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Held: Friday, 22 February 2008
Summary: We consider why and how you name values within a procedure, and limit access to those names to within the procedure.
lethas the form
(let ((name1 exp1) (name2 exp2) ... (namen expn)) body)
lethas the meaning:
letin a simple expression:
(define values (list 1 4 2 4 1 5 9)) (let ((largest (max values)) (smallest (min values))) (/ (+ largest smallest) 2))
letwithin a procedure.
let*has the form
(let* ((name1 exp1) (name2 exp2) ... (namen expn)) body)
let*has the meaning:
;;; Procedure: ;;; exact-average ;;; Parameters: ;;; num1, an exact number ;;; num2, an exact number ;;; Purpose: ;;; Average the two numbers. ;;; Produces: ;;; average, an exact number ;;; Preconditions: ;;; num1 is an exact number [Verified] ;;; num2 is an exact number [Verified] ;;; Postconditions: ;;; Guess. (define exact-average (lambda (num1 num2) (let ((verify? (lambda (val) (and (number? val) (exact? val))))) (cond ((not (verify? num1)) (throw "exact-average" "first parameter is a non-number")) ((not (verify? num2)) (throw "exact-average" "second parameter is a non-number")) (else (/ (+ num1 num2) 2))))))
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