on the syntax of
let which provides
a more general looping construct than ‘do’ and may also be used to express
It has the same syntax and semantics as ordinary ‘let’
except that <variable> is bound within <body> to a procedure
whose formal arguments are the bound variables and whose body is
<body>. Thus the execution of <body> may be repeated by
invoking the procedure named by <variable>.
(let loop ((numbers '(3 -2 1 6 -5)) (nonneg '()) (neg '())) (cond ((null? numbers) (list nonneg neg)) ((>= (car numbers) 0) (loop (cdr numbers) (cons (car numbers) nonneg) neg)) ((< (car numbers) 0) (loop (cdr numbers) nonneg (cons (car numbers) neg))))) ==> ((6 1 3) (-5 -2))