list-ref 入 return nth. 0-based. element of l #f if n>(length D) ( define( list-ref i n)…) (define (list-ref I n) cond ((null? D) (=0n) (car D) (else (list-ref (cdr D)(n 1)) sept.17,2003 16.410/16. More Scheme The define special form define-rule: (define <name> <value expression>) evaluate 2nd operand only, returning a value name in 1st operand position is bound to that value visible scheme versions differ world (define pi 3.14) P-->3.14 Environment executio name value world undefined 3.14 sept.17,2003 16.410/16. More SchemeSept. 17, 2003 11 list-ref • ; return nth, 0-based, element of l ; #f if n > (length l) (define (list-ref l n) … ) (define (list-ref l n) (cond ((null? l) #f) ((= 0 n) (car l)) (else Sept. 17, 2003 12 The Define special form • (define <name> <value expression>) – – – (define pi 3.14) eval define-rule print scheme versions differ • world • world "pi --> 3.14" undefined name pi 3.14 Environment define-rule: name in 1st operand position is bound to that value. returned value of the define expression is undefined visible execution value 16.410/16.413 More Scheme (list-ref (cdr l) (- n 1))))) 16.410/16.413 More Scheme evaluate 2nd operand only, returning a value