How to Design Programs phần 5

Đề án cung cấp nhiều phương pháp khác nhau cho các dữ liệu phức hợp. Trong phần này, chúng ta đối phó với các cấu trúc. Cấu trúc kết hợp một số cố định các giá trị vào một mảnh duy nhất củaLàm: N - N thậm chí; để tính toán thứ i-ngay cả số (xác định (làm, thậm chí i) (* 2)) | define y21 1 1 sqrt x21 y21 D 3 4 From here the evaluation proceeds according to the standard rules until we encounter a second nested local-expression in the expression that we are evaluating define D x y local define x2 x x define y2 y y sqrt x2 y2 define x21 0 define y21 1 1 local define x2 3 3 define y2 4 4 sqrt x2 y2 define D x y local define x2 x x define y2 y y sqrt x2 y2 define x21 0 define y21 1 define x22 9 define y22 16 1 sqrt x22 y22 By renaming x2 and y2 again we avoided clashes. From here the evaluation of the expression is straightforward 1 sqrt x22 y22 1 sqrt 9 y22 GxX u 1 sqrt 9 16 1 sqrt 25 XX v 1 5 XXx xXX 6 The result is 6 as Exercise . Since local definitions are added to the Definitions window during an evaluation we might wish to try to see their values by just typing in the variables into the Interactions window. Is this possible Why or why not Exercise . Evaluate the following expressions by hand 1. local define x 2 5 x y 3 y 2. local define f c 9 5 c 32 - f 0 f 10 3. local define odd n cond zero n false else even sub1 n define even n cond zero n true else odd sub1 n even 1 -226- TEAM FLY PRESENTS 4. local define f x g x 1 22 define g x y x y f 10 555 5. define h n cond n 0 empty else local define r n n cons r h - n 1 h 2 The evaluations should show all local-reductions. Pragmatics of local Part 1 The most important use of local-expressions is to ENCAPSULATE a collection of functions that serve one purpose. Consider for an example the definitions for our sort function from section sort list-of-numbers - list-of-numbers define sort alon cond empty alon empty cons alon insert first alon sort restOalon insert number list-of-numbers sorted - list-of-numbers define insert an alon cond empty alon list an k else cond an first alon cons an alon else cons first alon insert an rest alon The first definition defines sort per se and the second one defines an auxiliary function that inserts a number into a sorted list of .