In contrast to lists, structures deal with value extractions as a constant time operation.
In general, what we really wish to have in a programming language is:
1.a class of compound values size with constant lookup time,
2.based on ``keys.''
Because the problem is so common, Scheme and most other languages offer at least one built-in solution. (查看原文)
DrScheme also provides a vector analogue to build-list. It is called build-vector. Here is how it works:
1.(build-vector N f) = (vector (f 0) ... (f (- N 1)))
That is, build-vector consumes a natural number N and a function f on natural numbers. It then builds a vector of N items by applying f to 0, ..., N-1.
The operation vector-ref extracts a value from a vector in constant time, that is, for i between 0 and n (inclusive):
2.(vector-ref (vector V-0 ... V-n) i) = V-i
In short, extracting values from a vector is O(1).
If vector-ref is applied to a vector and a natural number that is smaller than 0 or larger than n, vector-ref signals an error.
The operation vector-length produces the number of items in a vector:
3.(vector-length (vector V-0 ... V-n)) = (+ n 1)
The operation vector? ... (查看原文)
平凡的老鱼
Noob
The MIT Press2001版
1 有用 figure9 2011-06-08 20:22:27
FP入门首选
0 有用 bysn 2021-02-23 14:15:06
很有营养的一本 思路清晰
10 有用 阅微草堂 2016-01-18 00:13:03
编程的本质 :把一个量和另一个量相关联 把值代换名进行关系计算;应用excel的过程本身就是一种广义的编程过程。程序就是数量之间的关系，对于特定的输入求取结果，从算术计算到变成对结构体 表 树的计算；程序逐步求精：先设计核心，在增加功能；工程总是由视图和程序组件模型两个部分 辅助函数原则 对于在表述中所提到的或在进行实例计算中所发现的每种依赖关系都使用一个辅助函数进行明确表达
2 有用 JmsAllen 2017-11-18 21:35:13
如果把精力放在Scheme的语法上，那说明根本没懂这本书是在讲什么。 这本书不是能用简单不简单来评价，它所描述的是程序的必要设计方式，对于任何时刻的编程都是极为重要的，和《sicp》一样，都是非常优秀的编程书籍。
0 有用 伯罗奔尼撒 2013-04-02 09:43:16
难得能在图书馆看到本lisp的书，不过只看了一点点。我也不会回头再看这个了，翻译略凶残……