Đang chuẩn bị liên kết để tải về tài liệu:
How to Design Programs phần 2

Các đầu vào của một hàm là hiếm khi một phép đo duy nhất (số), chuyển đổi một vị trí duy nhất (boolean), hoặc một tên duy nhất (biểu tượng). Thay vào đó, nó gần như luôn luôn là một phần dữ liệu đại diện cho một đối tượng với nhiều tài sản. Mỗi tài sản là một phần của thông tin. | Section 6 Compound Data Part 1 Structures The input of a function is seldom a single measurement number a single switch position boolean or a single name symbol . Instead it is almost always a piece of data that represents an object with many properties. Each property is a piece of information. For example a function may consume a record about a CD the relevant information might include the artist s name the CD title and the price. Similarly if we are to model the movement of an object across a plane with a function we must represent the position of the object in the plane its speed in each direction and possibly its color. In both cases we refer to several pieces of information as if they were one one record and one point. In short we COMPOUND several pieces of data into a single piece of data. Scheme provides many different methods for compounding data. In this section we deal with structures. A structure combines a fixed number of values into a single piece of data. In section 9 we will encounter a method for combining an arbitrarily large number of values into a single piece of data. O 6.1 Structures Suppose we wish to represent the pixe much like a Cartesian point. It has an horizontal direction and it has a y coo coloreddots on our computer monitors. A pixel is very oordinate which tells us where the pixel is in the nate which tells us where the pixel is located in the coor downwards vertical direction. Given the two numbers we can locate a pixel on the monitor and so can a computer program. DrScheme s teachpacks represent pixels with posn structures. A posn structure combines two numbers. That is a posn is a single value that contains two values. We can create a posn structure with the operation make-posn which consumes two numbers and makes a posn. For example make-posn 3 4 make-posn 8 6 make-posn 5 12 are posn structures. Each of these structures has the same status as a number as far as computations are concerned. Both primitive operations and functions .